Blood circulatory system of man

Class 12 Biology, unit 2, book pages 27–53

Practise 62 MCQs Take the unit quiz Review 59 cards

Every cell needs food and oxygen and must get rid of carbon dioxide and wastes, but the body is too large for simple diffusion to do this. The circulatory system, a muscular heart, blood vessels and blood, carries materials round the body, and the lymphatic system returns tissue fluid to the blood and defends the body. This unit covers the structure and working of the heart, its electrical control, the blood vessels and the routes of circulation, blood flow and pressure, the main cardiovascular disorders and their treatment, and the lymphatic system.

Learning outcomes (25)

State the location of heart in the body and define the role of pericardium.
Describe the structure of the walls of heart and rationalize the thickness of the walls of each chamber.
Trace the flow of blood through the heart as regulated by the valve.
State the phases of heartbeat.
Explain the roles of SA node, AV node and Purkinje fibers in controlling the heartbeat.
List the principles and uses of Electrocardiogram.
Describe the detailed structure of arteries, veins and capillaries.
Describe the role of arterioles in vasoconstriction and vasodilation.
Describe the role of pre-capillary sphincters in regulating the flow of blood through the capillaries.
Trace the path of the blood through the pulmonary and systemic circulation (coronary, hepatic-portal and renal circulation).
Compare the rate of blood flow through arteries, arterioles capillaries, venules and veins.
Define blood pressure and explain its periods of systolic and diastolic pressure.
State the role of baroreceptors and volume receptor in regulating the blood pressure.
Define the term thrombus and differentiate between thrombus and embolus.
Identify the factors causing atherosclerosis and arteriosclerosis.
Categorize Angina pectoris, heart attack, and heart failure as the stages of cardiovascular disease development.
State the congenital heart problem related to the malfunction of cardiac valves.
Describe the principles of angiography.
Outline the main principle of coronary bypass, angioplasty and open-heart surgery.
Define hypertension and describe the factor that regulate blood pressure and lead to hypertension and hypotension.
List the changes in lifestyles that can protect man from hypertension and cardiac problems.
Describe the formation, composition and function of intercellular fluid.
Compare the composition of intercellular fluid with that of lymph.
State the structure and role of lymph capillaries, lymph vessels and lymph trunks.
Describe the functions of lymph nodes and state the role of spleen as containing lymphoid tissue.

Why we need a circulatory system

All the cells of the body need food from the small intestine and oxygen from the lungs. Carbon dioxide and the other waste chemicals have to be removed from the cells, through the lungs and the kidneys. The body is too large for materials to simply diffuse in and out, so it has a system of internal transport, a circulatory system.

It transports oxygen and carbon dioxide.
It distributes nutrients to the body cells.
It conveys the waste products of metabolism to specific sites for disposal.

The cardiovascular system consists of a strong muscular heart, three kinds of blood vessels (arteries, capillaries and veins) and blood.

Angiology: The study of the diseases of the cardiovascular system.

2.1Human heartOften examined

Location

The size of the heart varies with the body size. The human heart is about the size of an adult's fist. It lies within the mediastinum, a region bordered laterally by the lungs, posteriorly by the backbone and anteriorly by the sternum.

Its base lies beneath the second rib.
Its distal end extends downward and to the left, ending as a bluntly pointed apex at the level of the fifth intercostal space.

Role of pericardium

The pericardium is a closed sac that surrounds the heart. It is a fibrous membrane, an external covering around the heart. It has two parts: the visceral layer (the inner part), which directly covers the heart, and the parietal layer (the outer part), which forms a sac around the outer region of the heart and contains the fluid of the pericardial cavity.

The pericardium in section. The two layers (visceral layer in accent) enclose the fluid-filled pericardial cavity around the heart.

Roles of the pericardium

It prevents overfilling. The pericardium as a whole is inelastic, so the heart cannot be overstretched or overfilled with blood.
It lubricates the heart. The pericardium produces a serous fluid that lubricates the heart and prevents friction between the heart and the surrounding organs.
It holds the heart in position in the chest.
It maintains a hollow space (the pericardial cavity) in which the heart can expand when it is full.

Layer of pericardium	Position	Feature
Visceral layer (inner)	Directly covers the heart	Also called the epicardium (the outer layer of the heart wall, section 2.2)
Parietal layer (outer)	Forms a sac around the outer region of the heart	Contains the fluid in the pericardial cavity

2.2Structure of walls of heartOften examined

The heart has four chambers: two atria (singular atrium, meaning entrance chamber) and two ventricles (meaning belly). The atria lie above the ventricles.

Three layers of the heart wall

Epicardium: A thin serous membrane made of the smooth outer surface of the heart (the outermost layer).
Myocardium: The thick middle layer of the heart, made of cardiac muscle cells.
Endocardium: The smooth inner surface of the heart chambers. It consists of simple squamous epithelium over a layer of connective tissue. The heart valves are formed by a fold of endocardium, making a double layer of endocardium with connective tissue in between.

Layer	Position	Made of
Epicardium	Outer	Thin serous membrane, the smooth outer surface of the heart
Myocardium	Middle (thickest)	Cardiac muscle cells
Endocardium	Inner (lines the chambers)	Simple squamous epithelium over connective tissue

Figure 2.1: Human heart, external view. The right side of the heart is on the left of the picture.

Figure 2.2: Dissection of a human heart, as seen from the front, with the ventral part of both atria and both ventricles removed.

Thickness of the walls of the chambers

The thickness of the wall is different in each chamber, and each difference matches the work the chamber does.

Chamber	Wall	Reason
Atria	Comparatively thin	They only have to force blood into the ventricles, which does not need much power.
Right ventricle	Thicker than an atrium but thinner than the left ventricle (ratio 1 : 3 compared with the left)	It pumps blood to the lungs, which are at a short distance from the heart.
Left ventricle	Thickest	It must pump blood around the whole body.

Worked example: why the left ventricle has the thickest wall

Explain why the wall of the left ventricle is thicker than the wall of the right ventricle and of the atria.

State the rule: the thickness of the muscular wall of a chamber depends on the force it must produce, which depends on where it sends the blood.
Atria: they push blood only a short way, into the ventricles below them, so they need little power and have thin walls.
Right ventricle: it sends blood through the pulmonary trunk to the lungs, which are close to the heart, so it has thicker walls than the atria.
Left ventricle: it sends blood through the aorta to the whole body, a very long path, so it must produce the greatest force. Its myocardium is the thickest, about three times that of the right ventricle (1 : 3).

Chamber	Where the blood goes	Wall
Atrium	Into the ventricle	Thin
Right ventricle	To the lungs (short distance)	Medium
Left ventricle	Round the whole body	Thickest

Chambers, septa and openings

The right atrium receives the superior vena cava, the inferior vena cava and the coronary sinus.
The left atrium receives the four pulmonary veins.
The two atria are separated from each other by the interatrial septum.
The atria open into the ventricles through the atrioventricular canals.
The right ventricle opens into the pulmonary trunk and the left ventricle opens into the aorta.
The two ventricles are separated from each other by the interventricular septum.

Heart valves

An atrioventricular (AV) valve is present in each atrioventricular canal. It is composed of cusps (flaps).

Valve	Position	Cusps
Tricuspid valve	Between the right atrium and the right ventricle	Three
Bicuspid (mitral) valve	Between the left atrium and the left ventricle	Two
Pulmonary semilunar valve	Where the pulmonary trunk leaves the right ventricle	Semilunar (half-moon) cusps
Aortic semilunar valve	Where the aorta leaves the left ventricle	Semilunar (half-moon) cusps

Each ventricle contains cone-shaped muscular pillars called papillary muscles. They are attached by thin, strong connective tissue strings, the chordae tendineae, to the cusps of the atrioventricular valves. The papillary muscles contract when the ventricles contract and pull on the chordae tendineae, which stops the valve cusps from opening back into the atria.

2.3Flow of blood through the heartOften examined

The path of blood through the heart is regulated by the valves, which let blood move in one direction only.

Path of blood through the heart

The superior vena cava and the inferior vena cava, both carrying deoxygenated blood, enter the right atrium.
The right atrium sends blood through the tricuspid valve into the right ventricle.
The right ventricle sends blood through the pulmonary semilunar valve into the pulmonary trunk and the two pulmonary arteries to the lungs.
Four pulmonary veins, carrying oxygenated blood from the lungs, enter the left atrium.
The left atrium sends blood through the bicuspid valve into the left ventricle.
The left ventricle sends blood through the aortic semilunar valve into the aorta, which carries it to the body parts.

The heart is a double pump: the right ventricle sends blood to the lungs, and the left ventricle sends blood throughout the body.

Figure 2.3: Passage of blood through the heart. Read the top row from left to right, the middle row from right to left and the bottom row from left to right.

Side of heart	Blood received	Blood sent	Type of blood
Right atrium and ventricle	From the body, by the venae cavae	To the lungs, by the pulmonary trunk	Deoxygenated
Left atrium and ventricle	From the lungs, by the pulmonary veins	To the body, by the aorta	Oxygenated

Worked example: trace one red blood cell

A red blood cell has just come back from the lungs. Name, in order, the vessels, chambers and valves it passes through before it enters the aorta.

Answer

The cell returns from the lungs in a pulmonary vein. Four pulmonary veins open into the left atrium.
It enters the left atrium.
It passes through the bicuspid (mitral) valve, the atrioventricular valve on the left.
It enters the left ventricle.
It is pushed through the aortic semilunar valve and enters the aorta.

Order: pulmonary vein, left atrium, bicuspid valve, left ventricle, aortic semilunar valve, aorta. The cell carries oxygenated blood all along this path. For the right side of the heart the same pattern holds: vena cava, right atrium, tricuspid valve, right ventricle, pulmonary semilunar valve, pulmonary trunk.

2.4Phases of heartbeatOften examined

In a continuous, rhythmic cycle the heart is passively filled with blood from the large veins, and then the heart actively contracts and propels blood through the body. The alternating relaxations and contractions make up the cardiac cycle. The cardiac cycle is the sequence of events in one heartbeat.

Cardiac cycle: The sequence of alternating relaxations and contractions of the heart, that is, the sequence of events that occurs during one complete heartbeat.
Systole: Contraction of the heart muscle.
Diastole: Relaxation (dilation) of the heart muscle.

Atrial systole is contraction of the atrial myocardium and atrial diastole is its relaxation. Likewise ventricular systole is contraction of the ventricular myocardium and ventricular diastole is its relaxation. When the words systole and diastole are used without naming a chamber, they mean ventricular systole and ventricular diastole.

Phases of heartbeat

The four phases

Atrial diastole. Blood enters the right atrium from the body through the venae cavae. At first the bicuspid and tricuspid valves are closed. As the atria fill, the pressure in them rises. Eventually it becomes greater than the pressure in the relaxed ventricles and the valves are pushed open.
Atrial systole. The two atria contract at the same time and push blood through the atrioventricular valves into the still relaxed ventricles. In this phase the semilunar valves are closed and the tricuspid and bicuspid valves are open.
Ventricular systole. Almost at once the ventricles contract. The pressure in them rises and closes the atrioventricular valves, which prevents blood from going back into the atria. The pressure forces the semilunar valves of the aorta and the pulmonary artery to open and blood enters these vessels. In this phase the tricuspid and bicuspid valves are closed.
Ventricular diastole. The high pressure in the aorta and pulmonary artery tends to force some blood back towards the ventricles and closes the semilunar valves, so back flow is prevented. The bicuspid and tricuspid valves are open, and the aortic and pulmonary semilunar valves are closed.

Figure 2.4: Cardiac cycle. (1) The heart is relaxed and the AV valves are open, (2) the atria contract, (3) the ventricles contract and the semilunar valves are open. Of the 0.8 s cycle, diastole lasts 0.4 s, atrial systole 0.1 s and ventricular systole 0.3 s. Contracting chambers are drawn with a thick outline.

Phase	Chambers contracting	AV valves (tricuspid and bicuspid)	Semilunar valves	Blood flow
Atrial diastole / heart relaxed	None	Closed at first, then pushed open	Closed	Venae cavae and pulmonary veins into the atria
Atrial systole	Atria	Open	Closed	Atria into ventricles
Ventricular systole	Ventricles	Closed	Open	Ventricles into pulmonary trunk and aorta
Ventricular diastole	None	Open	Closed (back pressure from artery)	Atria into ventricles again

The normal cardiac cycle takes 0.7 to 0.8 second, depending on the ability of the cardiac muscle to contract. The heart muscles rest 0.1 to 0.3 second between the beats.

Times from the book's Figure 2.4
Part of the cycle	Time
Atrial systole	0.1 s
Ventricular systole	0.3 s
Diastole (heart relaxed)	0.4 s
Whole cardiac cycle	0.8 s

Worked example: heart rate and the length of the cycle

A resting person has a heart rate of 80 beats per minute. Find the time of one cardiac cycle. If atrial systole lasts 0.1 s and ventricular systole 0.3 s, for how long is the heart relaxed in each cycle?

One beat is one cardiac cycle, so the time of a cycle is one minute divided by the number of beats in it.
Time of one cycle:
$\frac{60 s}{80} = 0.75 s$
Subtract the two systoles from the cycle to find the time of rest (diastole).
$0.75 - (0.1 + 0.3) = 0.35 s$
Check against the book: the normal cardiac cycle is 0.7 to 0.8 s, so 0.75 s is in the normal range.

cycle = 0.75 s; relaxed for 0.35 s

2.5Conducting system of the heartOften examined

The heart goes on beating after it has been cut out of the body. Cardiac muscles are myogenic: their rhythmic contraction arises from within the muscle itself. Cardiac muscle has an intrinsic rhythmicity, which allows the heartbeat to start and be conducted through the heart without outside (extrinsic) stimulation.

Conduction system: Specialised strands of interconnecting cardiac muscle tissue that coordinate cardiac contraction.
Myogenic: Describes a heart whose rhythmic contraction arises from within the muscle itself.

The conduction system has four components: (a) the sinoatrial (SA) node, (b) the atrioventricular (AV) node, (c) the atrioventricular bundle (bundle of His) and (d) the conducting myofibrils (Purkinje fibres).

