Urinary system of man: Biology unit 4 notes (FBISE class 12)

4.1Excretion

Body cells form a variety of wastes, and if these substances are allowed to accumulate, their effects are likely to be toxic. The body fluids, such as the blood and lymph, carry wastes away from the tissues that produce them. Other parts then remove these wastes from the blood and transport them to the outside. The respiratory system, for example, removes carbon dioxide from the blood, and the urinary system removes various salts and nitrogenous wastes.

The urinary system does more than remove waste. It also helps to maintain the normal concentrations of water and electrolytes within the body fluids, to regulate the pH and volume of body fluids, and to control red blood cell production and blood pressure.

Excretion: The removal of chemical waste from the body which is produced by the metabolic processes within cells.

Metabolism produces a number of toxic by-products, particularly the nitrogen containing compounds. These are the wastes the urinary system is mainly concerned with.

Two organ systems of excretion, as the book describes them
Waste	Removed mainly by
Carbon dioxide	Respiratory system (from the blood)
Salts and nitrogenous wastes	Urinary system

Wastes made by body cells are carried by the blood and lymph to the organ systems that remove them. The urinary system (accent) removes salts and nitrogenous wastes.

4.1.1Nitrogen as excretory product

Animals excrete a variety of nitrogenous waste products, but three predominate: ammonia, urea and uric acid. A major factor in determining the mode of nitrogen excretion is the availability of water in the environment.

Ammonia: excreted by animals with plenty of water.
Urea: excreted by animals with less freshwater available.
Uric acid: excreted by animals with a major water shortage problem.

4.1.2Relationship between excretory products and habitatsOften examined

The exact nature of the excretory product is determined mainly by the availability of water to the organism, which is based upon its habitat. The correlation with habitat is: (a) ammonia is the waste of aquatic animals, (b) urea is the waste of both aquatic and terrestrial animals, and (c) uric acid is the waste of terrestrial animals.

Ammonia

Ammonia is highly toxic because it tends to raise the pH of body fluids and interferes with membrane transport functions. It is highly soluble in water and diffuses rapidly across cell membranes, so it is excreted rapidly. One gram of nitrogen, in the form of ammonia, requires 500 mL of water to dissolve it to a nontoxic level. Such plenty of water can only be afforded by many aquatic organisms, particularly those in freshwater, for example most fishes, protozoans, sponges, cnidaria, echinoderms and crustaceans. Animals which excrete ammonia as their major nitrogenous waste product are called ammonotelic.

Urea

Organisms with less freshwater available, such as some marine organisms and all terrestrial organisms, remove most of their nitrogenous waste in the form of urea. They often invest some energy to convert the ammonia into urea, which is 100,000 times less toxic than ammonia. One gram of nitrogen, in the form of urea, requires 50 mL of water to dilute it to a nontoxic level. Animals which excrete urea as their major nitrogenous waste product are called ureotelic. Examples include mammals such as man, many amphibians and some marine animals like shark.

Uric acid

Uric acid is a by-product of the breakdown of purine. It is less toxic than urea and can precipitate from solution, allowing the 4 nitrogen atoms per uric acid molecule to be excreted. One gram of nitrogen, in the form of uric acid, requires just 1 mL of water for its excretion. It has evolved in two groups with major water shortage problems: terrestrial invertebrates (for example the land snail and many insects) and egg-laying vertebrates (reptiles such as snakes and lizards, and birds). These animals are called uricotelics. Examples include many terrestrial insects, reptiles and most birds.

The three nitrogenous wastes compared: toxicity, the water needed for each gram of nitrogen and the animals that excrete them. Man is ureotelic.

Comparison of ammonotelic, ureotelic and uricotelic animals
Feature	Ammonia	Urea	Uric acid
Type of animal	Ammonotelic	Ureotelic	Uricotelic
Toxicity	Highly toxic	100,000 times less toxic than ammonia	Least toxic; precipitates from solution
Water to excrete 1 g of nitrogen	500 mL	50 mL	1 mL
Habitat	Aquatic, especially freshwater	Aquatic (some marine) and terrestrial	Terrestrial, water shortage
Examples	Most fishes, protozoans, sponges, cnidaria, echinoderms, crustaceans	Mammals including man, many amphibians, shark	Land snail, many insects, reptiles (snakes, lizards), most birds

Worked example: the water cost of each waste

An animal has to excrete 4 g of nitrogen in a day. Use the book's figures to find how much water it needs if the nitrogen is excreted as ammonia, as urea or as uric acid. Which waste would suit a snake living in a dry place, and why?

The book gives the water needed to excrete 1 g of nitrogen: 500 mL as ammonia, 50 mL as urea, 1 mL as uric acid. Multiply each by 4 g.
$ammonia: 4 \times 500 urea: 4 \times 50 uric acid: 4 \times 1 = 2000 mL = 200 mL = 4 mL$
Compare. Urea needs one tenth of the water of ammonia, and uric acid needs only one fiftieth of the water of urea (one five hundredth of that of ammonia).
A snake is a reptile that lives where water is scarce. It cannot spend litres of water on its waste, so it needs the waste that costs the least water.
Uric acid is the waste that suits it. It is the least toxic of the three, and it can precipitate from solution, so the four nitrogen atoms in each molecule leave the body with almost no water.

Waste	Water per 1 g of nitrogen	Water for 4 g of nitrogen	Suited to
Ammonia	500 mL	2000 mL	Aquatic animals (plenty of water)
Urea	50 mL	200 mL	Mammals, many amphibians (less water)
Uric acid	1 mL	4 mL	Reptiles, birds, insects (very little water)

Ammonia 2000 mL, urea 200 mL, uric acid 4 mL. A snake in a dry habitat is uricotelic: uric acid saves the most water.

4.2Urinary system of manOften examined

The excretory system (urinary system) consists of the kidneys, ureters, urinary bladder and a tubular urethra. The kidneys lie on either side of the vertebral column between the twelfth thoracic and the third lumbar vertebrae.

Organs of the urinary system
Organ	Description	Function
Kidneys (two)	Bean-shaped, on either side of the vertebral column between the twelfth thoracic and third lumbar vertebrae	Filter the blood and form urine
Ureters (two)	Each is a tubular organ about 25 cm long. It begins as the funnel-shaped renal pelvis and extends downward, parallel to the vertebral column, to join the urinary bladder	Transport urine from the kidney to the urinary bladder
Urinary bladder	Hollow, distensible muscular organ in the pelvic cavity	Urine reservoir
Urethra	Tubular, leads out of the urinary bladder	Carries urine from the urinary bladder to the outside of the body

Figure 4.1: Excretory system of man

Path of urine

Each kidney filters the blood and makes urine.
Urine leaves the kidney through its ureter.
Urine is stored in the urinary bladder.
Urine leaves the body through the urethra.

4.2.1Structure of kidneyOften examined

Location

The kidneys are a pair of bean-shaped structures. They are located just below and posterior to the liver in the peritoneal cavity. The adrenal glands are on top of each kidney. The kidneys filter the blood and purify it. The filtrate coming out of the kidneys is called urine. The right kidney is slightly lower than the left because of the considerable space occupied by the liver on the right side.

External structure

Externally, each kidney is surrounded by three layers, from the outside to the inside:

Three layers around the kidney
Layer	Description
Renal fascia (outermost)	A tough connective tissue layer
Perirenal fat capsule (second)	Helps to anchor the kidney in place
Renal capsule (innermost)	Directly covers the kidney

Internal structure

Internally, the kidney has three regions: an outer cortex, a medulla in the middle, and the renal pelvis in the region called the hilum of the kidney. The hilum is the concave part of the bean-shape where blood vessels and nerves enter and exit the kidney; it is also the point of exit for the ureters.

Regions and parts of the kidney
Part	Description (as the book gives it)
Renal cortex	Granular, because of the presence of nephrons, the functional unit of the kidney
Renal medulla	Made of multiple pyramidal tissue masses, the renal pyramids
Renal columns	The spaces between the pyramids, through which the blood vessels pass
Renal papillae	The tips of the pyramids, pointing toward the renal pelvis
Lobes of the kidney	A renal pyramid together with the adjoining cortical region. There are on average eight pyramids in each kidney
Renal pelvis	Leads to the ureter on the outside of the kidney. Inside the kidney it branches into two or three extensions called major calyces, which branch into minor calyces
Ureter	Urine-bearing tube that exits the kidney and empties into the urinary bladder

Figure 4.2: Internal anatomy of the kidney

Where the urine goes inside the kidney (following the book's description)

Urine formed in the nephrons drains from each renal papilla into a minor calyx.
Minor calyces join to form the major calyces.
The major calyces open into the renal pelvis.
The renal pelvis leads into the ureter, which carries the urine to the urinary bladder.