Figure 2.5: Conducting system of the heart. (1) Action potentials start in the SA node and spread across the wall of the atrium to the AV node. (2) They pass through the AV node and along the AV bundle into the interventricular septum. (3) The AV bundle divides into right and left bundle branches and the action potentials pass down to the apex of each ventricle. (4) The Purkinje fibres carry them from the bundle branches to the ventricular walls.

Parts of the conduction system

Part	Position	Description
SA node (sinoatrial node)	Upper wall of the right atrium, close to where the venae cavae enter the atrium	A specialised plexus of cardiac muscle. It developed from the sinus venosus and became part of the atrium, hence sino-atrial. It is the pacemaker.
AV node (atrioventricular node)	Near the junction of the right atrium and the right ventricle	A second specialised group of cardiac muscle fibres. Action potentials pass through it slowly.
AV bundle (bundle of His)	In the ventricular septum; it passes through a small opening in the fibrous skeleton into the interventricular septum	A strand of specialised muscle connected to the AV node. It divides into right and left bundle branches.
Bundle branches	Beneath the endocardium on either side of the interventricular septum	Run to the apices of the right and left ventricles.
Purkinje fibres	Terminal (inferior) branches of the bundle branches, in the ventricular walls	Large-diameter cardiac muscle fibres with fewer myofibrils, which do not contract forcefully.

Why the SA node is the pacemaker

All cardiac muscle cells can generate spontaneous action potentials, but the cells of the SA node do so at a greater frequency. The SA node therefore sets the rhythm for the whole heart and is called the pacemaker of the heart.

Why the Purkinje fibres conduct quickly

Intercalated disks are well developed between the Purkinje fibres and contain numerous gap junctions. Because of these structural modifications, action potentials travel along the Purkinje fibres much more rapidly than through other cardiac muscle tissue, so the ventricles contract almost together.

Timing of the impulse

Path and timing of the heartbeat signal

Under resting conditions about 0.04 second is needed for action potentials to travel from the SA node to the AV node.
Within the AV node the action potentials are propagated slowly compared with the rest of the conducting system. There is a delay of 0.11 second from the time they reach the AV node until they pass to the AV bundle.
The total delay of 0.15 second (0.04 s + 0.11 s) allows the atria to finish contracting before the ventricles begin to contract.

Differences between the SA node and the AV node
Feature	SA node	AV node
Position	Upper wall of the right atrium, near the openings of the venae cavae	Near the junction of the right atrium and right ventricle
Frequency of spontaneous action potentials	Highest, so it starts every heartbeat	Lower than the SA node
Role	Pacemaker of the heart	Passes the impulse to the AV bundle after a delay of 0.11 s
Order in the path	First	Second

2.6ElectrocardiogramOften examined

The electrical impulses that pass through the conduction system of the heart during the cardiac cycle can be recorded as an electrocardiogram (ECG). The electrical changes result from the depolarization and repolarization of cardiac muscle fibres and can be detected on the surface of the skin by an instrument called the electrocardiograph.

Principle of the ECG

The wave deflections, named P, QRS and T, are produced as specific events of the cardiac cycle occur. A normal ECG shows that the heart is functioning properly.

Figure 2.6: Electrocardiogram (ECG). The trace shows the P wave, the QRS complex (Q, R and S), the T wave and the U wave, with the intervals and segments between them. Voltage is plotted upwards and time to the right.

Part of the ECG	What it shows
P wave	Depolarization of the atrial fibres, starting at the SA node. The ventricles are in diastole while the P wave is produced. It occurs just before the atria contract.
PR interval	The time from the start of the P wave to the beginning of the QRS complex: the time needed for the depolarization from the SA node to reach the ventricles.
QRS complex	Depolarization of the ventricles. It begins as a short downward deflection (Q), continues as a sharp upward spike (R) and ends as a downward deflection (S). The ventricles are in systole and blood is ejected. It occurs just before the ventricles contract.
ST segment	The period between the completion of ventricular depolarization and the start of repolarization.
T wave	Ventricular repolarization. It occurs just before the ventricles relax.
J (junction) point	The junction between the end of the QRS complex and the beginning of the ST segment.

Normal values printed with Figure 2.6
Measurement	Normal value
PR interval	0.12 to 0.20 s
QRS duration	0.08 to 0.10 s
QT interval	0.4 to 0.43 s
RR interval	0.6 to 1.0 s

Worked example: read an ECG trace and find the heart rate

The trace below has labelled parts A to F. (a) Name each part and say what it shows. (b) The distance F is 0.9 s. Find the heart rate and say if it is normal.

Trace with parts A to F marked.

Letter	Part	What it shows
A	P wave	Depolarization of the atria (from the SA node)
B	QRS complex	Depolarization of the ventricles, which are in systole
C	T wave	Repolarization of the ventricles, just before they relax
D	PR interval	Time for the impulse to travel from the SA node to the ventricles
E	ST segment	Period between the end of ventricular depolarization and the start of repolarization
F	RR interval	One complete heartbeat, from one R spike to the next

F is the time between two R waves, so it is the time of one heartbeat (one cardiac cycle): 0.9 s.
Heart rate is the number of beats in 60 s.
$heart rate = \frac{60}{0.9} = 66.7 \approx 67 beats per minute$
The book gives the normal RR interval as 0.6 to 1.0 s. Since 0.9 s lies in this range, the heart rate is normal (a RR of 0.6 s gives 100 beats per minute and a RR of 1.0 s gives 60 beats per minute).

\approx 67 beats per minute (normal)

Uses of the electrocardiogram

An ECG is a painless test that measures the electrical activity of the heart to show whether or not it is working normally. An ECG can detect:

arrhythmias (irregular heartbeat)
conduction defects
the size and position of the heart chambers
damage to the heart muscle
impaired blood flow to the heart muscle
the effect of cardiac medicines
the function of artificial pacemakers

2.7Blood vesselsOften examined

There are three types of blood vessels.

Blood vessel	Function
Arteries (and arterioles)	Carry blood away from the heart
Veins (and venules)	Return blood to the heart
Capillaries	Permit the exchange of materials with the tissues

The three kinds of blood vessel in the path of blood: arteries and arterioles lead away from the heart, capillaries exchange materials with the tissues, and venules and veins return blood to the heart.

2.7.1ArteriesOften examined

Arteries carry blood away from the heart. They are pink in colour and are situated within the muscles. Arteries vary in size. They branch into arterioles and then into capillaries. The lumens of arteries have no valves.

Wall of an artery

The wall of an artery has three layers or tunics: tunica adventitia, tunica media and tunica intima.

Layer	Position	Structure
Tunica adventitia	Outermost	White fibrous connective tissue
Tunica media	Middle	Has a variable amount of elastic fibres. It is many layered in thickness and consists of one or two layers of circular smooth muscle cells.
Tunica intima	Innermost	Simple squamous epithelium (endothelium) and elastic fibres composed of elastin

Figure 2.7: Artery. The three tunics are marked on the right. The wall is thick and the lumen is narrow.

Arterioles

Arterioles transport blood from small arteries to the capillaries.

Sizes given in the book
Vessel	Approximate diameter
Aorta	23 mm
Arteriole	0.2 mm

2.7.2CapillariesOften examined

The wall of a capillary consists primarily of endothelial cells. Most capillaries range from 7 to 9 µm in diameter and branch without a change in their diameter. A capillary is approximately 1 mm long. Red blood cells flow through most capillaries in a single file.

Figure 2.8: Capillary. The wall is a single layer of endothelial cells and the red blood cells pass through one behind another.

Feature of a capillary	Value in the book
Wall	Primarily endothelial cells
Diameter	7 to 9 µm (does not change as it branches)
Length	About 1 mm
Red blood cells	Flow in single file
Function (from section 2.7)	Permit exchange of materials with the tissues

2.7.3VeinsOften examined

The blood vessels that bring blood back to the heart are called veins. Veins are relatively not deep in the muscles and can be seen as blue vessels under the skin. A vein also consists of tunica adventitia, tunica media and tunica intima.

Layer	Structure in a vein
Tunica adventitia	Composed of collagenous connective tissue
Tunica media	A thin layer of circularly arranged smooth muscle cells, collagen fibres and a few sparsely distributed elastic fibres
Tunica intima	Consists of endothelial cells and a thin layer of elastic fibres

Figure 2.9: Vein. The wall is thinner than that of an artery and the lumen is wider.

Venules, with a diameter of 40 to 50 µm, are tubes composed of endothelium. The venules collect blood from the capillaries and transport it to the small veins.

Valves in veins

Veins with a diameter greater than 2 mm contain valves. The valves allow blood flow towards the heart but not in the opposite direction. Valves are present only in the lower part of the body, especially in the abdomen and hind limbs. In the upper region above the heart there is no valve.

Figure 2.10: Valves in veins. When blood flows towards the heart the cusps lie against the wall (valve open); if blood tries to flow back the cusps meet and close the vein (valve closed).

Blood pressure in the veins is comparatively low. The flow of blood in the veins is therefore helped by gravity, the semilunar valves and muscular contraction.

Arteries, capillaries and veins compared

Feature	Artery	Capillary	Vein
Direction of blood	Away from the heart	Between arterioles and venules	Towards the heart
Position	Within the muscles; pink	Among the tissue cells	Not deep in the muscles; seen as blue vessels under the skin
Wall	Three tunics; thick, with elastic fibres and smooth muscle in the tunica media	Primarily endothelial cells (one cell thick)	Three tunics; thinner, with a thin tunica media
Lumen	No valves	7 to 9 µm; red blood cells in single file	Valves in veins wider than 2 mm
Blood pressure	High, rising and falling with each heartbeat	Blood flows slowest here (0.26 cm/s on average)	Comparatively low

Worked example: differentiate between an artery and a vein

Differentiate between an artery and a vein in three points.

FBISE asks this as a three-point table. Choose points the book states clearly.

Point	Artery	Vein
Direction of blood flow	Carries blood away from the heart	Brings blood back to the heart
Valves	No valves in the lumen	Valves present (in veins over 2 mm), which allow flow only towards the heart
Wall	Thick wall; tunica media has elastic fibres and smooth muscle	Thinner wall; tunica media has a thin layer of smooth muscle, collagen and a few elastic fibres
Position	Situated within the muscles; pink	Not deep in the muscles; blue under the skin

Any three rows are enough for full marks; add a labelled diagram of the two walls (Figures 2.7 and 2.9) if you have time.

2.8Role of arterioles in vasoconstriction and vasodilationOften examined

The amount of blood flowing through a blood vessel can be regulated by the contraction or relaxation of the smooth muscle in the tunica media.

Vasoconstriction: A decrease in blood flow resulting from a decrease in the diameter of the blood vessel, caused by contraction of its smooth muscle.
Vasodilation: An increase in blood flow produced by an increase in the diameter of the blood vessel, because of relaxation of its smooth muscle.

Arterioles regulate blood flow. Left: smooth muscle contracts, the lumen narrows and less blood flows (vasoconstriction). Right: smooth muscle relaxes, the lumen widens and more blood flows (vasodilation).

Feature	Vasoconstriction	Vasodilation
Smooth muscle of the tunica media	Contracts	Relaxes
Diameter of the vessel	Decreases	Increases
Blood flow	Decreases	Increases
Chemical agents	Hormones: norepinephrine is an especially powerful vasoconstrictor, epinephrine is less potent. Some of the prostaglandins are vasoconstrictors.	Kinins, formed in the blood and tissue fluids of some organs, e.g. histamine. Most of the prostaglandins are vasodilators.

Blood circulation is also controlled by hormones (vasoconstriction agents) acting on the arterioles. Several substances called kinins (vasodilator agents) can cause powerful vasodilation.

Critical thinking: Justify how vasodilation and vasoconstriction are reflective of emotions

During emotional rage, such as apprehension and rage, vasodilation occurs because of the secretion of epinephrine, a hormone responsible for fear, flight and fright conditions. The sympathetic vasodilator fibres are part of a regulatory system that originates in the cerebral cortex and ends at postganglionic neurons in the blood vessels of skeletal muscles; they activate them to release acetylcholine, and vasodilation occurs. Blood is discharged through thoroughfare channels rather than capillaries, so heat loss occurs and the skin becomes hot and red. In vasoconstriction the blood supply to the skin becomes less, so heat is preserved and the skin becomes cold. Situations such as shock, hypotension and tachycardia occur by stimulation of arterial stretch receptors, while hypertension and bradycardia (slowness of the heart) occur by increased intracranial pressure.

2.9Role of pre-capillary sphinctersOften examined

Arterioles supply blood to the capillary network, and blood then flows through the capillary network into the venules.

Path of blood through a capillary bed

Blood flows from the arterioles through the metarterioles.
From a metarteriole the blood flows into a thoroughfare channel.
Several capillaries branch from the thoroughfare channels.
The flow into these capillaries is regulated by smooth muscle cells called precapillary sphincters, which are located at the origin of the branches.
Blood from the capillaries is collected by the venule.

Figure 2.11: Capillary network. A sphincter (coloured mark) sits at the start of each capillary branch and can open or close its entrance.

How the sphincters work

A precapillary sphincter can open and close the entrance to a capillary.
Precapillary sphincters are normally either completely open or completely closed.
The degree of constriction of the metarteriole also varies.
The sphincters and metarterioles often open and close cyclically several times per minute. The duration of the open phases is about proportional to the metabolic needs of the tissue.

Vasomotion: The cyclic opening and closing of the precapillary sphincters and metarterioles.

2.10Path of blood through the pulmonary and systemic circulationOften examined

Pulmonary circulation

The left atrium receives oxygenated blood from the lungs through two pairs of pulmonary veins, which open by a common aperture into it. From the left atrium the blood flows into the left ventricle. The superior and inferior venae cavae bring deoxygenated blood and open into the right atrium. The blood is then sent to the lungs for oxygenation by the pulmonary arch or trunk, which divides into two pulmonary arteries, each going to the lung of its own side.

Pulmonary circulation (circuit): The part of the circulation that carries blood between the heart and the lungs. The pulmonary arteries carry deoxygenated blood and the pulmonary veins carry oxygenated blood.

Pulmonary circulation in order

The right ventricle pumps deoxygenated blood into the pulmonary trunk.
The trunk divides into the right and left pulmonary arteries, one to each lung.
In the lungs the blood is oxygenated.
Two pairs of pulmonary veins bring oxygenated blood back to the left atrium.