4.2.2Relationship between structure and function of kidneyOften examined

The main parts of kidney anatomy with their related function are given below.

Structure and function of the parts of the kidney
Part	Structure	Function
Kidney capsule (renal capsule)	Three layers of connective tissue or fat that cover the kidney	Protects the kidney from injury, increases its stability and connects the kidney to the surrounding tissues
Renal artery	A large blood vessel	Controls blood flow into the kidney. For most people at rest, the kidneys pump a little over 5 cups (1.2 litres) of blood each minute
Renal cortex	The outer layer of the kidney, where the nephrons (blood-filtering units) begin	Filtration begins here. It also creates the hormone erythropoietin (EPO), which helps to make red blood cells in the bone marrow
Renal medulla	The inner part of the kidney. It contains most of the nephrons with their glomeruli and renal tubules	The renal tubules carry urine to the renal pelvis
Renal papillae	Pyramid-shaped structures	Transfer urine into the ureters
Renal pelvis	Funnel-shaped structure	Collects urine and passes it down the ureters. Urine travels from the ureters to the bladder, where it is stored
Renal vein	The main blood vessel that carries filtered blood out of the kidney	Takes the blood back to the heart. Each kidney has a renal vein

4.2.3Structure of a nephronOften examined

The nephron is the functional unit of the kidney. A nephron consists of a renal corpuscle and a renal tubule. A renal corpuscle is composed of a network of capillaries called glomerulus which is surrounded by a thin double-walled structure called Bowman's capsule. The Bowman's capsule is an expansion at the closed end of a renal tubule.

The renal tubule leads away from the Bowman's capsule and becomes highly coiled. This coiled portion of the tubule is called the proximal convoluted tubule. The proximal convoluted tubule dips toward the renal pelvis into the medulla, forming a sharp loop called the loop of Henle. The loop of Henle consists of a descending limb and an ascending limb. The ascending limb returns to the region of the renal corpuscle, where it becomes highly coiled again and is called the distal convoluted tubule, which is connected to the collecting duct. The collecting duct receives many nephrons. Many collecting ducts combine together to form larger collecting ducts which empty into the renal pelvis.

Path of the filtrate through a nephron

Renal corpuscle (glomerulus inside Bowman's capsule): the blood is filtered here.
Proximal convoluted tubule: the first coiled part of the renal tubule.
Loop of Henle: descending limb into the medulla, then ascending limb back up.
Distal convoluted tubule: the second coiled part, near the renal corpuscle.
Collecting duct: receives many nephrons; large ducts empty into the renal pelvis.

Figure 4.3: Structure of nephron

Worked example: identify the parts of a nephron

The figure shows a nephron with six parts marked A to F. Name each part and state what happens there.

A nephron with six parts marked A to F.

A is the tuft of capillaries inside the capsule. It is the glomerulus, where blood is filtered under pressure.
B is the thin double-walled cup around it. It is Bowman's capsule, which collects the filtrate. A and B together make the renal corpuscle.
C is the first highly coiled part of the renal tubule after the capsule. It is the proximal convoluted tubule, where water, ions and all organic nutrients are reabsorbed.
D is the limb that dips down into the medulla and E is the limb that comes back up. D is the descending limb and E is the ascending limb of the loop of Henle. Water is reabsorbed from the descending limb and sodium and chloride ions from the ascending limb.
F is the second highly coiled part, back near the renal corpuscle, before the tube joins the collecting duct. It is the distal convoluted tubule, where ions, acids, drugs and toxins are secreted.

A glomerulus, B Bowman's capsule, C proximal convoluted tubule, D descending limb of the loop of Henle, E ascending limb of the loop of Henle, F distal convoluted tubule.

Blood circulation to nephron

The renal artery within the kidney gives rise to branches that project into the cortex and give rise to afferent arterioles. The afferent arterioles supply blood to the glomerular capillaries of the renal capsule. Efferent arterioles arising from the glomeruli give rise to a plexus of capillaries called the peritubular capillaries around the proximal and distal tubules. A specialised part of the peritubular capillaries, called the vasa recta, courses into the medulla along with the loops of Henle and then back toward the cortex. The peritubular capillaries drain into the renal vein. The renal vein exits the kidney and connects to the inferior vena cava.

Blood supply of a nephron, in the order the blood flows (the glomerular capillaries, where filtration occurs, are highlighted).

Worked example: the journey of a urea molecule

Trace a molecule of urea from the blood entering the kidney to the outside of the body. Name every structure in order, assuming it is filtered and not reabsorbed.

The molecule arrives in the renal artery and passes into a branch in the cortex, then into an afferent arteriole.
The afferent arteriole delivers it to the glomerular capillaries. High hydrostatic pressure forces it out with water into the Bowman's capsule, because urea is a small solute.
In the glomerular filtrate it passes along the proximal convoluted tubule, the descending limb and the ascending limb of the loop of Henle, and the distal convoluted tubule.
It enters a collecting duct, which joins larger collecting ducts and empties at the renal papilla into a minor calyx.
From the minor calyx it goes to a major calyx, then to the renal pelvis, and out of the kidney along the ureter.
The ureter takes it to the urinary bladder, where it is stored. At urination it leaves through the urethra.

Renal artery, afferent arteriole, glomerulus, Bowman's capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule, collecting duct, minor calyx, major calyx, renal pelvis, ureter, urinary bladder, urethra.

4.3Urine formationOften examined

The formation of urine involves three processes: glomerular filtration, tubular reabsorption and tubular secretion.

Glomerular filtration

Glomerular filtration (pressure filtration) takes place in the renal capsule under pressure. The pressure comes from the blood pressure and is known as hydrostatic pressure. Glomerular capillaries have exceptionally high blood pressure compared with any other part of the capillary bed in the body.

The diameter of the efferent arteriole is half that of the afferent arteriole, so the blood enters the narrow capillaries and the pressure rises. Because of this high pressure, water and small solute molecules (ions, glucose and amino acids) are filtered out of the glomerular capillaries and are collected into the Bowman's capsule. Larger molecules like proteins, as well as red blood cells and platelets, are left behind in the blood. The filtered fluid in the capsule is called glomerular filtrate. It has a chemical composition similar to that of blood plasma.

Glomerular filtration: high pressure in the glomerular capillaries pushes water and small solutes into Bowman's capsule; blood cells, platelets and proteins stay in the blood.

Selective reabsorption

Selective reabsorption is the process by which certain substances that have been filtered out of the blood during ultrafiltration are reabsorbed. These substances include glucose, amino acids, vitamins, inorganic salts and some water. As only certain substances are reabsorbed, it is known as selective reabsorption.

Tubular secretion

Tubular secretion is the process by which certain substances, for example ammonium and hydrogen ions, are secreted mainly by the tubular epithelial cells of the loop of Henle into the lumen of the tubule. To some extent this process also occurs in the convoluted tubule. The main purpose of this secretion is to maintain the pH of the urine. Normal urine has a pH range from 4.8 to 7.5.

The three processes of urine formation
Process	What happens	Direction
Glomerular filtration	Water and small solutes leave the glomerular capillaries under hydrostatic pressure; proteins and blood cells stay behind	Blood to Bowman's capsule (to form filtrate)
Selective reabsorption	Glucose, amino acids, vitamins, inorganic salts and some water are taken back	Filtrate in the tubule to blood
Tubular secretion	Ammonium and hydrogen ions (and, in Figure 4.4, ions, acids, drugs and toxins) are added, which maintains the pH of urine	Blood and tubule cells into the tubule lumen

Figure 4.4: Urine formation. Each region of the nephron has its own job; the arrows show substances moving out of or into the tubule.

Worked example: reading a urine test

A urine test on a patient shows protein and red blood cells but no glucose. Which stage of urine formation has probably gone wrong, and which stage is working normally?

Recall what normally passes the glomerular filter. Water and small solutes (ions, glucose, amino acids) are filtered into Bowman's capsule. Larger molecules such as proteins, red blood cells and platelets are left behind in the blood.
Protein and red blood cells in the urine therefore mean that they got into the filtrate. The filtration step at the glomerulus is the fault (the filter is letting large particles through).
Glucose is filtered, but the proximal convoluted tubule reabsorbs it by selective reabsorption, so normal urine contains no glucose. 'No glucose' shows that reabsorption is working.

Substance	In blood?	Passes into the filtrate?	In normal urine?	Reason
Water	Yes	Yes	Yes (less than filtered)	Most is reabsorbed
Glucose	Yes	Yes	No	Reabsorbed in the proximal convoluted tubule
Amino acids	Yes	Yes	No	Selectively reabsorbed
Protein	Yes	No	No	Too large to be filtered
Red blood cells and platelets	Yes	No	No	Too large to be filtered

Glomerular filtration has gone wrong (large particles pass through); selective reabsorption is normal.