Figure 2.12: Cardiovascular system. Vessels carrying oxygenated blood are drawn in colour. The pulmonary circuit passes through the lungs; the systemic circuit supplies the head and arms, the liver and digestive tract, the kidneys, and the trunk and legs.

2.10Systemic circulationOften examined

The systemic circuit includes all the arteries and veins other than those involved in the pulmonary circuit. The largest artery in the systemic circuit is the aorta. The largest veins are the superior and inferior venae cavae.

Systemic circulation in order

The path of systemic blood to any organ begins in the left ventricle, which pumps blood into the aorta.
Branches from the aorta go to the organs and major body regions.
The superior vena cava collects blood from the head, the chest and the arms.
The inferior vena cava collects blood from the lower body regions.
Both venae cavae enter the right atrium.

The systemic circuit in order, from the left ventricle (accent) to the right atrium (accent).

The aorta and the venae cavae are the major pathways of the systemic circuit. In most instances the artery and the vein that serve the same organ are given the same name (for example the renal artery and renal vein).

Feature	Pulmonary circulation	Systemic circulation
Starts at	Right ventricle (pulmonary trunk)	Left ventricle (aorta)
Goes to	The lungs	All the organs and body regions except the lungs
Returns to	Left atrium, by pulmonary veins	Right atrium, by the venae cavae
Arteries carry	Deoxygenated blood	Oxygenated blood
Veins carry	Oxygenated blood	Deoxygenated blood

2.10Coronary circulationOften examined

The wall of the heart has its own supply of blood vessels to meet its vital needs. The myocardium is supplied with blood by the right and left coronary arteries.

Coronary circulation in order

The right and left coronary arteries supply the myocardium.
From the capillaries in the myocardium the blood enters the cardiac veins. Their course parallels that of the coronary arteries.
The cardiac veins converge to form the coronary sinus channel on the posterior surface of the heart.
The coronary venous blood enters the heart through an opening into the right atrium.

Coronary circulation: the heart wall is supplied by the coronary arteries and drained by the cardiac veins and coronary sinus.

2.10Hepatic portal systemOften examined

Portal system: A vascular system that begins and ends with capillary beds and has no pumping mechanism such as the heart.
Hepatic portal system: The portal system that begins with capillaries in the viscera and ends with capillaries in the liver.

The hepatic portal vein, the largest vein of the system, is formed by the union of all the veins coming from the digestive system.
Within the liver the blood flows through a series of dilated capillaries which empty into the hepatic veins.
The hepatic veins join the inferior vena cava.

Path of blood in the hepatic portal system

Capillaries in the viscera (digestive system) collect blood.
The veins from the digestive system unite to form the hepatic portal vein.
The hepatic portal vein enters the liver, where the blood flows through dilated capillaries.
The capillaries empty into the hepatic veins.
The hepatic veins join the inferior vena cava, which returns the blood to the right atrium.

Figure 2.13: Hepatic portal system. Veins from the stomach, spleen and intestines join to form the hepatic portal vein, which carries blood to the liver; the hepatic vein then joins the inferior vena cava.

2.10Renal circulation

Renal circulation in order

The renal artery enters the kidney and gives branches which pass through the medulla.
In the cortex the branches give rise to the afferent glomerular arterioles.
From these the blood enters the peritubular capillaries and the vasa recta.
From these capillary networks the blood is drained through veins and leaves the kidney as a single renal vein, which empties into the inferior vena cava.

Figure 2.14: Principal arteries and veins of the kidney.

2.11Rate of blood flow in blood vesselsOften examined

Blood flow means simply the quantity of blood that passes through a given point in the circulation in a given period. The overall blood flow in the circulation of an adult at rest is about 5000 ml/min. This is called the cardiac output: the amount of blood pumped by the heart in a unit time.

Cardiac output: The amount of blood pumped by the heart in a unit time (about 5000 ml/min in an adult at rest).

Comparison of the rate of blood flow through arteries, arterioles, capillaries, venules and veins

Blood travels over a thousand times faster in the aorta, about 30 cm/s on average, than in the capillaries, about 0.26 cm/s. One might expect blood to travel faster through capillaries because their diameter is very small. However, it is the total cross-sectional area of the capillaries that determines the flow rate.

Why blood slows in the capillaries

Each artery conveys blood to an enormous number of capillaries, so the total cross-sectional area of the capillary beds is much greater than that of any other part of the circulatory system.
Blood therefore slows substantially as it enters the arterioles from the arteries and slows further still in the capillary beds.
As blood leaves the capillaries and enters the venules and veins it speeds up again, because the total cross-sectional area is reduced.

Total cross-sectional area

Arteries

Arterioles

Capillaries

Venules

Veins

Vena cava

Type of blood vessel

Area (relative)

The total cross-sectional area is smallest in the aorta and greatest in the capillary beds, then falls again in the venules and veins. The shape is drawn from Figure 2.15; the values are relative, not measured.

Velocity of blood flow

Arteries

Arterioles

Capillaries

Venules

Veins

Vena cava

Type of blood vessel

Velocity (relative)

Blood velocity is the mirror image of the area curve: fast in the aorta (about 30 cm/s), slowest in the capillaries (about 0.26 cm/s) and faster again in the venules and veins.

Vessel	Total cross-sectional area	Velocity of blood
Aorta	Smallest (about 2.5 cm² for the aorta)	Highest (about 30 cm/s)
Arteries	Increasing	High, falling
Arterioles	Increasing	Falling
Capillaries	Greatest	Lowest (about 0.26 cm/s)
Venules	Decreasing	Rising
Veins	Decreasing	Rising

Worked example: blood speed and total area

The aorta has a cross-sectional area of about 2.5 cm² and blood flows through it at about 30 cm/s. (a) Find the volume of blood leaving the heart each second and each minute. (b) In the capillaries blood flows at 0.26 cm/s. What total cross-sectional area must all the capillaries have for the same volume to pass each second?

Flow is the area multiplied by the velocity.
$Q = A \times v = 2.5 c m^{2} \times 30 cm/s = 75 c m^{3} /s$
Per minute (1 cm³ = 1 ml):
$75 \times 60 = 4500 ml/min$
This is close to the cardiac output of about 5000 ml/min given in the book.
The same volume must pass through the capillaries every second, so use $Q = A v$ again.
$A = \frac{Q}{v} = \frac{75}{0.26} \approx 290 c m^{2}$
Compare with the aorta.
$\frac{290}{2.5} \approx 115$
So the total capillary area is about 115 times that of the aorta, and this is why the speed falls from 30 cm/s to 0.26 cm/s.

Q = 75 c m^{3} /s (4500 ml/min); A_{capillaries} \approx 290 c m^{2}

2.12Blood pressureOften examined

Blood pressure: The force exerted by the blood against any unit area on the inner walls of a blood vessel.
Systolic pressure: The highest arterial blood pressure, reached when the ventricles of the heart contract.
Diastolic pressure: The lowest arterial blood pressure, reached when the ventricles of the heart relax.

The standard reference for blood pressure is the mercury (Hg) manometer, which measures pressure in millimetres of mercury (mm Hg). If the blood pressure is 100 mm Hg, the pressure is great enough to lift a column of mercury 100 mm.

Arterial blood pressure

Arterial pressure rises to its highest value, the systolic pressure, when the ventricles contract and falls to its lowest value, the diastolic pressure, when the ventricles relax. 120/80 mm Hg is a normal reading. Drawn for illustration.

Period	Heart event	Arterial pressure
Systolic pressure	Ventricles contract	Highest
Diastolic pressure	Ventricles relax	Lowest

2.13Baroreceptors and volume receptorsOften examined

Baroreceptors can be divided into two categories, based on the type of blood vessel in which they are located: high-pressure arterial baroreceptors and low-pressure baroreceptors or volume receptors.

High-pressure arterial baroreceptors

These are mechanoreceptors located in the walls of the aorta and the carotid sinus in the carotid arteries. They sense the blood pressure and relay the information to the brain, so that a proper blood pressure can be maintained.

Baroreceptor control

When blood pressure is high, the parasympathetic nerves in these areas are stimulated. This causes vasodilation throughout the body, and a consequent reduction in blood pressure, as well as a slowing of the heart rate.
The opposite occurs when blood pressure is low. A fall in blood pressure increases nerve impulse transmission along sympathetic nerves. This causes body-wide vasoconstriction and a rise in blood pressure.

Baroreflex. Arterial baroreceptors in the aorta and carotid sinus keep blood pressure steady by a negative feedback loop.

Baroreflex: The negative feedback system in which the arterial baroreceptors act immediately to keep blood pressure steady.

Low-pressure baroreceptors or volume receptors

Low-pressure baroreceptors, or volume receptors, are found in the atria of the heart and the carotid arteries. When these receptors detect a decrease in blood volume in the atria, a signal is transmitted from the receptors to the hypothalamus in the brain. The hypothalamus in turn increases the production of antidiuretic hormone (ADH), which causes water retention in the kidney. This increases the blood volume, resulting in an increase in blood pressure.

Volume receptors restore blood volume and pressure after a fall in volume.

Differences between the baroreceptor and the volume receptor
Feature	High-pressure arterial baroreceptors	Low-pressure baroreceptors (volume receptors)
Position	Walls of the aorta and the carotid sinus	Atria of the heart (and carotid arteries)
Detect	Blood pressure	A decrease in blood volume
Response	Nerves (parasympathetic or sympathetic) change the vessel diameter and heart rate immediately	Signal to the hypothalamus, which raises ADH production so the kidney retains water

2.14Cardiovascular disordersOften examined

Cardiovascular disorders or diseases (CVD) are the diseases of the heart and blood vessels. CVD are the leading cause of untimely death.

Thrombosis and embolism

Thrombosis: The formation of a clotted mass of blood within a vessel or the heart during life.
Thrombus: The clotted mass of blood within a vessel or the heart formed during life.
Embolism: The occlusion (closing of an opening) of some part of the cardiovascular system by any mass transported to the site through the blood stream.
Embolus (plural: emboli): A detached intravascular solid, liquid or gaseous mass that is carried by the blood to a site distant from its point of origin.
Thromboembolus: An embolus that arises from the dislodgement of a thrombus. About 99% of emboli arise this way and are therefore called thromboemboli.

A thrombus is attached at the place where it formed; an embolus is detached and carried by the blood to a distant site.

Differences between thrombus and embolus
Feature	Thrombus	Embolus
What it is	A clotted mass of blood	A detached solid, liquid or gaseous mass
Where it is	Formed within a vessel or the heart, and stays at its place of formation	Carried by the blood stream to a site distant from where it started
Origin	Forms by clotting during life	About 99% arise from the dislodgement of thrombi (thromboemboli)
Effect	Can block the vessel; can cause death	Occludes (closes) a part of the cardiovascular system; can cause death

2.15Heart problemsOften examined

Atherosclerosis and arteriosclerosis

Atherosclerosis: The deposition of plaque of cholesterol in the arteries, which narrows the lumen of the arteries.
Arteriosclerosis: The later stage in which fibres are deposited in the cholesterol plaque and it often starts to calcify and become hard.

The plaque first forms a thrombus and may detach to form an embolus.

Atherosclerosis: cholesterol plaque narrows the lumen of an artery; later fibres and calcium harden the plaque (arteriosclerosis).

Stages

Cholesterol is deposited as a plaque in the wall of an artery, so the lumen becomes narrow (atherosclerosis).
Fibres are deposited in the cholesterol plaque.
The plaque begins to calcify and becomes hard (arteriosclerosis).
The plaque first forms a thrombus and may detach to form an embolus.

Factors that cause atherosclerosis and arteriosclerosis

Major factors	Minor risk factors
Hypercholesterolemia (hyperlipidemia)	Increasing age
Hypertension	Lack of exercise
Cigarette smoking	Stressful competitive life
Diabetes mellitus	Obesity

2.16Stages of cardiovascular disease developmentOften examined

The stages are angina pectoris, heart attack and heart failure. Congenital heart problems are also described.

Angina pectoris

Due to atherosclerosis a person may feel occasional chest pain, a condition known as angina pectoris. Angina is most likely to occur when the heart is labouring hard because of physical or emotional stress. Angina is a signal that part of the heart is not receiving a sufficient supply of oxygen and that part of the heart could suffer a heart attack in future.

Heart attack

Many heart attacks occur without warning. A blood clot may completely block a coronary artery, or atherosclerosis may reach a critical level causing massive damage to the heart muscle. All of a sudden the person feels a heavy squeezing ache or discomfort in the centre of the chest. The pain may radiate to the shoulder, arm, neck or jaw. Other symptoms may include sweating, nausea, shortness of breath and dizziness or fainting.

Myocardial infarction (heart attack): Death of heart muscle due to lack of oxygen.

When heart muscles die they are not replaced, because cardiac muscles do not divide. If a person survives a heart attack, scar tissue (a type of connective tissue) grows into the areas where the heart muscles have died. The scar tissue cannot contract as cardiac muscle, so the damaged heart is permanently weakened.

Heart failure

Congestive heart failure is the inability of the heart to pump all the blood coming to it. The cardiac output is unable to keep pace with the venous return.

Congenital heart problem

Congenital heart problems result from abnormalities in the embryonic development. It can be related to the malfunctioning of cardiac valves, e.g. valvular stenosis.

Stages in the development of cardiovascular disease, in the order given in the book.

Stage	What happens	Cause or note
Angina pectoris	Occasional chest pain, a signal of too little oxygen supply to part of the heart	Atherosclerosis; most likely when the heart labours hard
Heart attack (myocardial infarction)	Heart muscle dies from lack of oxygen; scar tissue forms	A clot blocks a coronary artery, or atherosclerosis reaches a critical level
Heart failure	Heart cannot pump all the blood that comes to it	Cardiac output cannot keep pace with venous return

2.17Diagnosis of cardiovascular disordersOften examined

Modern research efforts have resulted in improved diagnosis of CVD, their treatment and prevention.

a. Principles of angiography

Cardiac catheterization is a technique in which a specially designed catheter is inserted into a vein or artery and advanced into the heart under radiographic fluoroscopic guidance. This allows the operator to obtain angiograms by injecting contrast media into an area of interest. It is used to evaluate disease of the mitral valve, the aortic valve and the aorta, and to determine the size and function of the left ventricle. Coronary angiography is used to detect stenosis (narrowing of a tube) and to guide revascularisation procedures such as balloon angioplasty and stenting.