Two types of nephrons

There are two types of nephrons, cortical nephrons and juxtamedullary nephrons. Cortical nephrons are found in the cortex. They have their renal corpuscle in the superficial renal cortex and have relatively short loops of Henle. 70 to 80% of the nephrons in the human kidney are cortical. Under normal conditions of water availability, the cortical nephrons deal with the control of blood volume by forming diluted urine. Juxtamedullary nephrons have their renal corpuscle close to the junction of the cortex and medulla. They have a long loop of Henle which extends deep into the medulla. These nephrons are relatively rare and comprise only 20 to 30% of the nephrons in the human kidney. When water is in short supply, increased water retention occurs through the juxtamedullary nephrons.

The two types of nephron: the cortical nephron has its corpuscle in the superficial cortex and a short loop of Henle; the juxtamedullary nephron has its corpuscle near the cortex-medulla junction and a long loop of Henle.

Cortical and juxtamedullary nephrons
Feature	Cortical nephron	Juxtamedullary nephron
Position of renal corpuscle	Superficial renal cortex	Close to the junction of cortex and medulla
Loop of Henle	Relatively short	Long, extends deep into the medulla
Share of nephrons in man	70 to 80%	20 to 30% (relatively rare)
Role	Control blood volume by forming diluted urine under normal water availability	Increased water retention when water is in short supply

4.3.1Mechanism of urine concentrationOften examined

Water is reabsorbed along the whole length of the nephron, but the formation of hypertonic urine depends on the reabsorption of water from the loop of Henle and the collecting duct. This is achieved by the countercurrent multiplier mechanism.

Because of the counter current, that is, the filtrate moving in the limbs of the loop of Henle and the blood moving in the capillaries of the vasa recta, water is greatly (approximately 99.5%) reabsorbed. As the fluid travels up the ascending limb, sodium chloride is transported actively out of the limb into the surrounding area. This movement is controlled by aldosterone. It causes an increase in the concentration of water in the filtrate and a decrease in the concentration of water in the kidney interstitium (the space within a tissue or organ). As a result, water passes out of the descending limb by osmosis. This movement of water is also promoted by the anti-diuretic hormone (ADH), which is secreted from the posterior lobe of the pituitary.

Countercurrent multiplier mechanism

Filtrate flows down the descending limb into the medulla, and then up the ascending limb. Blood in the vasa recta flows alongside in the opposite direction (a counter current).
In the ascending limb sodium chloride is transported actively out of the filtrate into the surrounding kidney interstitium (aldosterone controls this movement).
The interstitium therefore has a high concentration of solutes, that is, a low concentration of water, while the filtrate in the ascending limb has a higher concentration of water.
Water passes out of the descending limb by osmosis into the concentrated interstitium. ADH from the posterior pituitary promotes this movement of water.
The vasa recta carry the reabsorbed water away, so that about 99.5% of the filtered water is reabsorbed and hypertonic urine is formed.

Figure 4.5: Counter current mechanism. Water leaves the descending limb by osmosis, sodium chloride is pumped out of the ascending limb (green), and urea moves between the collecting duct, the medullary fluid and the descending limb.

Worked example: urine after a hot day and after drinking much water

Two people give urine samples. Person X has been working in the heat without drinking and is dehydrated. Person Y has just drunk a large amount of water. Compare their urine and explain, using the kidney's osmoregulatory function.

The kidneys are osmoregulatory organs: they form concentrated urine during dehydration and diluted urine when there is plenty of water (flooding).
Person X needs to save water. The reabsorption of water from the loop of Henle and the collecting duct is high (helped by antidiuretic hormone), so the urine is hypertonic: small in volume and concentrated.
Person Y has excess water. Less water is reabsorbed, so the urine is diluted (hypotonic) and larger in volume.

Feature	Person X (dehydrated)	Person Y (drank much water)
Kidney response	Conserves water	Gets rid of extra water
Water reabsorbed from the loop of Henle and collecting duct	More	Less
Urine concentration	Concentrated (hypertonic)	Diluted (hypotonic)
Urine volume	Small	Large

X forms small amounts of concentrated urine and Y forms large amounts of dilute urine, because the kidneys adjust water reabsorption to keep the water balance of the blood.

Table 4.1Glomerular capillaries and peritubular capillaries

The kidney has two major capillary beds, the glomerular capillaries and the peritubular capillaries. They do different jobs.

Table 4.1: Comparison of function of glomerular capillaries and peritubular capillaries
Glomerular capillaries	Peritubular capillaries
These function as a filtration unit of the kidney.	They filter waste from the blood so the waste can leave the body through urine.
The glomerular function ensures that essential plasma proteins are retained in the blood and the filtrate is passed on as urine.	Peritubular capillaries also reabsorb nutrients the body needs to work properly, such as minerals.
Blood cells and proteins pass through the glomerulus harmlessly.	The peritubular capillaries reabsorb essential minerals and nutrients, along with excess water.

A summary of the two capillary beds (position and pressure from sections 4.2.3 and 4.3)
Point	Glomerular capillaries	Peritubular capillaries
Position	Between the afferent and efferent arterioles, inside Bowman's capsule	Around the proximal and distal tubules, arising from the efferent arteriole (the vasa recta run with the loops of Henle)
Main function	Filtration of blood	Reabsorption of needed substances (and removal of waste)
Blood pressure	Exceptionally high	Lower (they drain into the renal vein)

4.4Functions of kidneyOften examined

Kidneys function as excretory as well as osmoregulatory organs.

Excretory function: The filtration of nitrogenous wastes from the blood and their removal outside the body in the form of urine.
Osmoregulatory function: Being osmoregulatory organs, the kidneys are concerned with the formation of diluted urine during the state of flooding and the formation of concentrated urine during the state of dehydration.

Excretory and osmoregulatory functions
Function	What the kidney does	When
Excretory	Filters nitrogenous wastes from the blood and removes them in the urine	Always
Osmoregulatory	Forms diluted urine	During flooding (excess water)
Osmoregulatory	Forms concentrated urine	During dehydration

The four jobs of the kidneys named in this section: excretion, osmoregulation, control of red blood cell formation by EPO and control of blood pressure by renin.

In addition to their excretory and osmoregulatory role, the kidneys also help to control the red blood cell formation by secreting the hormone erythropoietin (EPO), and help to regulate blood pressure by secreting the enzyme renin.

4.5Disorders of urinary tract

The normal aging process in humans affects kidney function in various ways. Urinary tract infections (UTI) are fairly common. Urology is the branch of medicine which deals with diseases and abnormalities of the urinary tract and their treatment.

Disorders of the urinary tract in this chapter
Disorder	Section	Treatment named in the book
Urinary tract infection	4.5.1	Antibiotic therapy
Kidney stones	4.5.2	ESWL and PCNL
Kidney failure	4.5.3	Dialysis or kidney transplant

4.5.1Urinary tract infection

Although males can get a urinary tract infection, the condition is fifty times more common in women. In general, the higher risk in women is mostly due to the shortness of the female urethra, which is 1.5 inches compared with 8 inches in men. Bacteria from faecal matter at the anal opening can be easily transferred to the opening of the urethra.

Almost all parts of the urinary tract are affected by the infection except the ureters, which are rarely the site of infection. The types of UTIs depending upon the site are: urethritis is an infection of the urethra, cystitis involves the bladder, and if the kidneys are infected the infection is called pyelonephritis.

Types of urinary tract infection
Type of UTI	Site of infection
Urethritis	Urethra
Cystitis	Urinary bladder
Pyelonephritis	Kidneys

Sites of urinary tract infection: the kidneys (pyelonephritis), the bladder (cystitis) and the urethra (urethritis). The ureters are rarely infected.

Table 4.2: Urinary tract infection (UTI) caused by bacteria
Bacteria	Diseases
1. E. coli	1. UTI
2. N. gonorrhoeae	2. Urethritis, gonorrhoea
3. T. palladium	3. Syphilis

Since the infection is caused by bacteria, it is curable by antibiotic therapy. For prevention, one should drink lots of water to flush out bacteria. Personal hygiene is especially important too.

4.5.2Kidney stonesOften examined

Urinary stones are hard, crystalline mineral materials that stick together to form small "pebbles" within the kidney or urinary tract. They can be as small as grains of sand or as large as golf balls. They may stay in the kidneys or travel out of the body through the urinary tract. The condition of having stones in the kidney is termed nephrolithiasis.

Figure 4.6: Kidney stone. Stones may lie in a calyx (calyceal stone), in the renal pelvis (renal pelvic stone) or in the ureter (upper ureteral stone).

Worked example: which kind of stone from the urine test?

Using the book's hypothesis for kidney stones, predict the type of stone in each patient. Patient 1: urine is acidic (low pH). Patient 2: urine is alkaline (high pH). Patient 3: urine is persistently acidic. Patient 4: the blood calcium is raised (hypercalcemia) and the urine is rich in calcium.

Patient 1 has acidic urine. By the book's rule, acidic (low pH) urine goes with a stone of calcium oxalate.
Patient 2 has alkaline (high pH) urine. The stone is calcium phosphate.
Patient 3 has persistently acidic urine. The stone is of the uric acid type, which is linked with hyperuricemia, the increased amount of uric acid in the blood.
Patient 4 has hypercalcemia, which causes high calcium in the urine (hypercalciuria). Calcium combined with oxalate gives calcium oxalate stones, the commonest type (70% of patients).

Patient	Finding	Likely stone
1	Acidic urine (low pH)	Calcium oxalate
2	Alkaline urine (high pH)	Calcium phosphate
3	Persistently acidic urine	Uric acid
4	Hypercalcemia and hypercalciuria	Calcium oxalate (70% of cases)

1 calcium oxalate, 2 calcium phosphate, 3 uric acid, 4 calcium oxalate.

Symptoms

Common symptoms include severe pain in the lower back, blood in the urine, nausea, vomiting, fever and chills, or urine that smells bad or looks cloudy.

Causes

Kidney stones may be caused by an increased calcium level in the blood, which is termed hypercalcemia. It, in turn, causes high calcium in the urine, the hypercalciuria. An increased oxalate level in the urine is called hyperoxaluria. Hypercalciuria and hyperoxaluria cause the calcium oxalate type of kidney stones, which are present in 70% of kidney stone patients. Hyperuricemia is the increased amount of uric acid in the blood and it causes the uric acid type of kidney stones, which are found in 10% of kidney stone patients. A high concentration of cysteine and phosphates in urine also causes kidney stones. A continuous state of dehydration increases the chances of kidney stone formation.

Causes of kidney stones
Condition	Meaning	Stone type caused
Hypercalcemia	Increased calcium level in the blood (leads to hypercalciuria, high calcium in the urine)	Calcium oxalate (with hyperoxaluria)
Hyperoxaluria	Increased oxalate level in the urine	Calcium oxalate (70% of patients)
Hyperuricemia	Increased amount of uric acid in the blood	Uric acid (10% of patients)
High cysteine and phosphates in urine	High concentration of cysteine and phosphates in the urine	Kidney stones
Continuous dehydration	Too little water in the body	Raises the chance of any stone forming

Treatment

Extracorporeal shock wave lithotripsy (ESWL) and percutaneous nephrolithotripsy (PCNL) are common methods for kidney stone treatment. In ESWL, an instrument called a lithotripter is used to generate shock waves from outside the patient's body, focused on the stone, breaking it into small pieces. Most of the fragments then pass spontaneously via the urethra.

In the case of a larger stone, PCNL is preferred. A tube is inserted from the patient's back into the kidney to create a tract. A scope is run through the tract to directly visualize the stone inside the kidney. Ultrasound equipment can then be inserted to break up the stone. While watching the stone through the scope, the stone fragments can be grasped with special equipment and pulled through the tract out of the kidney. Open surgery is now almost never needed, except for a large bladder stone.

Figure 4.7: Extracorporeal shock wave lithotripsy (ESWL). The lithotripter outside the body sends shock waves that are focused on the stone and break it into small pieces. (The book shows a photograph; this is a simplified drawing.)

ESWL and PCNL compared
Feature	ESWL	PCNL
Full name	Extracorporeal shock wave lithotripsy	Percutaneous nephrolithotripsy
Method	Lithotripter outside the body generates shock waves focused on the stone	A tube is inserted from the back into the kidney to make a tract; a scope and ultrasound equipment are passed through it
How the stone is removed	It breaks into small pieces, most of which pass spontaneously via the urethra	The stone is broken up and the fragments are grasped and pulled out through the tract
Used for	Common method	Larger stones

4.5.3Kidney failure

A general term for a decline in kidney function, particularly in the efficiency of the filtering process, is called kidney failure or renal failure. Chronic renal failure is the irreversible deterioration in renal function. It is a gradual, slowly progressive condition and occurs over a period of years.

Causes of kidney failure
Type	Causes given in the book
Chronic renal failure	(a) Bacterial infection of the pelvis and surrounding tissue. (b) Nephritis (inflammation of glomeruli). (c) Damage due to high blood pressure. (d) Diabetes mellitus.
Acute renal failure	(a) Haemorrhage due to trauma. (b) Vomiting, diarrhoea. (c) Diuresis (excess excretion of urine). (d) Obstruction of the ureters, bladder or urethra, for example by a kidney stone. (e) Severe nephritis.

Kidney failure is chronic or acute. The chronic type (accent) is irreversible and can progress to end-stage renal disease and uremia, which is treated by dialysis or a transplant.

4.5.4Dialysis: mechanismOften examined

A procedure to filter toxins from the blood by artificial methods when the kidneys are unable to perform this function is called renal dialysis. Dialysis works on the principle of the kidneys, although it is not as effective, efficient or thorough as the natural processes performed by the kidneys. There are two general types of renal dialysis: haemodialysis and peritoneal dialysis.

Haemodialysis

Haemodialysis removes wastes and water by circulating blood outside the body through an external filter, called a dialyzer, which consists of tubes of semipermeable membrane. A catheter is inserted into a blood vessel, usually in the arm. It routes the blood circulation externally through a machine that removes wastes. The cleansed blood then returns to the body through a second catheter. The haemodialysis machine consists of a pump and a container in which a network of synthetic tubes made up of cellophane membrane, called the dialyzer, is situated.

Haemodialysis step by step

A catheter in a blood vessel of the arm (an artery) takes blood out of the body.
The blood pump moves the blood into the tubes of the dialyzer from the top.
In the dialyzer the blood circulates through tubes of cellophane (semipermeable) membrane. The dialysate pours into the machine from the bottom and flows around the membranous tubes, leaving from the top.
The dialysate attracts certain substances (minerals, electrolytes and waste by-products) to cross the membrane from the blood, so the dialysate absorbs these substances.
The blood leaves the machine from the bottom and is transfused back into the body through a second catheter (to a vein).

Figure 4.8: Haemodialysis. Blood (green) goes from the artery through the pump into the dialyzer from the top and returns to the vein through the bubble trap; dialysate (white) enters the dialyzer from the bottom and leaves from the top.

Inside the dialyzer. Blood flows down and dialysate flows up, on either side of the partially permeable dialysis membrane. Urea and excess salts (accent) diffuse from the blood into the dialysate; blood cells are too big to pass.

Peritoneal dialysis

Peritoneal dialysis involves the use of a natural membrane in the body, the peritoneum, which encloses the abdominal cavity. In this process, two catheters are surgically inserted into the abdominal cavity that serve as the portals (any entrance) through which dialysate enters and leaves the cavity. During circulation, when blood passes through the blood capillaries within the peritoneum, the dialysate attracts certain molecules to cross the membrane into the dialysate.

Figure 4.9: Peritoneal dialysis. Dialysis fluid runs into the peritoneal cavity through one catheter; waste products move from the capillaries of the peritoneal membrane into the cavity, and the fluid plus waste is drained out through the other catheter.

Haemodialysis and peritoneal dialysis compared
Feature	Haemodialysis	Peritoneal dialysis
Membrane	Artificial: cellophane (semipermeable) membrane of the dialyzer	Natural: the peritoneum, which encloses the abdominal cavity
Where blood is cleaned	Blood is taken outside the body through an external filter (dialyzer)	Blood stays in the capillaries of the peritoneum inside the body
Catheters	One in a blood vessel (usually in the arm) takes blood out and a second returns it	Two surgically inserted into the abdominal cavity, for dialysate to enter and leave
Dialysate	Pours in from the bottom of the machine, circulates around the tubes, and leaves from the top	Enters the abdominal cavity and then leaves it

4.5.5Kidney transplant: process and problemsOften examined

Kidney transplantation is the surgical procedure of placing a fully functioning kidney into a person with chronic kidney failure.

Principles of kidney transplant

The kidney graft is taken from a deceased donor or from a related or unrelated person. ABO blood group compatibility between donor and recipient is essential. It is usual to select donor kidneys on the basis of human leucocyte antigen (HLA) matching, as this improves graft survival. A person can live normally with just one kidney.

Problems associated with kidney transplant

The two problems are rejection and the toxic effects of cyclosporine. These problems are usually treated simultaneously by adding extra doses of steroids. Patients are required to take medications such as cyclosporine etc., to suppress their immune system in order to prevent rejection of the transplanted kidney. If at any point a recipient stops taking the medications, rejection can occur, even ten or fifteen years after the transplant.