Figure 2.16: Coronary angiogram, schematic of the vessels and branches. The catheter reaches the opening of the coronary arteries and a narrowing (stenosis) is seen in a branch.

b. Coronary bypass

A coronary bypass is a surgical procedure that relieves the effects of obstruction in the coronary arteries. The technique involves taking healthy segments of blood vessel from other parts of the patient's body, usually a vein from the leg or an artery of the thorax, to bypass the obstructions in the coronary arteries.

Figure 2.17: Coronary artery bypass graft. The grafts join the aorta to the coronary artery beyond the blocked part.

c. Angioplasty

In angioplasty a cardiologist threads a plastic tube into an artery of an arm or a leg and guides it through a major blood vessel towards the heart. When the tube reaches the region of plaque in a coronary artery, a balloon attached to the end of the tube is inflated, forcing the vessel open. However, the artery may not remain open, so slotted tubes called stents are expanded inside the artery to keep it open. Stents are coated with heparin to prevent blood clotting and with chemicals to prevent arterial closing.

Angioplasty and stenting: a balloon on a catheter is inflated at the plaque and a stent is left to keep the artery open.

d. Open heart surgery

This is a surgery in which the patient's chest is opened. The surgery is performed on the heart. The term open refers to the chest, not to the heart itself: the heart may or may not be opened depending on the type of surgery. Heart surgery is used to correct heart problems in children and adults.

Steps of open heart surgery

An incision is made through the breastbone (sternum) while the patient is under general anaesthesia.
Tubes re-route the blood through a special pump called a heart-lung bypass machine. This machine adds oxygen to the blood and keeps the blood warm and moving through the rest of the body while the surgeon is repairing the heart.
Using the machine allows the heart to be stopped, which makes it possible to repair the heart muscle itself, the heart valves, or the blood vessels outside the heart.
After the repair the heart is started again and the machine is removed.
The breastbone and the skin incision are closed.

Procedure	Principle
Angiography	Catheter plus contrast media gives an angiogram that shows stenosis
Coronary bypass	Healthy vein (from the leg) or artery (of the thorax) is grafted to bypass the blocked coronary artery
Angioplasty	A balloon at the plaque opens the vessel; a stent keeps it open
Open heart surgery	The chest is opened, a heart-lung machine takes over, and the heart is repaired while stopped

2.18HypertensionOften examined

Blood pressure (BP) is the measurement of the pressure or force of blood pushing against blood vessel walls. A BP reading has two numbers.

Number	Name	What it measures
Top number	Systolic blood pressure	The pressure on the artery walls when the heart beats or contracts
Bottom number	Diastolic blood pressure	The pressure on the artery walls between beats, when the heart is relaxing

Physicians measure blood pressure in millimetres of mercury (mmHg). Your blood pressure may be different at different times of the day. High blood pressure is also called hypertension. When the blood pushes harder against the walls, your blood pressure goes up.

Hypertension: Blood pressure higher than 140/90 mmHg. A diagnosis may be made when one or both readings are high.

120/80 mmHg is normal blood pressure.

Readings given in the book
Reading	Meaning
120/80 mmHg	Normal blood pressure
Higher than 140/90 mmHg	Hypertension (high blood pressure)
Lower than 90/60 mm Hg	Hypotension (low blood pressure)

2.18.1Factors regulating blood pressureOften examined

Blood pressure is influenced by various factors. They can be broadly categorised into controllable and uncontrollable risk factors, with other physiological factors.

Controllable factors

Factor	How it affects blood pressure
Diet	High sodium intake, a diet low in potassium, and excess saturated fat can elevate blood pressure
Physical activity	Lack of regular exercise is a significant risk factor
Weight	Being overweight or obese increases the risk of high blood pressure
Stress	High stress levels can lead to temporary increases in blood pressure
Sleep	Insufficient or poor quality sleep can affect blood pressure regulation
Medications	Some medications, including cough and cold medicines, can interfere with blood pressure control

Uncontrollable factors

Factor	How it affects blood pressure
Age	The risk of high blood pressure increases with age
Genetics	A family history of hypertension can increase the predisposition
Medical conditions	Conditions like kidney disease, thyroid problems and obstructive sleep apnea (breathing stops and restarts many times during sleep) can affect blood pressure

Other factors

Factor	Role
Baroreceptors	Sensory receptors in blood vessels that monitor blood pressure and send signals to the brain to adjust heart rate and blood vessel constriction
Cardiac output	The amount of blood pumped by the heart with each beat; it is influenced by heart rate and stroke volume
Total peripheral resistance	The resistance to blood flow in the blood vessels; it is affected by factors like blood vessel diameter and blood viscosity
Blood volume	The amount of blood in the circulatory system also impacts blood pressure
Blood viscosity	The thickness of blood can affect its flow and pressure

The factors that regulate blood pressure, grouped as in the book.

2.18.2Hypotension

Low blood pressure is a condition in which the force of the blood pushing against the artery walls is too low. It is also called hypotension. Blood pressure is measured in millimetres of mercury (mm Hg). In general, low blood pressure is a reading lower than 90/60 mm Hg.

Condition	Reading
Normal	120/80 mm Hg
Hypertension	Higher than 140/90 mm Hg
Hypotension	Lower than 90/60 mm Hg

2.18.3Protection from hypertension and cardiac problemOften examined

To protect yourself from hypertension and cardiac problems, make significant lifestyle changes.

Change	What to do
1. Heart-healthy diet	Focus on fruits, vegetables, whole grains and lean protein (rich in nutrients and fibre, which help lower blood pressure and cholesterol). Limit saturated and trans fats (they can raise LDL cholesterol, increasing the risk of heart disease). Reduce sodium intake (high sodium contributes to high blood pressure). Choose anti-inflammatory foods such as dark green leafy vegetables, orange-coloured vegetables and legumes, which can help reduce the inflammation linked to heart disease.
2. Regular physical activity	Aim for at least 150 minutes of moderate-intensity aerobic activity or 75 minutes of vigorous activity per week (walking, running, swimming or dancing). Include strength training exercises, which help build muscle, improve overall metabolism and help manage weight.
3. Maintain a healthy weight	Calculate your Body Mass Index (BMI) to see whether you are at a healthy weight: being overweight or obese increases the risk of hypertension and heart disease. Combine a healthy diet with regular physical activity to lose weight and maintain it, which helps lower blood pressure and cholesterol levels.
4. Quit smoking	Smoking damages blood vessels and increases the risk of heart attack and stroke. Quitting smoking is one of the most important steps to improve heart health.
5. Manage stress	Find healthy ways to manage stress, such as exercise, yoga, meditation or spending time with loved ones, because chronic stress can contribute to high blood pressure and other health problems. Get enough sleep: aim for 7 to 9 hours of quality sleep per night to allow the body to recover and manage stress.

2.19Intercellular fluid and lymphOften examined

The lymphatic system includes lymph, lymphocytes, lymphatic vessels, lymph nodes, tonsils, spleen and the thymus gland. About one sixth of the body consists of spaces between the cells, which collectively are called the interstitium. The fluid in these spaces is the interstitial fluid or intercellular fluid.

Interstitial fluid

Aspect	Description
Formation	The fluid in the interstitium is derived by filtration and diffusion from the capillaries.
Composition	Almost the same constituents as plasma, except for much lower concentrations of proteins, because proteins do not pass outward through the walls of the capillaries with ease.
Function	Allows rapid transport of water molecules, electrolytes, nutrients, cellular excreta, oxygen, carbon dioxide and so on. Materials are exchanged between the blood and the interstitial fluid and between the interstitial fluid and the body cells. To get from the blood to the body cells or vice versa, materials must pass through the interstitial fluid.

Figure 2.19: Relationship between a blood capillary, interstitial fluid and tissue cells. Oxygen and dissolved food move from the capillary into the interstitial fluid and on to the cells; excretory waste products move the other way.

2.19.2Comparison of the composition of interstitial fluid and lymphOften examined

The composition of intercellular fluid is primarily water, along with electrolytes, sugars, acids, hormones, neurotransmitters and cell waste products. The tissue fluid that has entered the lymphatic capillaries is called lymph and passes through the lymphatic vessels back to the blood. In addition to water, lymph contains solutes such as ions, nutrients, gases and some proteins, hormones, enzymes and waste products. As lymph moves through the lymphatic system it accumulates more white blood cells (lymphocytes) and proteins.

Figure 2.20: Lymphatic vessels. Tissue fluid enters the blind end of a lymph capillary and becomes lymph, which flows into larger lymphatic vessels.

Table 2.1: Differences between composition of intercellular fluid and lymph
Feature	Intercellular fluid	Lymph
Major electrolytes	High potassium, high phosphate, moderate magnesium, low sodium	High sodium, high chloride, moderate potassium, moderate phosphate
Proteins	Higher concentration of proteins	Lower concentration of proteins than plasma
White blood cells	None given	Significantly higher, particularly lymphocytes
Red blood cells	None given	Absent normally
Water	High water content	High water content
Fats	None given	Can be high in lymph from the digestive system (chyle)
Origin	Fluid inside cells	Derived from blood plasma and interstitial fluid

2.20Lymphatic system of manOften examined

The lymphatic system includes lymph, lymphocytes, lymphatic vessels, lymph nodes, tonsils, spleen and the thymus gland.

Figure 2.21: Lymphatic system of man.

Lymph and lymphocytes
Lymphatic capillaries, vessels and trunks, ending in the thoracic duct and the right lymphatic duct
Lymph nodes (cervical, axillary, parasternal, superficial cubital and inguinal groups)
Tonsils, thymus gland and spleen
Peyer patches (aggregated lymphoid nodules) in the intestinal wall and the red bone marrow, as labelled in the figure

2.20.1Lymph vesselsOften examined

Lymphatic pathways begin as lymphatic capillaries. These tiny tubes emerge to form larger lymphatic vessels, which in turn lead to the collecting ducts that unite with the veins in the thorax.

a. Lymphatic capillaries

Lymph capillaries are microscopic, closed-ended tubes. They extend into the interstitial spaces, forming complex networks that parallel the networks of blood capillaries. The walls of the lymphatic capillaries, like those of blood capillaries, consist of a single layer of squamous epithelial cells. This thin wall makes it possible for interstitial fluid from the interstitial spaces to enter the lymphatic capillary. Once the fluid is inside a lymphatic capillary it is called lymph. Each villus contains a lymph capillary called a lacteal. The lacteal absorbs fats and other substances from the digestive tract.

b. Lymphatic vessels

Lymphatic vessels are formed by the merging of lymphatic capillaries. They have walls similar to those of veins. Like veins, lymphatic vessels have flap-like valves, which help to prevent the backflow of lymph. Typically the lymphatic vessels lead to specialised organs called lymph nodes. After leaving these structures the vessels merge to form still larger lymphatic trunks.

c. Lymphatic trunks

Lymphatic trunks drain lymph from relatively large portions of the body and are named for the region they serve. The lymphatic trunks then join one of two collecting ducts: the thoracic duct or the right lymphatic duct.

Collecting duct	Receives lymph from	Empties into
Thoracic duct (the larger and longer one)	The lower body regions, the left side, and the left side of the head and neck	The left subclavian vein, near the junction of the left jugular vein
Right lymphatic duct	The right side of the head and neck, the right arm and the right thorax	The subclavian vein, near the junction of the right jugular vein

Path of lymph

Interstitial fluid enters a lymph capillary and becomes lymph.
Capillaries merge into lymphatic vessels (with valves).
Vessels pass through lymph nodes and merge into lymphatic trunks.
The trunks join the thoracic duct or the right lymphatic duct.
The ducts empty into the subclavian veins, returning lymph to the blood.

The path of lymph from the tissues back to the blood. Lymph returns to the blood through the subclavian veins (accent).

Differences between blood capillaries and lymph capillaries
Feature	Blood capillary	Lymph capillary
Ends	Joined to arterioles and venules	Closed-ended (blind) tubes
Wall	Single layer of endothelial cells	Single layer of squamous epithelial cells
Contents	Blood	Lymph (tissue fluid that has entered it)
Function	Exchange of materials with the tissues	Takes up interstitial fluid; lacteals absorb fats from the digestive tract

2.20.2Lymph nodes and spleenOften examined

a. Lymph nodes

The lymph nodes are structures located along the lymphatic pathways.

They contain a large number of lymphocytes.
They act against foreign particles such as bacteria and viruses.
They contain macrophages, which engulf and destroy foreign substances, damaged cells and cellular debris.

b. Spleen

The spleen helps to get rid of germs. It contains white blood cells called lymphocytes and macrophages. These cells attack and destroy germs and remove them from the blood that passes through the spleen.

It filters blood by removing cellular waste and getting rid of old or damaged blood cells.
It stores blood and iron for future use.
It makes white blood cells and antibodies that help to fight infection.
It maintains the levels of fluid in the body.

Feature	Lymph node	Spleen
Position	Along the lymphatic pathways	An organ with lymphoid tissue
Cells	Lymphocytes and macrophages	Lymphocytes and macrophages
Functions	Act against bacteria and viruses; macrophages engulf foreign substances, damaged cells and debris	Destroys germs, filters blood, removes old or damaged blood cells, stores blood and iron, makes white blood cells and antibodies, maintains fluid levels

2.20.2STEAM activity 2.1: pulse and heart rate

When we are active, the heart pumps faster to help blood move around our bodies more quickly. As the heart beats and pumps blood around the body, you can feel a slight throbbing or thumping in some parts where an artery comes close to the surface of the skin. This is called your pulse. Your pulse tells you how fast your heart is beating; this is called your heart rate. The heart rate is measured in beats per minute (BPM).

There are two main places where the pulse can be felt: the neck and the wrist.

Place	How to find the pulse
Neck	Put two fingers of your left hand onto the side of the windpipe in your throat. Push down gently and you will find a pulse that feels like a small thump. It can be tricky to find, so you may have to move your fingers around.
Wrist	Using the pointer and middle fingers of your right hand, slide from the base of your left thumb (the squishy part of your palm) to where your hand meets your wrist. Move the fingers around until you find the right spot. Count how many times you feel it while timing 30 seconds. Multiply the number by 2 to calculate your heart rate in beats per minute.

Pulse experiment

Material required: a stopwatch, timer or clock with a second hand, pencil and record sheet.

Procedure

Find your resting heart rate, how fast your heart beats each minute when you are not doing physical activity. Set a timer for one minute. Sit down and relax, taking slow deep breaths.
Set a timer for 30 seconds and take your pulse on the wrist or neck. Record the number in the table and multiply it by 2 to calculate your resting heart rate in beats per minute.
Jog on the spot for 30 seconds. Set your timer and take your pulse again for 30 seconds. Record it and calculate beats per minute.
Rest for one minute to allow your heart rate to slow down again.
Continue the activities named on the record sheet for 30 seconds, taking your pulse after each to find BPM. Take one minute of rest after each time you take your pulse.

Record sheet
Name of the activity	Length of the activity	Heart rate (BPM)
Sitting	60 seconds	x 2 = ........ BPM
Jogging on the spot	30 seconds	x 2 = ........ BPM
Stretching	30 seconds	x 2 = ........ BPM
Running as fast as on the spot	30 seconds	x 2 = ........ BPM

Key terms

Angiology: The study of the diseases of the cardiovascular system.
Cardiovascular system: The strong muscular heart, the three kinds of blood vessels (arteries, capillaries, veins) and the blood.
Mediastinum: The region of the chest in which the heart lies, bordered laterally by the lungs, posteriorly by the backbone and anteriorly by the sternum.
Pericardium: A closed fibrous sac around the heart that stops overfilling, lubricates it with serous fluid and holds it in position.
Epicardium: The thin serous membrane forming the smooth outer surface of the heart wall.
Myocardium: The thick middle layer of the heart wall, made of cardiac muscle cells.
Endocardium: The smooth inner lining of the heart chambers: simple squamous epithelium over connective tissue.
Atrioventricular valve: A valve of cusps in each atrioventricular canal: the tricuspid valve (right) and the bicuspid or mitral valve (left).
Semilunar valves: The aortic and pulmonary valves at the exits of the left and right ventricles.
Papillary muscles: Cone-shaped muscular pillars in the ventricles, attached by chordae tendineae to the cusps of the atrioventricular valves.
Chordae tendineae: Thin strong connective tissue strings joining the papillary muscles to the cusps of the atrioventricular valves.
Cardiac cycle: The sequence of events in one heartbeat: alternating relaxation and contraction of the heart.
Systole: Contraction of the heart muscle.
Diastole: Relaxation (dilation) of the heart muscle.
Myogenic: Describes cardiac muscle whose rhythmic contraction arises from within the muscle itself.
Conduction system: Specialised strands of cardiac muscle tissue that coordinate cardiac contraction.
SA node: The sinoatrial node in the upper wall of the right atrium; its cells fire at the greatest frequency, so it is the pacemaker of the heart.
AV node: The atrioventricular node near the junction of the right atrium and ventricle; delays the impulse by about 0.11 s.
Bundle of His: The atrioventricular bundle in the ventricular septum, which divides into right and left bundle branches.
Purkinje fibres: Large-diameter conducting fibres at the ends of the bundle branches that carry impulses rapidly to the ventricular walls.
Cardiac arrhythmia: A disturbance in the electrical rhythm of the heart: bradycardia (less than 40 beats per minute) or tachycardia (more than 100 beats per minute).
Electrocardiogram (ECG): A recording of the electrical changes (depolarization and repolarization) of the heart, made with an electrocardiograph.
P wave: ECG wave produced by depolarization of the atria.
QRS complex: ECG wave produced by depolarization of the ventricles.
T wave: ECG wave produced by repolarization of the ventricles.
Artery: A blood vessel that carries blood away from the heart; wall of tunica adventitia, media and intima; no valves.
Capillary: A blood vessel 7 to 9 µm wide with a wall of endothelial cells; allows exchange of materials with the tissues.
Vein: A blood vessel that returns blood to the heart; has valves when wider than 2 mm.
Vasoconstriction: A decrease in blood flow caused by contraction of smooth muscle and a decrease in vessel diameter.
Vasodilation: An increase in blood flow caused by relaxation of smooth muscle and an increase in vessel diameter.
Precapillary sphincter: Smooth muscle at the origin of a capillary branch that opens and closes its entrance.
Vasomotion: The cyclic opening and closing of the precapillary sphincters and metarterioles.
Pulmonary circulation: The circuit of blood from the right ventricle to the lungs and back to the left atrium.
Systemic circulation: The circuit of blood from the left ventricle through the aorta to the body and back by the venae cavae.
Portal system: A vascular system that begins and ends with capillary beds and has no pump.
Hepatic portal system: The portal system from capillaries in the viscera to capillaries in the liver, through the hepatic portal vein.
Cardiac output: The amount of blood pumped by the heart in a unit time (about 5000 ml/min at rest).
Blood pressure: The force exerted by the blood against a unit area of the inner wall of a blood vessel, in mm Hg.
Systolic pressure: The highest arterial pressure, when the ventricles contract.
Diastolic pressure: The lowest arterial pressure, when the ventricles relax.
Baroreceptors: Mechanoreceptors in the aorta and carotid sinus that sense blood pressure.
Baroreflex: The negative feedback system of the arterial baroreceptors.
Volume receptors: Low-pressure baroreceptors in the atria that detect a fall in blood volume.
Thrombus: A clotted mass of blood formed within a vessel or the heart during life.
Embolus: A detached solid, liquid or gaseous mass carried by the blood to a distant site.
Atherosclerosis: Deposition of cholesterol plaque in the arteries, narrowing the lumen.
Arteriosclerosis: Hardening of the plaque by deposited fibres and calcification.
Angina pectoris: Occasional chest pain from too little oxygen supply to part of the heart.
Myocardial infarction: Death of heart muscle due to lack of oxygen (heart attack).
Heart failure: Inability of the heart to pump all the blood that comes to it.
Angiography: Imaging by a catheter and contrast media to detect stenosis.
Stent: A slotted tube expanded inside an artery to keep it open after angioplasty.
Hypertension: Blood pressure higher than 140/90 mmHg.
Hypotension: Blood pressure lower than 90/60 mm Hg.
Interstitial fluid: The fluid in the spaces between cells, derived from the capillaries by filtration and diffusion.
Lymph: Tissue fluid that has entered the lymphatic capillaries.
Thoracic duct: The larger collecting duct, draining the lower body and the left side of the head and neck into the left subclavian vein.
Lymph node: A structure along the lymphatic pathways containing lymphocytes and macrophages.
Spleen: A lymphoid organ that destroys germs, filters blood and stores blood and iron.

Multiple choice questionsp. 50–52

Each question has one correct key. The reasons say why the tempting option is wrong.

11 mark

Which one of the following has the thickest wall?

Aright ventricle
Bleft ventricleAnswer
Cright atrium
Dleft atrium

Why

The left ventricle pumps blood around the whole body, so its wall is the thickest of the four chambers. The right ventricle is the tempting choice, but its wall is only about one third as thick as the left (ratio 1:3), because it pumps blood only to the nearby lungs.

The atria have the thinnest walls because they only push blood into the ventricles, which needs little force.

21 mark

The reason why tricuspid and bicuspid valves are closed is

Aventricular relaxation
Bventricular filling
Catrial systole
Dattempted backflow of blood into the atriaAnswer

Why

When the ventricles contract, the pressure inside them rises above the pressure in the atria. This closes the atrioventricular (tricuspid and bicuspid) valves so that blood cannot flow back into the atria.

Ventricular filling (B) and atrial systole (C) happen while these valves are open, so they cannot be the reason for closure.

31 mark

Tricuspid valve is present between

Aventricle and pulmonary artery
Bventricle and aorta
Cleft auricle and left ventricle
Dright auricle and right ventricleAnswer

Why

The tricuspid valve guards the opening from the right atrium (right auricle) into the right ventricle. Options A and B describe the semilunar valves at the exits of the ventricles, and option C describes the bicuspid valve on the left side.

41 mark

From where does the aorta originate?

Aright ventricle
Bleft atrium
Cright atrium
Dleft ventricleAnswer

Why

The left ventricle opens into the aorta through the aortic semilunar valve, and the aorta carries oxygenated blood to the body. The right ventricle gives rise to the pulmonary trunk, not the aorta, and the atria do not give rise to any great artery.

51 mark

How many aortic valves are present?

A1Answer
B2
C3
D4

Why

There is one aortic semilunar valve, at the base of the aorta where it leaves the left ventricle. The number 3 is tempting because the valve has three flaps, but the question asks for the number of valves, not flaps.

61 mark

The epicardium:

Ais also known as the parietal pericardium
Bis a layer of cardiac muscle
Cis the visceral pericardiumAnswer
Dlines the heart chambers

Why

The epicardium is the thin serous outer layer that covers the outside of the heart. It is the visceral layer of the pericardium, so the two names describe the same tissue.

Option A names the wrong layer, since the parietal layer forms the outer sac. Option B confuses it with the myocardium, and option D describes the endocardium, which lines the chambers.

71 mark

The tissue which forms a loose-fitting sac around the heart is the:

Avisceral pericardium
Bparietal pericardiumAnswer
Cmyocardium
Depicardium

Why

The parietal layer of the pericardium forms a sac around the outer region of the heart, and the pericardial cavity between the two layers contains a serous fluid that lubricates the heart.

The visceral layer (A) and the epicardium (D) lie directly on the heart and are not the sac. The myocardium (C) is the muscular middle wall of the heart.

81 mark

The valve between the left ventricle and the blood vessel leaving the left ventricle is the:

Abicuspid valve
Btricuspid valve
Cpulmonary semilunar valve
Daortic semilunar valveAnswer

Why

The blood vessel leaving the left ventricle is the aorta, and the valve at its base is the aortic semilunar valve. The bicuspid valve lies between the left atrium and the left ventricle, so it is the tempting but wrong choice.

91 mark

During the period of ejection in the cardiac cycle, the atrioventricular valves are ........ and the semilunar valves are ........

Aclosed, closed
Bclosed, openAnswer
Copen, closed
Dopen, open

Why

Ejection happens during ventricular systole. The ventricles contract, their pressure closes the atrioventricular valves, and the pressure then pushes the semilunar valves open so that blood leaves for the aorta and the pulmonary trunk.

Option D would let blood flow back into the atria, and option A would leave the blood inside the ventricles.

101 mark

The second heart sound, described as "dupp", is actually the sound of the

Aatria contracting
Bventricles contracting
Catrioventricular valves closing
Dsemilunar valves closingAnswer

Why

The second heart sound is higher pitched and comes from the closing of the aortic and pulmonary semilunar valves near the end of systole, when the high pressure in the aorta and pulmonary artery pushes these valves shut.

The first sound, the low-pitched "lub", comes from the closing of the atrioventricular valves (option C).

111 mark

Cardiac output is determined by:

Aheart rate
Bstroke volume
Cblood flow
Dheart rate and stroke volumeAnswer

Why

Cardiac output is the amount of blood the heart pumps in a unit period. It depends on how many beats occur per minute (heart rate) and how much blood leaves the ventricle with each beat (stroke volume), so both must be included.

Options A and B each name only one of the two factors. Blood flow (C) is the cardiac output itself, not the thing that determines it.

121 mark

Which one is the definition of cardiac cycle?

AThe contraction of the atria.
BCirculation of blood in the heart.
CThe contraction and relaxation of the ventricles.
DIt is a sequence of events that occurs during a complete heartbeat.Answer

Why

The cardiac cycle is the whole sequence of events in one heartbeat, made of alternating relaxation and contraction of the atria and the ventricles. Options A and C describe only a part of it, and option B describes the circulation rather than the cycle.

131 mark

For how long is the cardiac cycle, if the heart rate is 75 beats per minute?

A0.7 s
B0.8 sAnswer
C0.5 s
D0.4 s

Why

One cardiac cycle lasts one heartbeat, so its length is 60 s divided by 75 beats: 60/75 = 0.8 s. This value lies in the normal range of 0.7 to 0.8 s for a resting heart.

The value 0.7 s would need about 86 beats per minute, so it is the nearest wrong answer for a student who rounds the calculation down.

141 mark

The fluid that passes through the lymphatic vessels

Aflows toward the lungs
Bpasses from the lymphatic vessels into the arteries
Centers the left ventricle of the heart through the right thoracic duct
Dmoves in a single direction toward the heartAnswer

Why

Lymph moves in one direction only, toward the heart. Valves in the lymphatic vessels prevent backflow, and the vessels finally empty into veins near the heart, which return the fluid to the blood.

Option C is wrong because the thoracic duct is on the left side and it empties into a vein (the left subclavian vein), not directly into the ventricle. Lymph does not pass into arteries, so option B is also wrong.

151 mark

Lymph nodes may be located in the human body in the tissues of the

Astomach and brain
Bgroin and neckAnswer
Cventricle and atrium
Dthyroid gland and adrenal gland

Why

Lymph nodes lie along the lymphatic pathways. Among the given choices, the cervical (neck) nodes and the inguinal (groin) nodes are the examples shown in the lymphatic system figure.

The ventricles and atria (option C) are heart chambers, and the thyroid and adrenal glands (option D) are endocrine glands. Option A is wrong because the stomach and brain have no lymph nodes of this kind.

161 mark

A red blood cell, entering the right side of the heart, passes by or through the following structures: 1. atrioventricular valve 2. semilunar valve 3. right atrium 4. right ventricle 5. pulmonary trunk. In which order will the red blood cell pass the structures?

A2 \to 3 \to 1 \to 4 \to 5
B3 \to 1 \to 4 \to 2 \to 5Answer
C3 \to 5 \to 1 \to 2 \to 4
D5 \to 3 \to 1 \to 4 \to 2

Why

Blood enters the right atrium (3) from the vena cavae.
It passes through the tricuspid valve, the atrioventricular valve on the right (1), into the right ventricle (4).
It leaves the right ventricle through the pulmonary semilunar valve (2) and enters the pulmonary trunk (5).

The order 3, 1, 4, 2, 5 is option B. Option A starts at a valve, and option D puts the pulmonary trunk first, which is downstream of the right ventricle.