Kidney transplant: principles and problems
Principles	Problems
Graft from a deceased donor or from a related or unrelated person	Rejection of the transplanted kidney
ABO blood group compatibility is essential	Toxic effects of cyclosporine
HLA matching of donor kidneys improves graft survival	Treated at the same time with extra doses of steroids
A person can live normally with just one kidney	Medicines to suppress the immune system must be taken, because rejection can occur even 10 or 15 years later if they are stopped

Activity 4.1STEAM activity 4.1: blood dialysis and kidney functions

This experiment, called an experiment to show blood dialysis and kidney functions, uses dialysis tubing as a model of the dialysis membrane.

Requirements

Three small beakers or jars (not larger than 250 mL size)
Distilled or deionized water
Scissors, ruler, timer or clock, paper towels, string (optional)
Dialysis tubing, a transfer pipet and the three samples (Normal, Patient 1, Patient 2)

Procedure

Cut the dialysis tubing into three 4-inch segments.
Soak the segments of dialysis tubing in a jar or beaker filled with distilled water for at least one hour.
Add 75 to 100 mL of distilled or deionized water to three separate beakers or jars. Label them "Normal", "P1" and "P2". These are the water baths.
Carefully tie a single knot at one end of all three dialysis tubing segments, as close to the ends as possible (string may also be used).
Fill the first dialysis tubing with 2 mL of the Normal Patient Sample: (a) roll the untied end of the tubing between your thumb and pointer finger to open the end; (b) squeeze the transfer pipet bulb and fill the tip to the 2 mL mark; (c) gently insert the tip of the pipet deep into the tubing and slowly transfer the sample by applying gentle pressure to the bulb; (d) remove any trapped air without displacing the sample; (e) carefully tie the second knot at the open end; (f) rinse the filled-tied tube with water, especially near the tied ends, and place the bag on a paper towel in front of the beaker labelled "Normal" (do the same for P1 and P2 when you fill those).
In a separate container, rinse the transfer pipet with water by repeatedly squeezing the bulb and flushing the pipette.
Repeat steps 5 and 6 for Patients 1 and 2. Quickly place all dialysis tubes in their respective water baths.
Monitor the three dialysis water baths every 5 minutes or so, taking notes on any changes in the colour of the water outside the dialysis bag. Allow the bags to soak for up to 30 minutes.
After you have recorded your results, you can cut or untie the dialysis tubing, empty it and rinse it well to reuse for another experimental trial.

Record the colour of the water outside the bag, as in the book's results table
	Normal	Patient 1	Patient 2
Colour at 5 min
Colour at 10 min
Colour at 15 min

Key terms

Excretion: The removal of chemical waste from the body which is produced by the metabolic processes within cells.
Ammonia: A highly toxic, highly soluble nitrogenous waste that raises the pH of body fluids; 1 g of nitrogen needs 500 mL of water to dilute it. Excreted by aquatic animals.
Urea: A nitrogenous waste 100,000 times less toxic than ammonia; 1 g of nitrogen needs 50 mL of water. Excreted by mammals including man, many amphibians and some marine animals like shark.
Uric acid: A less toxic by-product of the breakdown of purine that can precipitate from solution; 1 g of nitrogen needs 1 mL of water. Excreted by terrestrial insects, reptiles and most birds.
Ammonotelic: Animals which excrete ammonia as their major nitrogenous waste product.
Ureotelic: Animals which excrete urea as their major nitrogenous waste product.
Uricotelic: Animals which excrete uric acid as their major nitrogenous waste product.
Urinary system: The excretory system, consisting of the kidneys, ureters, urinary bladder and urethra.
Ureter: A tubular organ about 25 cm long that carries urine from the renal pelvis of the kidney to the urinary bladder.
Urinary bladder: A hollow, distensible muscular organ in the pelvic cavity that serves as the urine reservoir.
Urethra: The tube that carries urine from the urinary bladder to the outside of the body.
Renal fascia, perirenal fat capsule and renal capsule: The three layers around the kidney: the tough connective tissue outer layer, the fat layer that anchors the kidney, and the innermost capsule that protects it.
Renal cortex: The outer region of the kidney, granular because of the nephrons; it also makes erythropoietin.
Renal medulla: The middle region of the kidney, made of the renal pyramids.
Hilum: The concave part of the bean-shaped kidney where blood vessels and nerves enter and exit; the point of exit for the ureters.
Renal pyramids: Pyramidal tissue masses of the medulla; there are on average eight in each kidney.
Renal columns: The spaces between the renal pyramids through which the blood vessels pass.
Renal papillae: The tips of the renal pyramids, which point toward the renal pelvis.
Lobes of the kidney: A renal pyramid together with the adjoining cortical region.
Renal pelvis: The funnel-shaped structure that collects urine and passes it to the ureter; it branches inside the kidney into the major calyces.
Major and minor calyces: Two or three major calyces branch from the renal pelvis and divide into the minor calyces.
Nephron: The functional unit of the kidney, consisting of a renal corpuscle and a renal tubule.
Renal corpuscle: The glomerulus together with the Bowman's capsule that surrounds it.
Glomerulus: The network of capillaries in the renal corpuscle where blood is filtered.
Bowman's capsule: The thin double-walled expansion at the closed end of a renal tubule that surrounds the glomerulus.
Proximal convoluted tubule: The first highly coiled part of the renal tubule after the Bowman's capsule.
Loop of Henle: The sharp loop of the renal tubule dipping into the medulla, with a descending limb and an ascending limb.
Distal convoluted tubule: The highly coiled part of the renal tubule after the ascending limb, connected to the collecting duct.
Collecting duct: The duct that receives many nephrons; many ducts combine to form larger ducts that empty into the renal pelvis.
Afferent arteriole: The arteriole from a branch of the renal artery that supplies blood to the glomerular capillaries.
Efferent arteriole: The arteriole leaving the glomerulus (half the diameter of the afferent arteriole) that gives rise to the peritubular capillaries.
Peritubular capillaries: The plexus of capillaries around the proximal and distal tubules that drains into the renal vein.
Vasa recta: The specialised part of the peritubular capillaries that runs into the medulla along with the loops of Henle and back toward the cortex.
Glomerular filtration: Pressure filtration in the renal capsule in which water and small solutes pass from the glomerular capillaries into Bowman's capsule.
Hydrostatic pressure: The pressure that drives glomerular filtration, which comes from the blood pressure.
Glomerular filtrate: The fluid filtered into Bowman's capsule, with a composition similar to that of blood plasma.
Selective reabsorption: The process by which glucose, amino acids, vitamins, inorganic salts and some water are taken back from the filtrate.
Tubular secretion: The process by which substances such as ammonium and hydrogen ions are secreted into the lumen of the tubule, mainly to maintain the pH of urine.
Countercurrent multiplier mechanism: The mechanism, based on the opposite flow of filtrate in the limbs of the loop of Henle and blood in the vasa recta, that lets water be reabsorbed to form hypertonic urine.
Aldosterone: The hormone that controls the active transport of sodium chloride out of the ascending limb.
Anti-diuretic hormone (ADH): The hormone from the posterior lobe of the pituitary that promotes the movement of water out of the filtrate.
Cortical nephrons: The 70 to 80% of nephrons with their renal corpuscle in the superficial cortex and short loops of Henle.
Juxtamedullary nephrons: The 20 to 30% of nephrons with their renal corpuscle near the cortex-medulla junction and long loops of Henle.
Osmoregulation: The kidney function of forming diluted urine during flooding and concentrated urine during dehydration.
Erythropoietin (EPO): The hormone made by the kidney that helps to make red blood cells in the bone marrow.
Urology: The branch of medicine that deals with diseases and abnormalities of the urinary tract and their treatment.
Urethritis: An infection of the urethra.
Cystitis: An infection that involves the bladder.
Pyelonephritis: An infection of the kidneys.
Nephrolithiasis: The condition of having stones in the kidney.
Hypercalcemia: Increased calcium level in the blood; leads to hypercalciuria, high calcium in the urine.
Hyperoxaluria: Increased oxalate level in the urine.
Hyperuricemia: Increased amount of uric acid in the blood; it causes uric acid kidney stones.
Lithotripter: The instrument that generates shock waves from outside the body, focused on a kidney stone, in ESWL.
Kidney failure (renal failure): A decline in kidney function, particularly in the efficiency of the filtering process.
Chronic renal failure: The irreversible, gradual and slowly progressive deterioration of renal function over a period of years.
Renal dialysis: A procedure to filter toxins from the blood by artificial methods when the kidneys are unable to do so.
Haemodialysis: Dialysis in which blood circulates outside the body through an external filter (dialyzer) of semipermeable tubes.
Peritoneal dialysis: Dialysis that uses the peritoneum, which encloses the abdominal cavity, as the membrane.
Kidney transplant: The surgical procedure of placing a fully functioning kidney into a person with chronic kidney failure.

Multiple choice questionsp. 82–84

1

Excretion of hypotonic urine in humans is associated best with the:

Aglomerular capsule
Bproximal convoluted tubule
Cloop of the HenleAnswer
Ddistal convoluted tubule

Why

The ascending limb of the loop of Henle pumps sodium chloride out of the filtrate but is impermeable to water, so the fluid leaving the loop is dilute (hypotonic). The glomerular capsule only filters blood, and the proximal convoluted tubule reabsorbs water and solutes in similar proportions, so its fluid stays about as concentrated as blood plasma. The distal convoluted tubule adjusts the final volume of urine, but the dilution itself is produced in the loop.

Note. The book prints no key for this exercise. Among the options, the loop of Henle is the site where the filtrate is diluted, so it is the answer chosen here. The distal convoluted tubule is the tempting wrong option because it also changes urine volume.

2

The walls of the ______ are made more or less permeable to water, depending on the need to conserve water:

Aureter
Burethra
Cfibrous capsule
Dcollecting ductAnswer

Why

Antidiuretic hormone (ADH) controls how permeable the walls of the collecting duct are to water. When the body needs to conserve water, more ADH makes the duct walls more permeable, so more water is reabsorbed into the blood and the urine becomes more concentrated. The ureter and urethra only carry urine, and the fibrous capsule is a covering around the kidney.

3

Which of the following will cause a decrease in ADH production?

Adehydration
Ban increase in osmotic pressure of blood
Cdrinking waterAnswer
Dabnormally low blood pressure

Why

Drinking water dilutes the blood and lowers its osmotic pressure. The fall is detected and less ADH is released. Dehydration and a rise in blood osmotic pressure both increase ADH, the opposite effect. Low blood pressure also increases ADH release rather than reducing it.

4

The function of glomerulus and Bowman's capsule of the nephron is to:

Areabsorb water into the blood
Beliminate ammonia from the body
Creabsorb salts and amino acids
Dfilter the blood and capture the filtrateAnswer

Why

The glomerulus is a knot of capillaries under high pressure, which forces water and small solutes out of the blood into Bowman's capsule. This is glomerular filtration, and the capsule captures the filtrate. Reabsorption of water, salts and amino acids happens later, in the tubule, so options A and C describe the tubule and not this part.

5

In man, glucose is present in blood plasma but not in urine. This is because glucose molecules are:

Aactively transported from the proximal convoluted tubule to blood capillariesAnswer
Boxidised to supply energy for ultrafiltration
Cstored in the kidney
Dtoo large to enter Bowman's capsule

Why

Glucose is filtered into Bowman's capsule with the filtrate, so it does reach the tubule. In the proximal convoluted tubule it is reabsorbed back into the blood capillaries by active transport, so almost none is left to pass into the urine. Glucose is small enough to be filtered (option D is false), and it is not used for filtration (option B) or stored in the kidney (option C).

6

Evidence for glomerular filtration in the kidney could be obtained by comparing the sizes of the molecules present in Bowman's capsule with those in the:

Aafferent blood vesselAnswer
Bcollecting duct
Cloop of Henle
Dproximal tubule

Why

Filtration separates molecules by size. If the fluid in Bowman's capsule is compared with the blood entering the glomerulus through the afferent arteriole, the large plasma proteins and blood cells are present in the blood but absent from the capsule. The collecting duct, loop of Henle and proximal tubule contain fluid that has already been changed by reabsorption and secretion, so they cannot show what the filter did.

7

The site and principal mechanism for the passage of glucose into the bloodstream in the human kidney is the:

Acollecting duct, by active secretion
Bglomerulus, by selective reabsorption
Cglomerulus, by ultrafiltration
Dproximal convoluted tubule, by selective reabsorptionAnswer

Why

Glucose returns to the bloodstream from the kidney tubule by selective reabsorption in the proximal convoluted tubule. In the glomerulus, glucose moves the other way, out of the blood into the capsule by ultrafiltration, so options B and C describe the opposite movement. Glucose is not secreted in the collecting duct (option A).

8

A drug reduces mitochondrial activity in kidney nephrons. Which chemical will be present in increased amounts in the urine?

Aammonia
BglucoseAnswer
Curic acid
Durea

Why

Mitochondria release the energy (ATP) that cells need. Reabsorption of glucose from the proximal tubule is an active process that needs this energy, so less mitochondrial activity means less glucose is reabsorbed and more of it is lost in the urine. Normally no glucose appears in urine. Ammonia, uric acid and urea are nitrogenous wastes that the kidney excretes whatever the energy supply, so they do not rise because of this drug.

9

The water content of human blood is regulated by ADH. In which part of the nephron does regulation occur?

Aascending limb of loop of Henle
Bdescending limb of loop of HenleAnswer
CBowman's capsule
Dproximal convoluted tubule

Why

Water leaves the descending limb of the loop of Henle by osmosis, and the book's account of this movement says that ADH promotes it. The ascending limb is impermeable to water, Bowman's capsule only filters, and the proximal convoluted tubule reabsorbs water without hormonal control. Among the options, the descending limb is the only site where ADH controls water movement.

Note. Strictly, ADH acts mainly on the collecting duct, which is not among the options. The descending limb is the best option because the book's text ties ADH to water leaving that limb.

10

The reason why the right kidney is slightly lower than the left is:

Athe left kidney is bigger than right
Bconsiderable space occupied by the heart
Cconsiderable space occupied by the liver on the right sideAnswer
Dthe right kidney is bigger than the left

Why

The liver is large and lies on the right side of the abdomen, above the right kidney, so the right kidney sits slightly lower. The heart is not on the right side, and the size of the two kidneys is not the reason for the difference in height.

11

Which of the following is known as the functional unit of the kidney and is found in the cortex?

Acalyces
Brenal pyramids
CnephronsAnswer
Dmedulla

Why

Each nephron is the functional unit that filters blood and forms urine, and its renal corpuscle lies in the cortex. The calyces collect urine, the renal pyramids and the medulla are inner regions of the kidney, and only the nephrons are the filtering units.

12

This artery passes blood to the kidney:

Acommon iliac
Bcystic
CrenalAnswer
Dcoeliac

Why

The renal artery branches from the aorta and carries blood into the kidney at the hilum. The other three arteries supply other organs and not the kidney.

13

Which structure carries urine from the kidneys to the bladder?

Anephron
BureterAnswer
Cbladder
Durethra

Why

Each ureter is a tube about 25 cm long that begins as the renal pelvis and carries urine down to the urinary bladder. The nephron makes urine but does not carry it away, the bladder stores urine, and the urethra carries urine from the bladder out of the body.

14

The blood vessel which carries blood to the glomerulus is the:

Arenal artery
Bafferent arterioleAnswer
Crenal vein
Dperitubular capillaries

Why

Each glomerulus is supplied by one afferent arteriole, which branches from the renal artery. The efferent arteriole carries blood away from the glomerulus, the renal vein carries blood out of the kidney, and the peritubular capillaries come after the efferent arteriole.

15

The basic unit of the renal system is the:

Acortex
Bneuron
CnephronAnswer
Dmedulla

Why

The nephron is the functional unit of the kidney, so it is the basic unit of the renal (excretory) system. The cortex and medulla are regions of the kidney that contain nephrons, and a neuron is a nerve cell that belongs to the nervous system.

16

The centre of the kidney where urine collects before leaving the kidneys is the:

Aglomerulus
Bproximal tubule
Cdistal tubule
Drenal pelvisAnswer

Why

The renal pelvis is the funnel-shaped structure at the centre of the kidney. Urine from the calyces collects there and passes into the ureter. The glomerulus and the tubules form urine in the nephrons but do not collect it at the centre of the kidney.

17

An organ or structure that is not a component of the urinary system is the:

Aurethra
Burinary bladder
Cureter
Dadrenal glandAnswer

Why

The urinary system is made of the kidneys, ureters, urinary bladder and urethra. The adrenal glands sit on top of the kidneys but are endocrine glands that make hormones, so they are not part of the urinary tract.

18

All of the following belong to the urinary system except:

Aurethra
Bureter
Cbladder
DprostateAnswer

Why

The prostate is a gland of the male reproductive system. The urethra, ureter and bladder are parts of the urinary system, which carry, store or pass out urine.

19

What portion of the nephron extends into the medulla?