171 mark

The rhythmic beating of cardiac muscle in the mammalian heart is initiated by the

Aatrio-ventricular node
Bparasympathetic nervous system
CPurkinje tissue
Dsino-atrial nodeAnswer

Why

The sino-atrial (SA) node is the pacemaker. Its cells generate action potentials most frequently, so the whole heart beats at its rate. The AV node only passes on and delays the impulse, and the Purkinje fibres conduct it to the ventricles.

The parasympathetic nerves can slow the heart, but they do not start each beat.

181 mark

What produces systolic blood pressure?

Acontraction of the right atrium
Bcontraction of the right ventricle
Ccontraction of the left atrium
Dcontraction of the left ventricleAnswer

Why

Systolic pressure is the highest arterial pressure, reached when the ventricles contract. The left ventricle pumps blood into the aorta and around the whole body, so its contraction produces the systolic pressure in the arteries of the body.

The atria (A and C) push blood only into the ventricles and do not produce the arterial pressure, while the right ventricle (B) sends blood only to the lungs.

191 mark

Human heart is

AmyogenicAnswer
Bneurogenic
Ccardiogenic
Ddigenic

Why

A myogenic heart beats because its rhythmic contraction arises from within the cardiac muscle itself. The heart keeps beating even when it is cut out of the body. A neurogenic heart would need nerve impulses to start each beat, so option B is the tempting wrong answer.

201 mark

Pacemaker is situated in heart

Ain the wall of right atriumAnswer
Bon interauricular septum
Con interventricular septum
Din the wall of left atrium

Why

The pacemaker is the SA node, which is embedded in the upper wall of the right atrium close to where the vena cavae enter it.

The interventricular septum (C) carries the bundle of His and the bundle branches, which conduct the impulse but do not start it. The interauricular septum (B) and the wall of the left atrium (D) contain no pacemaker tissue.

Short answer questionsp. 52

13 marks

Why do we have a circulatory system?

Solution

The body is too large for materials to diffuse in and out of the cells fast enough, so a circulating fluid moves substances over long distances.
Blood carries oxygen from the lungs and nutrients from the small intestine to the body cells.
Blood carries carbon dioxide and other waste products away from the cells to the lungs and kidneys for removal.

23 marks

What are the contraction and relaxation of human heart called?

Solution

Contraction of the heart chambers is called systole, and relaxation (dilation) is called diastole.
Each chamber has its own systole and diastole. Atrial systole is the contraction of the atria and ventricular systole is the contraction of the ventricles.
One complete heartbeat, made of alternating systole and diastole, is called the cardiac cycle.

33 marks

Where is SA node, AV node, Purkinje fibre, Bundle of His located?

Solution

SA node: in the upper wall of the right atrium, close to where the vena cavae enter it.
AV node: near the junction of the right atrium and the right ventricle.
Bundle of His (atrioventricular bundle): it leaves the AV node, passes through the fibrous skeleton into the interventricular septum and divides into the right and left bundle branches.
Purkinje fibres: the terminal branches of the bundle branches, which run beneath the endocardium on either side of the septum to the apex of each ventricle and into the ventricular walls.

43 marks

Why action potentials travel along the Purkinje fibres more rapidly than through other muscle fibres?

Solution

Purkinje fibres are large-diameter cardiac muscle fibres, and a wider fibre conducts an impulse faster than a narrow one.
They have fewer myofibrils than most cardiac muscle cells and do not contract forcefully. Their job is to conduct the impulse, not to pump.
Intercalated disks between Purkinje fibres are well developed and contain numerous gap junctions, which pass the impulse rapidly from one cell to the next.

53 marks

Name the artery supplying blood to the heart.

Solution

The heart muscle is supplied by the right coronary artery and the left coronary artery, which branch from the aorta.
The coronary arteries carry oxygenated blood to the cardiac muscle (myocardium).
The blood from the heart wall drains through the cardiac veins into the coronary sinus, which opens into the right atrium.

63 marks

What is blood pressure?

Solution

Blood pressure is the force exerted by the blood against a unit area of the inner walls of a blood vessel.
It is measured in millimetres of mercury (mm Hg) with a mercury manometer. A pressure of 100 mm Hg lifts a column of mercury 100 mm high.
Systolic pressure is the highest arterial pressure, produced when the ventricles contract. Diastolic pressure is the lowest, when the ventricles relax.

73 marks

Why SA node is called pacemaker of the heart?

Solution

The SA node is made of specialized cardiac muscle cells that generate action potentials spontaneously and more frequently than any other part of the conducting system.
Its impulses spread across the atria to the AV node and on through the conducting system, so every heartbeat starts at the SA node and the whole heart beats at its rate.
The SA node therefore sets the heart rhythm. If it fails, an artificial pacemaker can supply the electrical impulses at a set rate.

83 marks

What is a cardiac cycle?

Solution

A cardiac cycle is the sequence of events that occurs during one complete heartbeat.
It consists of alternating relaxation and contraction of the atria and the ventricles: atrial systole (about 0.1 s), ventricular systole (about 0.3 s) and complete diastole (about 0.4 s).
A normal cardiac cycle lasts about 0.7 to 0.8 s, depending on the capability of the cardiac muscle to contract.

93 marks

What is an arterial pulse? What is the normal human pulse rate?

Solution

The arterial pulse is the rhythmic throbbing felt when an artery lying close to the skin expands with each heartbeat.
It can be felt at the neck and at the wrist, where the artery comes near the surface, and it shows how fast the heart is beating (the heart rate, in beats per minute).
A normal resting pulse rate is about 60 to 100 beats per minute. Tachycardia is a rate above 100 and bradycardia is a rate below 40 beats per minute.

Note. No single normal pulse rate is printed in the text. The range of 60 to 100 beats per minute follows from its RR interval of 0.6 to 1.0 s on the electrocardiogram.

103 marks

Why AV node is essential for the conduction of cardiac impulse?

Solution

Action potentials from the SA node pass through the AV node and along the AV bundle into the ventricles. The AV bundle passes through the fibrous skeleton, so the AV node and bundle are the route by which the impulse reaches the ventricles.
The impulse is delayed for 0.11 s in the AV node before it passes to the AV bundle, so the total delay from the SA node is 0.15 s.
This delay lets the atria finish contracting and empty into the ventricles before the ventricles contract, so the ventricles start their contraction with a full chamber.

113 marks

What are the risks associated with atherosclerosis?

Solution

Plaque narrows the lumen of the artery, which reduces the blood supply to the tissue beyond it. Later the plaque may calcify and harden, which is arteriosclerosis.
A thrombus (clot) can form on the plaque and may break away as an embolus, which can block a vessel at a distant site.
In the coronary arteries, a narrowed supply causes angina pectoris (chest pain when the heart labours), and a blocked supply causes a heart attack (myocardial infarction), which kills heart muscle.

123 marks

Why can you feel your pulse in arteries but not in veins? If there is no pulse in your veins what pushes the blood in veins back to the heart?

Solution

Arteries have thick elastic walls, which expand and recoil with the surge of blood from each ventricular contraction. This expansion is the pulse. Veins have thin walls and low blood pressure, so they do not pulse.
Blood in the veins is pushed back to the heart mainly by the contraction of the skeletal muscles around the veins, which squeezes the blood along the vessel.
Valves in the veins prevent backflow, and gravity also helps the flow of blood from the upper body. Valves are present only in the lower part of the body.

133 marks

Define the term thrombus and differentiate between thrombus and embolus.

Solution

A thrombus is a clotted mass of blood that forms within a vessel or the heart during life. The table shows how it differs from an embolus.

Feature	Thrombus	Embolus
Definition	A clotted mass of blood forming inside a vessel or the heart during life.	A detached intravascular solid, liquid or gaseous mass carried to a site away from its point of origin.
Origin	Forms in place, on the wall of a vessel or the heart.	Usually a piece dislodged from a thrombus (about 99% of emboli).
Movement	Stays at the site where it formed.	Travels in the blood stream.
Effect	Blocks the vessel where it formed.	Blocks a vessel at a site distant from its origin. Thrombus and embolus can both cause death.

143 marks

Identify the factors causing atherosclerosis and arteriosclerosis.

Solution

Major factors: hypercholesterolemia (high cholesterol in the blood), hypertension (high blood pressure), cigarette smoking and diabetes mellitus.
Other risk factors: increasing age, lack of exercise, a stressful competitive life and obesity.
Arteriosclerosis develops from atherosclerosis, as fibres are deposited in the cholesterol plaque and it hardens and calcifies, so the same factors contribute to both.

153 marks

List the advantages and disadvantages of coronary bypass.

Solution

Advantages	Disadvantages
It relieves the effects of an obstruction in a coronary artery, so the blood supply to the heart muscle is restored past the blockage.	It is a major surgical procedure, which the patient must survive and recover from.
The graft is the patient's own healthy tissue, taken from a vein of the leg or an artery of the thorax.	A second operation site is needed to take the graft from the leg or the thorax, which adds to the wound and recovery time.
The graft provides a new route for the blood, so the blocked part of the artery is bypassed.	The graft only goes round the obstruction. It does not remove the atherosclerosis that caused it, so new blockages can form.

163 marks

List the changes in the life styles that can protect man from hypertension and cardiac problems.

Solution

Heart-healthy diet: eat more fruits, vegetables, whole grains and lean protein, limit saturated and trans fats, reduce sodium and choose anti-inflammatory foods.
Regular physical activity: at least 150 minutes of moderate aerobic activity or 75 minutes of vigorous activity per week, plus strength training.
A healthy weight: check the body mass index and combine a healthy diet with regular exercise to lose weight and keep it off.
Quit smoking, which damages the blood vessels and increases the risk of heart attack and stroke.
Manage stress and aim for 7 to 9 hours of quality sleep each night.

173 marks

What is the major feature of human lymphatic system?

Solution

The major feature is that the lymphatic system is a one-way drainage system. It begins as closed-ended lymph capillaries that collect the interstitial fluid from the tissue spaces.
The lymph vessels have valves that prevent backflow, so the lymph moves through the lymph nodes towards the thorax.
The lymph returns to the blood at the subclavian veins through the thoracic duct and the right lymphatic duct, so the system returns fluid that would otherwise be left in the tissues.

183 marks

Justify why blood circulatory system is dependent on lymphatic system.

Solution

Interstitial fluid is formed by filtration and diffusion from the blood capillaries. Lymphatic capillaries collect the fluid that the blood capillaries cannot take back, and the lymph returns it to the blood.
Without the lymphatic system the fluid would stay in the tissues and the volume of blood would fall, so the circulation would not work properly.
The lymphatic system also carries fats absorbed from the digestive tract (chyle) and the lymphocytes into the blood, so it links the digestive system and the defence of the blood with the circulation.

193 marks

Interpret why the swelling of the lymph node is cause of concern.

Solution

Lymph nodes contain many lymphocytes and macrophages, which attack bacteria, viruses and other foreign substances, and destroy damaged cells and cellular debris.
A swollen node shows that it is working harder than normal, usually because it has many more cells to fight an infection or a disease in the area it drains.
Because the swelling is a sign of infection or disease, it is a warning that needs medical examination to find and treat the cause.

203 marks

Write the differences between the following pairs.

Solution

(a) bicuspid valve and tricuspid valve

Feature	Bicuspid valve	Tricuspid valve
Cusps	Two cusps	Three cusps
Position	Between the left atrium and the left ventricle	Between the right atrium and the right ventricle
Other name	Mitral valve	No other name

(b) systole and diastole

Feature	Systole	Diastole
Meaning	Contraction of the heart muscle	Relaxation (dilation) of the heart muscle
Effect on blood	Pushes blood out of the chamber	Lets the chamber fill with blood
Example	Ventricular systole ejects blood into the aorta and pulmonary trunk	Ventricular diastole lets blood enter from the atria

Feature	SA node	AV node
Position	Upper wall of the right atrium, near the vena cavae	Near the junction of the right atrium and right ventricle
Role	Starts each heartbeat; the pacemaker	Passes the impulse on to the ventricles after a delay
Speed of impulse	Generates impulses most frequently	Delays the impulse by 0.11 s

(d) P-wave and T-wave of ECG

Feature	P-wave	T-wave
Produced by	Depolarization of the atrial fibres	Repolarization of the ventricles
Occurs	Just before the atria contract	Just before the ventricles relax
Meaning	Atrial excitation	Ventricular recovery

(e) blood capillaries and lymph capillaries

Feature	Blood capillaries	Lymph capillaries
Ends	Open at both ends, between arterioles and venules	Closed-ended, starting in the tissue spaces
Contents	Blood, carried in a continuous circuit	Lymph (interstitial fluid that has entered)
Wall	A single layer of endothelial cells	A single layer of squamous epithelial cells, which lets fluid in

(f) baroreceptor and volume receptor

Feature	Baroreceptor (high pressure)	Volume receptor (low pressure)
Site	Walls of the aorta and the carotid sinus	Atria of the heart (and carotid arteries)
Stimulus	Blood pressure in the arteries	Decrease in the blood volume
Response	Changes heart rate and vasoconstriction through the nerves (baroreflex)	Signal to the hypothalamus, which increases the release of ADH and water retention in the kidney

Note. The book also lists the carotid arteries as a site of low-pressure receptors, although it places the high-pressure carotid sinus in the same arteries. The volume receptors described here are the atrial receptors.

Extensive answer questionsp. 53

18 marks

Draw, label and describe the external structure of human heart.

Solution

Position and size

The heart is about the size of an adult fist. It lies in the mediastinum, between the two lungs, behind the sternum and in front of the backbone, enclosed in the pericardium. Its base lies beneath the second rib and its pointed apex lies at the level of the fifth intercostal space.

Figure 2.1: External view of the human heart

Parts of the external structure

Superior vena cava and inferior vena cava: the two large veins that bring deoxygenated blood into the right atrium.
Right atrium: the upper right chamber. It receives the superior and inferior vena cava and the coronary sinus.
Left atrium: the upper left chamber at the back. It receives the four pulmonary veins carrying oxygenated blood from the lungs.
Right ventricle: the lower right chamber, forming most of the front of the heart. It opens into the pulmonary trunk.
Left ventricle: the lower left chamber, forming the apex. It opens into the aorta.
Pulmonary trunk: leaves the right ventricle and divides into the right and left pulmonary arteries.
Aorta: leaves the left ventricle and arches over the top of the heart. Its branches are the brachiocephalic trunk, the left common carotid artery and the left subclavian artery.
Right and left coronary arteries: supply the heart wall. The cardiac veins drain the heart wall and join the coronary sinus.