Anephron loopAnswer
Bproximal convoluted tubule
Cdistal convoluted tubule
Dpapillary duct

Why

The loop of Henle is the part of the nephron that dips down into the medulla and turns back toward the cortex. The proximal and distal convoluted tubules lie in the cortex. The papillary duct carries urine to the minor calyx and is not part of the nephron.

20

Fibrous connective tissue that surrounds each kidney is the:

Acortex
Bhilum
Cmedulla
Drenal capsuleAnswer

Why

The renal capsule is the connective tissue layer that surrounds each kidney, protecting it and giving it stability. The cortex and medulla are regions inside the kidney, and the hilum is the concave opening where the blood vessels and ureter enter and leave.

Note. The book describes three covering layers. The outermost is a tough connective tissue layer called the renal fascia, which is not among the options, so the renal capsule is the best answer.

Short answer questionsp. 84

13 marks

Name the organs of the urinary system and write their major functions.

Solution

Organ	Major function
Kidneys	Filter the blood, form urine and remove nitrogenous wastes; regulate water and salts
Ureters	Carry urine from the kidneys to the urinary bladder
Urinary bladder	Stores urine until it is released
Urethra	Carries urine from the bladder to the outside of the body

The kidneys also produce erythropoietin, which helps make red blood cells, and renin, which helps regulate blood pressure.

23 marks

Describe glomerular filtration.

Solution

Blood pressure in the glomerulus is high because the afferent arteriole is wider than the efferent arteriole (the efferent arteriole has about half its diameter). The blood is held under pressure in the glomerular capillaries.
Water and small solutes (ions, glucose and amino acids) are forced out through the capillary walls into Bowman's capsule. This fluid is the glomerular filtrate.
Large molecules such as plasma proteins, red blood cells and platelets are too big to pass, so they stay in the blood. The filtrate therefore has a composition similar to blood plasma, without its proteins and cells.

33 marks

Describe the countercurrent multiplier mechanism.

Solution

Filtrate flows down the descending limb and up the ascending limb of the loop of Henle, while blood flows in the opposite direction in the vasa recta. Flows in opposite directions are called countercurrent.
The thick ascending limb actively pumps sodium chloride out into the surrounding medulla. The thin descending limb is permeable to water, so water leaves it by osmosis into this salty region. The medulla becomes more concentrated toward the papilla, and this gradient is multiplied along the loop.
The vasa recta carries blood in the opposite direction, so it keeps the gradient instead of washing it out. When ADH makes the collecting duct permeable to water, water leaves the duct down this gradient and the urine becomes hypertonic (concentrated).

43 marks

Name the parts of a nephron and trace the blood supply to the nephron.

Solution

Parts of a nephron:

Renal corpuscle: glomerulus and Bowman's capsule
Proximal convoluted tubule
Loop of Henle: descending limb and ascending limb
Distal convoluted tubule
Collecting duct (receives urine from several nephrons)

Blood supply to the nephron, in order:

Renal artery
Afferent arteriole
Glomerular capillaries (inside Bowman's capsule)
Efferent arteriole
Peritubular capillaries and vasa recta around the tubules
Venules, then the renal vein
Inferior vena cava

53 marks

Name general processes which are involved with urine formation.

Solution

Process	Where it happens	What moves
Glomerular filtration	Glomerulus into Bowman's capsule	Water, ions, glucose, amino acids and urea pass out of the blood; proteins and blood cells stay
Selective reabsorption	Proximal tubule, loop of Henle, distal tubule and collecting duct	Glucose, amino acids, vitamins, salts and water return to the blood
Tubular secretion	Loop of Henle and distal convoluted tubule	Hydrogen and ammonium ions, drugs and toxins move from the blood into the tubule

Note. The book places hydrogen and ammonium ion secretion mainly in the loop of Henle and says it also occurs to some extent in the convoluted tubules. Most texts place it mainly in the proximal and distal convoluted tubules.

63 marks

Describe urinary tract infection.

Solution

A urinary tract infection (UTI) is an infection of the urinary system caused by bacteria, most often Escherichia coli. The bacteria come from faecal matter near the anal opening and reach the urethra.

Urethritis is an infection of the urethra. Cystitis is an infection of the bladder. Pyelonephritis is an infection of the kidney, which occurs when the infection reaches the kidneys.
Infection is more common in women because the female urethra is short. Almost any part of the urinary tract can be infected, but the ureters are rarely affected.
Because the cause is bacterial, UTI is treated with antibiotics. It is prevented by drinking plenty of water to flush out bacteria and by good personal hygiene.

73 marks

Name three urinary tract infections and the bacteria responsible.

Solution

Infection	Bacterium
Urinary tract infection (UTI), for example cystitis	Escherichia coli
Urethritis and gonorrhoea	Neisseria gonorrhoeae
Syphilis	Treponema pallidum

Note. The book's Table 4.2 prints the third bacterium as 'T. palladium'; the correct name is Treponema pallidum.

82026 Annual I, Set B, Q.2 (xi)3 marks

What are the causes of kidney failure?

Solution

Type of failure	Causes
Chronic (gradual, irreversible over years)	Bacterial infection of the pelvis and surrounding tissue; nephritis (inflammation of the glomeruli); damage due to high blood pressure; diabetes mellitus
Acute (sudden)	Haemorrhage due to trauma; vomiting, diarrhoea or sweating (loss of fluid); diuresis (excess urine output); obstruction of the ureters, bladder or urethra, for example by a kidney stone; severe nephritis

93 marks

By what physical processes do solutes enter or leave the blood during dialysis?

Solution

Solutes move by diffusion, from a higher concentration to a lower concentration. Waste products and excess minerals and electrolytes leave the blood and move into the dialysate, which has a lower concentration of them.
The dialysis membrane is semipermeable (cellophane tubes in haemodialysis, the peritoneum in peritoneal dialysis). Small molecules pass through it, but blood cells and large proteins do not.
Fresh dialysate is constantly supplied, so the concentration gradient is kept. Water can also move by osmosis when the dialysate is more concentrated than the blood.

103 marks

Why do blood and dialysate flow in opposite directions?

Solution

The blood and the dialysate flow in opposite directions (countercurrent flow), so along the whole length of the membrane the blood is always more concentrated in wastes than the dialysate.
If both flowed in the same direction, the two fluids would reach equal concentrations partway along the membrane and diffusion would stop.
The gradient is kept along the whole membrane, so more waste is removed and the blood leaves the machine cleaner.

113 marks

Suggest two problems that might occur if the dialysate was pure water.

Solution

Pure water is hypotonic to blood, so water enters the red blood cells by osmosis. They swell and may burst (haemolysis).
Useful substances such as glucose, amino acids, salts and minerals diffuse out of the blood into the pure water, because the dialysate would have none of them. The blood loses essential substances and its electrolyte balance is upset.

123 marks

Why are women more likely to get urinary tract infection as compared to men?

Solution

The female urethra is much shorter, about 1.5 inches, compared with about 8 inches in men.
The opening of the female urethra is close to the anal opening, so bacteria from faecal matter reach it easily.
Bacteria therefore travel a short distance to the bladder, and UTI is about fifty times more common in women than in men.

13

Write the differences between the following.

Solution

Each pair is answered in a table with three points of difference.

(a) osmoregulation and osmoconformers

Feature	Osmoregulation	Osmoconformers
Internal osmotic concentration	Kept steady whatever the surroundings	Changes to match the surroundings
Control	Active control of water and salt balance, for example by the kidneys	No active control; the body fluids follow the environment
Energy	Energy is spent pumping salts and producing urine	Little energy is needed to control the internal concentration

(b) ammonotelic and ureotelic

Feature	Ammonotelic	Ureotelic
Main nitrogenous waste	Ammonia	Urea
Water needed to excrete 1 g of nitrogen	About 500 ml	About 50 ml
Habitat and energy	Aquatic (freshwater); no energy spent on conversion	Aquatic and terrestrial; energy spent converting ammonia to urea

(c) ureotelic and uricotelic

Feature	Ureotelic	Uricotelic
Main nitrogenous waste	Urea	Uric acid
Water needed to excrete 1 g of nitrogen	About 50 ml	About 1 ml
Habitat	Aquatic and terrestrial	Terrestrial (for example insects, reptiles and birds)

(d) proximal and distal convoluted tubule

Feature	Proximal convoluted tubule	Distal convoluted tubule
Position	Follows Bowman's capsule and leads into the loop of Henle	Follows the ascending limb, lies near the renal corpuscle and leads into the collecting duct
Main function	Reabsorbs water, ions and all organic nutrients	Secretes ions, acids, drugs and toxins
Connected to	Loop of Henle	Collecting duct

(e) afferent and efferent arterioles

Feature	Afferent arteriole	Efferent arteriole
Direction of blood	Carries blood into the glomerulus	Carries blood away from the glomerulus
Diameter	Wider	About half the diameter of the afferent arteriole
Capillaries supplied	Glomerular capillaries	Peritubular capillaries around the tubules

(f) hypercalcemia and hyperuricemia

Feature	Hypercalcemia	Hyperuricemia
Meaning	Increased calcium in the blood	Increased uric acid in the blood
Urine change	High calcium in urine (hypercalciuria), with excess oxalate, forming calcium oxalate stones	Uric acid crystallises from the urine, forming uric acid stones
Share of kidney stones	About 70% (calcium oxalate type)	About 10% (uric acid type)

(g) extracorporeal shock wave lithotripsy and percutaneous nephrolithotripsy

Feature	ESWL	PCNL
Method	Shock waves are generated outside the body by a lithotripter and break the stone	A tube is inserted through the back into the kidney; a scope and ultrasound break the stone
Stone size	Used for stones that are small enough to break up	Preferred for larger stones
Fragments leave by	Passing spontaneously through the urethra	Being grasped with special equipment and pulled out through the tract

The question prints 'nephronlithotripsy'; the correct term is nephrolithotripsy.