Wall thickness

The thickness of each wall matches the work done by the chamber. The atria have thin walls because they only push blood into the ventricles. The ventricles have thicker walls, and the left ventricle is the thickest because it pumps blood around the whole body. The right ventricle wall is about one third as thick as that of the left ventricle (ratio 1:3), since it pumps blood only to the lungs.

28 marks

Describe the flow of blood through human heart as regulated by the valves.

Solution

Blood flows through the heart in one direction only. Four valves regulate the flow: the tricuspid valve (between the right atrium and right ventricle), the bicuspid or mitral valve (between the left atrium and left ventricle), the pulmonary semilunar valve (at the exit of the right ventricle) and the aortic semilunar valve (at the exit of the left ventricle).

Figure 2.3: Passage of blood through the heart

Path of the blood

The superior and inferior vena cava bring deoxygenated blood into the right atrium.
Blood passes through the tricuspid valve, which is open during ventricular diastole, into the right ventricle.
When the right ventricle contracts, blood passes through the pulmonary semilunar valve into the pulmonary trunk and the pulmonary arteries, which carry it to the lungs.
Oxygenated blood returns through the four pulmonary veins into the left atrium.
Blood passes through the bicuspid valve into the left ventricle.
When the left ventricle contracts, blood passes through the aortic semilunar valve into the aorta, which sends it to the body parts. The coronary arteries leave the aorta to supply the heart wall.

How the valves regulate the flow

The atrioventricular valves (tricuspid and bicuspid) open when the atria fill the ventricles, because the pressure in the atria then exceeds the pressure in the relaxed ventricles.
When the ventricles contract, their pressure closes the atrioventricular valves. Papillary muscles pull on the chordae tendineae, which stop the cusps from bulging back into the atria.
The semilunar valves open when the ventricles contract and push the pressure above that in the aorta or pulmonary trunk, so blood is ejected.
When the ventricles relax, the high pressure in the aorta and pulmonary artery pushes blood back against the semilunar valves, which close and prevent backflow into the ventricles.

38 marks

State the phases of heartbeat in man.

Solution

One heartbeat is one cardiac cycle. It is a sequence of alternating contraction (systole) and relaxation (diastole) of the atria and the ventricles, and it lasts about 0.7 to 0.8 s. The heart muscle rests for 0.1 to 0.3 s between beats.

Figure 2.4: Cardiac cycle

The four phases

Phases of one heartbeat at 75 beats per minute
Phase	Duration	Atrioventricular valves	Semilunar valves	What happens
Atrial diastole	Most of the cycle	Open	Closed	Blood enters the atria from the veins and the pressure in the atria rises until it pushes the atrioventricular valves open.
Atrial systole	0.1 s	Open	Closed	The two atria contract at the same time and push the blood through the open valves into the relaxed ventricles.
Ventricular systole	0.3 s	Closed	Open	The ventricles contract. Their pressure closes the atrioventricular valves, which gives the first heart sound. The pressure then opens the semilunar valves and blood is ejected into the aorta and pulmonary trunk.
Ventricular diastole	0.4 s	Open	Closed	The ventricles relax. The high pressure in the aorta and pulmonary trunk closes the semilunar valves, which gives the second heart sound. The atrioventricular valves open and the ventricles fill.

The two heart sounds

The first heart sound ("lub") is low pitched. It is caused by the closing of the atrioventricular valves near the beginning of ventricular systole.
The second heart sound ("dub") is higher pitched. It comes from the closing of the aortic and pulmonary semilunar valves near the end of systole.

48 marks

Describe the conducting system of human heart.

Solution

The heart beats without any outside stimulation because its cardiac muscle is myogenic. Specialized strands of cardiac muscle, called the conducting system, coordinate the contraction. The conducting system consists of the SA node, the AV node, the atrioventricular bundle (bundle of His), the bundle branches and the Purkinje fibres.

Figure 2.5: Conducting system of the heart

Parts of the conducting system

Part	Location	Role
SA node (sinoatrial node)	Upper wall of the right atrium, close to where the vena cavae enter	Starts each impulse; it is the pacemaker
AV node (atrioventricular node)	Near the junction of the right atrium and right ventricle	Receives the impulse from the atria and delays it before passing it on
Atrioventricular bundle (bundle of His)	Leaves the AV node, passes through the fibrous skeleton into the interventricular septum	Carries the impulse from the AV node to the bundle branches
Bundle branches	Right and left branches on either side of the interventricular septum	Carry the impulse down the septum towards the apex
Purkinje fibres	Beneath the endocardium, from the bundle branches to the apex and into the ventricular walls	Spread the impulse through the ventricular muscle so both ventricles contract

Steps of one heartbeat

The SA node generates an action potential, which spreads across the walls of the atria. The atria contract.
The impulse reaches the AV node in about 0.04 s.
Within the AV node the impulse is delayed for 0.11 s. The total delay of 0.15 s lets the atrial contraction finish before the ventricles contract.
The impulse passes along the AV bundle through the fibrous skeleton into the interventricular septum, where it divides into the right and left bundle branches.
The bundle branches carry the impulse down the septum to the apex of each ventricle.
The Purkinje fibres spread the impulse into the ventricular walls, so the ventricles contract together.

Why the Purkinje fibres are fast

The Purkinje fibres are large-diameter cells with few myofibrils and well-developed intercalated disks that contain many gap junctions. These features let action potentials travel much faster along them than through other cardiac muscle fibres.

58 marks

Explain electrocardiogram with the help of diagram.

Solution

What the ECG records

The electrical impulses that pass through the conducting system during the cardiac cycle can be recorded as an electrocardiogram (ECG). The changes come from the depolarization and repolarization of the cardiac muscle fibres. They are detected on the skin by an instrument called an electrocardiograph.

Figure 2.6: Electrocardiogram (ECG)

The waves, segments and intervals

Feature	What it shows	Normal value
P wave	Depolarization of the atrial fibres; the atria contract just after it	Small upward wave
PR interval	Time for the impulse to pass from the SA node to the ventricles	0.12 to 0.20 s
QRS complex	Depolarization of the ventricles; the ventricles contract just after it	0.08 to 0.10 s
ST segment	Period between the end of ventricular depolarization and the start of repolarization	Flat section after the S wave
T wave	Ventricular repolarization; occurs just before the ventricles relax	Rounded upward wave
QT interval	Total time of ventricular electrical activity	0.40 to 0.43 s
RR interval	Time between two successive R peaks, one full heartbeat	0.6 to 1.0 s

Uses of the electrocardiogram

It is a painless test of the electrical activity of the heart, which shows whether it is working normally.
It detects arrhythmias (irregular heartbeats) and conduction defects in the conducting system.
It shows the size and position of the heart chambers and damage to the heart muscle, and impaired blood flow to the heart muscle.
It shows the effect of cardiac medicines and whether an artificial pacemaker is working.

68 marks

Describe the structure of blood vessels in man.

Solution

There are three types of blood vessels. Arteries, with arterioles, carry blood away from the heart. Veins return blood to the heart. Capillaries allow exchange of materials between the blood and the tissues. Arteries and veins have three layers, called tunics, although the walls differ in thickness.

Artery

The wall of an artery has three layers. The outermost layer is the tunica adventitia, made of white fibrous connective tissue. The middle layer is the tunica media, which is thick and contains one or two layers of circular smooth muscle cells and a variable amount of elastic fibres. The innermost layer is the tunica intima, made of simple squamous epithelium (endothelium) with elastic fibres. The lumen of an artery has no valves.

Figure 2.7 to 2.9: Layers of the walls of an artery, a vein and a capillary (not to scale)

Vein

Veins have a thinner wall than arteries. The tunica media is a thin layer of circularly arranged smooth muscle cells with collagen fibres and a few elastic fibres, and the tunica adventitia is made of collagenous connective tissue. Veins with a diameter greater than 2 mm contain valves, which allow blood to flow only towards the heart. Valves are present only in the lower part of the body, so blood in the upper body returns with the help of gravity.

Capillary

The capillary wall is made of a single layer of endothelial cells. Capillaries are 7 to 9 \mum in diameter and about 1 mm long, and red blood cells pass through most of them in a single file.

Feature	Artery	Vein	Capillary
Wall layers	Adventitia, media and intima	Adventitia, media and intima, thinner	One layer of endothelial cells
Tunica media	Thick, with circular smooth muscle and elastic fibres	Thin, with smooth muscle and collagen	Absent
Valves	None	Present in veins over 2 mm in diameter, in the lower body	None
Diameter	Aorta about 23 mm; arterioles about 0.2 mm	Venules 40 to 50 \mum; veins larger	7 to 9 \mum

Note. The book describes the tunica intima of a vein as "a smooth muscle". It is a smooth lining of endothelial cells with a thin layer of elastic fibres, as the same paragraph goes on to say.

78 marks

What is the role of precapillary sphincter?

Solution

A precapillary sphincter is a ring of smooth muscle at the origin of a capillary branch. It opens and closes the entrance to the capillary, so it controls how much blood flows into each capillary of a capillary network.

Figure 2.11: Capillary network

Path of the blood through a capillary network

The arteriole supplies blood to the capillary network.
Blood flows from the arteriole through the metarteriole into a thoroughfare channel.
Several capillaries branch from the thoroughfare channel. A precapillary sphincter at the origin of each branch decides whether blood enters it.
When a sphincter is open, blood flows through the capillary and exchanges materials with the tissue cells. When it is closed, blood flows straight through the thoroughfare channel to the venule.

Role of the sphincter

It regulates the flow of blood through the capillary network according to the needs of the tissue.
Precapillary sphincters are normally either completely open or completely closed.
The sphincters and metarteriole open and close cyclically several times per minute. This cyclic opening and closing is called vasomotion.
The duration of the open phases is roughly proportional to the metabolic needs of the tissue, so an active tissue receives more blood.

82026 Annual I, Set B, Q.58 marks

Describe pulmonary circulation and systemic circulation.

Solution

Blood circulates through two circuits that begin and end at the heart. The pulmonary circulation carries blood between the heart and the lungs. The systemic circulation carries blood between the heart and the rest of the body. Together they make the heart a double pump.

Figure 2.12: Cardiovascular system

Pulmonary circulation

Deoxygenated blood from the body enters the right atrium through the superior and inferior vena cava.
It passes through the tricuspid valve into the right ventricle, and from there through the pulmonary semilunar valve into the pulmonary trunk.
The pulmonary trunk divides into the two pulmonary arteries, one going to each lung. Gas exchange takes place in the lungs.
Oxygenated blood returns through the two pairs of pulmonary veins, which open into the left atrium.

Systemic circulation

Oxygenated blood enters the left atrium from the pulmonary veins and passes through the bicuspid valve into the left ventricle.
The left ventricle pumps blood through the aortic semilunar valve into the aorta.
Branches of the aorta carry blood to the organs and major body regions. The head, chest and arms are supplied by the superior vena cava return, and the lower body by the inferior vena cava.
Both vena cavae return the blood to the right atrium.

Comparison

Feature	Pulmonary circulation	Systemic circulation
Starts at	Right ventricle	Left ventricle
Ends at	Left atrium	Right atrium
Blood in the arteries	Deoxygenated	Oxygenated
Blood in the veins	Oxygenated	Deoxygenated
Main vessels	Pulmonary trunk, pulmonary arteries and pulmonary veins	Aorta, superior and inferior vena cava

Within the systemic circuit there are named circulations for particular organs. The coronary circulation supplies the heart wall, the hepatic portal system carries blood from the digestive organs to the liver, and the renal circulation supplies the kidneys.

98 marks

Describe hepatic portal system.

Solution

A portal system is a vascular system that begins and ends in capillary beds and has no pumping mechanism of its own. The hepatic portal system begins with capillaries in the visceral organs of the digestive system and ends with capillaries in the liver.

Figure 2.13: Hepatic portal system

Path of the blood

Blood passes through capillaries in the digestive organs (the stomach, small intestine and appendix) and in the spleen. These capillaries drain into veins.
The veins from the digestive system unite to form the hepatic portal vein, which is the largest vein of the system.
The hepatic portal vein enters the liver, where it breaks up into a series of dilated capillaries.
These capillaries empty into the hepatic veins, which join the inferior vena cava and carry the blood to the right atrium.

Comparison of the two veins

Vein	Carries blood from	Carries blood to
Hepatic portal vein	The digestive organs and spleen	The capillaries of the liver
Hepatic veins	The capillaries of the liver	The inferior vena cava

108 marks

Give an account of blood pressure in man.

Solution

Definition

Blood pressure is the force exerted by the blood against a unit area of the inner walls of a blood vessel. The standard reference is the mercury manometer, which measures the pressure in millimetres of mercury (mm Hg).

Systolic and diastolic pressure

Systolic pressure is the highest arterial pressure. It is produced when the ventricles contract and pump blood into the aorta and pulmonary trunk.
Diastolic pressure is the lowest arterial pressure. It occurs when the ventricles relax and no blood is being ejected.

Normal and abnormal values

Category	Reading (mm Hg)
Normal	120/80
Hypertension (high blood pressure)	Higher than 140/90, when one or both readings are high
Hypotension (low blood pressure)	Lower than 90/60

Regulation

Blood pressure is kept steady by the cardiac output, which depends on heart rate and stroke volume, and by the resistance of the vessels, which depends on their diameter and the viscosity of the blood. The baroreceptors in the aorta and carotid sinus adjust the heart rate and the diameter of the vessels moment to moment. Question 13 gives the full list of regulating factors.

118 marks

Compare the rate of blood flow through arteries, arterioles, capillaries, venules and veins.

Solution

Blood flow is the quantity of blood that passes a given point in a given period. The rate of flow in each type of vessel depends on the total cross-sectional area of that group of vessels, not on the diameter of one vessel.

Vessel	Total cross-sectional area	Rate of blood flow
Arteries	Small, and grows as the arteries branch	High; about 30 cm/s in the aorta
Arterioles	Larger than the arteries, because there are many of them	Falls as blood enters the arterioles
Capillaries	Largest of all, because each artery supplies a huge number of capillaries	Lowest; about 0.26 cm/s
Venules	Shrinks as the capillaries join to form venules	Increases again
Veins	Smaller than the capillary beds, as the venules merge	Increases further, towards the heart

Each artery conveys blood to an enormous number of capillaries, so the total cross-sectional area of the capillary beds is much greater than that of any other part of the circulation. Although each capillary is very narrow, the blood slows substantially as it enters the arterioles and slows further in the capillary beds. As the blood leaves the capillaries and enters the venules and veins, it speeds up again because the total cross-sectional area is reduced.

Note. The book says blood travels over a thousand times faster in the aorta than in the capillaries. The figures it gives, about 30 cm/s in the aorta and about 0.26 cm/s in the capillaries, give a ratio of about 115.

128 marks

Explain the following: a. Principle of angiography b. Coronary bypass c. Angioplasty d. Open heart surgery.

Solution

(a) Principle of angiography

Angiography uses a specially designed catheter to show the blood vessels of the heart on X-ray images. Cardiac catheterization is the technique.

The catheter is inserted into a vein or artery and advanced into the heart under radiographic fluoroscopic guidance.
The operator injects a contrast medium into the area of interest, which makes the vessels visible on the images.
The resulting angiogram shows the narrowing (stenosis) of a vessel and the condition of the mitral valve, aortic valve and aorta. It also gives the size and function of the left ventricle.
Coronary angiography guides the revascularization procedures, such as balloon angioplasty and stenting.

(b) Coronary bypass

A coronary bypass relieves the effects of an obstruction in the coronary arteries by providing a new route for the blood around the blocked part.

Figure 2.17: Coronary artery bypass graft

A healthy segment of blood vessel is taken from another part of the patient's body, usually a vein from the leg or an artery of the thorax.
One end of the graft is joined to the aorta and the other end is joined to the coronary artery beyond the obstruction.
Blood now flows from the aorta through the graft and bypasses the blocked part of the coronary artery.

Angioplasty opens a narrowed coronary artery with a balloon, and a stent keeps it open.

Figure 2.18: Angioplasty of a coronary artery

A cardiologist threads a plastic tube (catheter) into an artery of the arm or leg and guides it through a major blood vessel towards the heart.
When the tube reaches the plaque in a coronary artery, a balloon attached to its end is inflated, which forces the vessel open.
The artery may not stay open, so slotted tubes called stents are expanded inside it.
Stents are coated with heparin, which prevents blood clotting, and with chemicals that prevent the artery from closing again.

(d) Open heart surgery

Open heart surgery is surgery in which the chest is opened. The heart may or may not be opened, depending on the type of surgery. It is used to correct heart problems in children and adults.

An incision is made through the breastbone (sternum) while the patient is under general anesthesia.
Tubes re-route the blood through a heart-lung bypass machine. The machine adds oxygen to the blood, keeps it warm and keeps it moving through the rest of the body.
Using the machine allows the heart to be stopped, so the surgeon can repair the heart muscle, the heart valves or the blood vessels outside the heart.
After the repair the heart is started again and the machine is removed. The breastbone and the skin incision are then closed.

Note. The book captions Figure 2.18 as a coronary artery bypass graft, but the drawing with the balloon and stent inset shows angioplasty.

138 marks

Explain hypertension and hypotension. What are the factors that regulate blood pressure?

Solution

Hypertension

Hypertension is high blood pressure. It is defined as a blood pressure higher than 140/90 mm Hg. The diagnosis is made when one or both readings are high. A normal blood pressure is 120/80 mm Hg. The top number is the systolic pressure, and the bottom number is the diastolic pressure.

Hypotension

Hypotension is low blood pressure, in which the force of the blood against the artery walls is too low. In general, a reading lower than 90/60 mm Hg indicates low blood pressure.

Factors that regulate blood pressure

Factor	How it affects blood pressure
Cardiac output	Depends on the heart rate and the stroke volume. A higher cardiac output raises the blood pressure.
Total peripheral resistance	Depends on the diameter of the blood vessels and the viscosity of the blood. A narrower vessel raises the resistance.
Blood volume	More blood in the circulation raises the blood pressure.
Blood viscosity	Thicker blood affects its flow and pressure.
Diet	A high sodium intake and a diet low in potassium, and excess saturated fat, can raise the pressure.
Physical activity, weight and stress	A lack of exercise, being overweight and high stress levels raise the pressure.
Sleep and medication	Insufficient sleep, and some medicines such as cough and cold medicines, can interfere with blood pressure control.
Age and genetics	The risk of high blood pressure increases with age, and a family history increases the predisposition.
Medical conditions	Kidney disease, thyroid problems and obstructive sleep apnea can affect blood pressure.

Reflex regulation

High-pressure arterial baroreceptors in the aorta and the carotid sinus sense the blood pressure. When the pressure rises, the parasympathetic nerves cause vasodilation throughout the body and slow the heart rate, so the pressure falls.
When the blood pressure falls, the sympathetic nerves increase their impulses, which cause vasoconstriction and a rise in blood pressure.
Low-pressure volume receptors in the atria detect a fall in blood volume. They signal the hypothalamus, which increases the production of antidiuretic hormone (ADH). ADH increases water retention in the kidney, so the blood volume and the pressure rise.

Hormones and other substances

Norepinephrine is a powerful vasoconstrictor hormone, and epinephrine is less potent. Kinins such as histamine are powerful vasodilators, and most prostaglandins are vasodilators, although some are vasoconstrictors.

Note. The book says that stimulation of the parasympathetic nerves in the baroreceptor areas is "caused by cardiac output". The stimulus is the rise in blood pressure that an increased cardiac output produces.

148 marks

Describe the lymphatic system of man.

Solution

The lymphatic system includes the lymph, lymphocytes, lymphatic vessels, lymph nodes, tonsils, spleen and thymus gland. Its main work is to drain the fluid from the tissue spaces back into the blood and to defend the body against infection.

Figure 2.21: Lymphatic system of man (schematic)

Lymph capillaries and vessels

Lymphatic pathways begin as lymph capillaries. These are microscopic, closed-ended tubes with a thin wall of one layer of squamous epithelial cells, which lets interstitial fluid enter. Once inside, the fluid is called lymph.
Each villus of the small intestine contains a lacteal, a lymph capillary that absorbs fats and other substances from the digestive tract.
Lymphatic vessels form by merging the lymph capillaries. Their walls are similar to those of veins and they have valves that prevent backflow.
The lymphatic vessels lead to the lymph nodes, and after leaving the nodes they merge into larger lymphatic trunks.

Lymphatic trunks and ducts

The lymphatic trunks drain the lymph from the regions of the body they serve. They join one of two collecting ducts. The thoracic duct is the larger and longer one. It receives lymph from the lower body, the left side of the body and the left side of the head and neck, and empties into the left subclavian vein near the junction with the left jugular vein. The right lymphatic duct receives lymph from the right side of the head and neck, the right arm and the right side of the thorax, and empties into the right subclavian vein.

Lymph nodes and spleen

Lymph nodes are located along the lymphatic pathways, for example in the neck, the armpit (axilla) and the groin. They contain many lymphocytes, which act against foreign particles such as bacteria and viruses.
Lymph nodes also contain macrophages, which engulf and destroy foreign substances, damaged cells and cellular debris.
The spleen contains lymphocytes and macrophages that remove germs and damaged blood cells from the blood that passes through it. It stores blood and iron, makes white blood cells and antibodies, and helps to maintain the fluid levels in the body.

FBISE 2026 questions

Questions from the FBISE HSSC-II 2026 (1st Annual) papers that belong to this unit, answered the way an examiner expects.

2026 Annual I, Set B, MCQ 81 mark

The valve that prevents backflow of blood from left ventricle into left atrium:

AAortic valve
BTricuspid valve
CPulmonary valve
DBicuspid valveAnswer

Why

The bicuspid (mitral) valve lies between the left atrium and the left ventricle and shuts when the ventricle contracts, so blood cannot flow back into the atrium. The aortic valve is the tempting option, but it stops blood falling back from the aorta into the left ventricle. The tricuspid valve is on the right side, and the pulmonary valve guards the exit of the right ventricle.

2026 Annual I, Set B, Q.2 (iv) OR1+2 = 3 marks

a. How can ECG help in detecting heart abnormalities? b. State any two conditions that can be diagnosed using ECG.

Solution

a. How ECG detects abnormalities (1 mark)

An electrocardiogram records the electrical changes (depolarization and repolarization) of the heart as the P wave, QRS complex and T wave. A normal ECG shows that the heart is working properly, so a trace whose waves, timing or rhythm differ from the normal values (for example PR interval 0.12 to 0.20 s, QRS 0.08 to 0.10 s) points to a fault in the heart.

b. Two conditions (2 marks)

Arrhythmias (irregular heartbeat).
Conduction defects (a fault in the pathway of the impulse).
Damage to the heart muscle, impaired blood flow to the heart muscle, the size and position of the chambers, and the effect of cardiac medicines or pacemakers can also be shown.

2026 Annual I, Set B, Q.2 (vii) OR1+2 = 3 marks

a. What is meant by Double-Walled sac in context of pericardium? b. State any two functions of pericardium.

Solution

a. Double-walled sac (1 mark)

The pericardium is a closed sac around the heart made of two layers: the visceral layer (inner), which covers the heart directly (the epicardium), and the parietal layer (outer), which forms the sac around the heart and contains the fluid of the pericardial cavity between the layers.

b. Two functions (2 marks)

It prevents overfilling: the pericardium is inelastic, so the heart cannot be overstretched with blood.
It lubricates the heart with serous fluid, preventing friction with the surrounding organs.
(Also: it holds the heart in position and keeps a hollow space in which the heart can expand.)

2026 Annual I, Set B, Q.54+3 = 7 marks

Discuss the path of blood through pulmonary and systemic circulation. Draw diagram as well.

Solution

Path of blood (4 marks)

Pulmonary circulation (heart to lungs and back)

Deoxygenated blood from the body enters the right atrium by the superior and inferior venae cavae.
It passes through the tricuspid valve into the right ventricle.
The right ventricle pumps it through the pulmonary semilunar valve into the pulmonary trunk, which divides into the right and left pulmonary arteries to the lungs.
In the lungs the blood is oxygenated.
Pulmonary veins carry the oxygenated blood to the left atrium.

Systemic circulation (heart to body and back)

The left atrium sends blood through the bicuspid valve into the left ventricle.
The left ventricle pumps it through the aortic semilunar valve into the aorta, whose branches supply all organs except the lungs.
Oxygen is given up in the capillaries of the tissues.
The superior vena cava collects blood from the head, chest and arms and the inferior vena cava from the lower body; both return it to the right atrium.

Feature	Pulmonary circulation	Systemic circulation
Starts at	Right ventricle	Left ventricle
Goes to	Lungs	All organs except the lungs
Returns to	Left atrium (pulmonary veins)	Right atrium (venae cavae)
Arteries carry	Deoxygenated blood	Oxygenated blood

Labelled diagram (3 marks)

Double circulation: the pulmonary circuit through the lungs and the systemic circuit through the other organs (accent arrows: oxygenated blood).

2026 Annual I, Set D, MCQ 91 mark

Blood enters the right atrium from the body through the:

AVena cavaAnswer
BPulmonary artery
CPulmonary vein
DAorta

Why

The superior and inferior venae cavae bring deoxygenated blood from the body into the right atrium. The pulmonary vein brings blood from the lungs into the left atrium, the pulmonary artery carries blood from the right ventricle to the lungs, and the aorta carries blood away from the left ventricle.

2026 Annual I, Set D, Q.2 (iv) OR3 marks

Compare arteries and veins. (any three points)

Solution

Feature	Artery	Vein
Direction of blood	Away from the heart	Towards the heart
Wall	Thick; tunica media has elastic fibres and smooth muscle	Thinner; thin tunica media
Valves	No valves	Valves in veins wider than 2 mm
Blood pressure	High, rising and falling with each heartbeat	Comparatively low
Position	Within the muscles; pink	Not deep; seen as blue vessels under the skin

2026 Annual I, Set D, Q.2 (vii) OR3 marks

State the role of baroreceptors and volume receptors in regulating the blood pressure.

Solution

Baroreceptors (high-pressure, 1.5 marks)

These mechanoreceptors in the walls of the aorta and the carotid sinus sense blood pressure and tell the brain. When pressure is high, parasympathetic nerves cause vasodilation and slow the heart, so pressure falls. When pressure is low, sympathetic impulses increase and cause body-wide vasoconstriction, so pressure rises. This negative feedback is the baroreflex.

Volume receptors (low-pressure, 1.5 marks)

These receptors in the atria (and carotid arteries) detect a decrease in blood volume and signal the hypothalamus. It increases the production of antidiuretic hormone (ADH), so the kidney retains water, blood volume rises and blood pressure rises.

Exam focus

Describe the path of blood through the pulmonary and systemic circulation, with a labelled diagram. (long)
A 7-mark Section C question in 2026 (Set B Q.5); book long question 8.
Go to answer
Compare arteries and veins (any three points) and describe the structure of blood vessels. (short)
Set D Q.2 (iv) OR in 2026; book long question 6.
Go to answer
Heart valves and chambers: which valve lies where and which vessel enters or leaves which chamber. (mcq)
Set B MCQ 8 and Set D MCQ 9 in 2026; book exercise MCQs 3, 4, 8 and long questions 1 and 2.
Go to answer
Electrocardiogram: waves, what the ECG detects and conditions diagnosed. (short)
Set B Q.2 (iv) OR in 2026; book long question 5.
Go to answer
Pericardium as a double-walled sac and its functions. (short)
Set B Q.2 (vii) OR in 2026; book exercise MCQ 7.
Go to answer
Role of baroreceptors and volume receptors in controlling blood pressure. (short)
Set D Q.2 (vii) OR in 2026; book short question 20 (f).
Go to answer
Conducting system of the heart: SA node as pacemaker, AV node delay, Purkinje fibres. (short)
Not asked in the 2026 papers, but book short questions 3, 4, 7 and 10, long question 4 and MCQs 17 and 20 test it.
Go to answer
Blood pressure, hypertension and hypotension. (long)
No paper question in 2026; book long questions 10 and 13.
Go to answer