(h) chronic renal failure and acute renal failure

Feature	Chronic renal failure	Acute renal failure
Onset	Gradual and slowly progressive over years	Sudden
Nature	Irreversible deterioration of kidney function	Can improve when the cause is treated
Main causes	Infection of the pelvis, nephritis, high blood pressure, diabetes mellitus	Haemorrhage, vomiting or diarrhoea, diuresis, obstruction such as a kidney stone

(i) peritoneal dialysis and haemodialysis

Feature	Peritoneal dialysis	Haemodialysis
Membrane	The natural peritoneum lining the abdominal cavity	An artificial dialyser of cellophane tubes
Blood outside the body	No; blood stays in the body	Yes; blood is pumped through the machine
Access	Two catheters into the abdominal cavity	A catheter into a blood vessel, usually in the arm

(j) renal cortex and renal medulla

Feature	Renal cortex	Renal medulla
Position	Outer layer of the kidney	Inner part of the kidney, made of renal pyramids
Structures	Renal corpuscles and convoluted tubules; granular in appearance	Loops of Henle and collecting ducts
Function	Where nephrons begin filtration; produces erythropoietin	Concentrates urine through the countercurrent mechanism

(k) vasodilation and vasoconstriction

Feature	Vasodilation	Vasoconstriction
Change in the vessel	Widening of the arterioles	Narrowing of the arterioles
Effect on blood flow	Blood flow through the vessel increases	Blood flow through the vessel decreases
Effect on pressure	Blood pressure in the vessel falls	Blood pressure in the vessel rises

In the kidney, changes in the diameter of the afferent and efferent arterioles change the pressure in the glomerulus and so the rate of filtration.

(l) dialyzer and dialysate

Feature	Dialyzer	Dialysate
What it is	The artificial filter: a network of semipermeable cellophane tubes in the machine	The dialysis fluid that flows around the outside of the tubes
Role	Holds the membrane across which wastes move from the blood	Takes up wastes and minerals from the blood by diffusion
Path	Blood flows through the tubes	Enters from the bottom and leaves from the top of the machine

Extensive answer questionsp. 84

12026 Annual I, Set D, Q.6 OR6 marks

Discuss relationship between excretory products and habitats.

Solution

Principle

Nitrogenous wastes are toxic, so an animal must remove them. The form in which nitrogen is excreted depends on how much water the animal can spare: a toxic and soluble waste needs a lot of water to dilute it, while a less toxic, less soluble waste can be excreted with very little water.

Excretory products and their habitats

Nitrogenous excretory products
Excretory product	Toxicity	Water needed to excrete 1 g of nitrogen	Habitat	Animal group
Ammonia	Highly toxic; raises the pH of body fluids	About 500 ml	Aquatic	Ammonotelic: most freshwater fishes, protozoans, sponges, cnidarians, echinoderms and crustaceans
Urea	About 100,000 times less toxic than ammonia; the animal spends energy converting ammonia into urea	About 50 ml	Aquatic and terrestrial	Ureotelic: mammals (including man), many amphibians and some marine animals such as sharks
Uric acid	Less toxic than urea; it can precipitate from solution	About 1 ml	Terrestrial	Uricotelic: land snails, many insects, reptiles and most birds

Water needed to excrete 1 g of nitrogen: the bars are drawn to scale, so the uric acid bar is almost invisible.

Conclusion

Aquatic animals with plenty of water excrete ammonia, the most toxic product, because it needs the most water and the least energy to make.
Animals that have some water but cannot waste much excrete urea, which is less toxic and needs less water than ammonia.
Land animals that cannot spare water excrete uric acid, which needs almost no water. So the habitat, through water availability, decides the excretory product.

28 marks

Describe the structure and function of human kidney.

Solution

Location and covering

The kidneys are a pair of bean-shaped organs lying just below and behind the liver and against the back wall of the abdomen, on either side of the vertebral column between the twelfth thoracic and third lumbar vertebrae. Each kidney is covered by three layers: the outer renal fascia (a tough connective tissue layer), the perirenal fat capsule and the inner renal capsule, which protects the kidney and holds it in place.

Internal structure

A longitudinal section of the kidney shows three regions. The outer cortex is granular in appearance. The inner medulla is made of cone-shaped renal pyramids, separated by renal columns. The renal pelvis lies in the hilum, the concave opening where the renal artery and renal vein, nerves and the ureter enter and leave.

Figure 4.2: Internal anatomy of the kidney (longitudinal section)

Parts and their functions

Part	Structure	Function
Renal capsule	Connective tissue or fat layers around the kidney	Protects the kidney from injury and anchors it to surrounding tissues
Renal artery	Large vessel that enters at the hilum	Carries blood into the kidney (about 1.2 litres per minute at rest) and controls the blood flow
Renal cortex	Outer, granular layer where the nephrons begin	Filters the blood; produces erythropoietin (EPO), which helps make red blood cells
Renal medulla and pyramids	Inner tissue masses containing most nephron loops and collecting ducts	Carry urine to the renal pelvis; form concentrated urine
Renal pelvis	Funnel-shaped structure in the hilum, with major and minor calyces	Collects urine from the pyramids and passes it into the ureter
Ureter	Tube about 25 cm long	Carries urine from the renal pelvis to the urinary bladder
Renal vein	Vein leaving the hilum	Carries filtered blood out of the kidney to the inferior vena cava

Function

Excretion: filters nitrogenous wastes from the blood and removes them in urine.
Osmoregulation: forms diluted urine when there is a surplus of water and concentrated urine when the body is dehydrated.
Hormone production: makes erythropoietin for red blood cells and renin, an enzyme that helps regulate blood pressure.

32026 Annual I, Set B, Q.38 marks

Describe the structure of human nephron.

Solution

Definition

The nephron is the functional unit of the kidney. Each nephron consists of a renal corpuscle and a renal tubule, and the tubule drains into a collecting duct that is shared with other nephrons.

Parts in order

Renal corpuscle: a knot of capillaries called the glomerulus, enclosed by the double-walled Bowman's capsule at the closed end of the tubule. Blood enters through the afferent arteriole and leaves through the efferent arteriole.
Proximal convoluted tubule: a highly coiled tubule in the cortex that leads from the capsule. It dips toward the renal pelvis.
Loop of Henle: a sharp loop that dips into the medulla. It has a descending limb, which carries filtrate down, and an ascending limb, which carries it back up toward the cortex.
Distal convoluted tubule: a second coiled tubule in the cortex, close to the renal corpuscle of its own nephron, that leads into the collecting duct.
Collecting duct: receives filtrate from many nephrons. Several collecting ducts join to form larger ducts that empty into the renal pelvis.

Figure 4.3: Structure of a nephron and its blood supply

Blood supply to the nephron

The renal artery branches into afferent arterioles that supply the glomerular capillaries. The efferent arteriole is narrower, so pressure builds up in the glomerulus. The efferent arteriole then breaks up into peritubular capillaries around the proximal and distal tubules. A specialised part of these, the vasa recta, runs down into the medulla with the loop of Henle and back toward the cortex. The capillaries drain into the renal vein.

Two types of nephron

Feature	Cortical nephron	Juxtamedullary nephron
Share of nephrons in the human kidney	70 to 80%	20 to 30%
Position of renal corpuscle	Superficial, in the outer cortex	Close to the junction of the cortex and medulla
Loop of Henle	Short	Long, extending deep into the medulla
Main role	Control of blood volume by forming diluted urine	Water retention when water is in short supply

48 marks

Discuss the urine formation and mechanism of urine concentration in man.

Solution

Urine formation

Urine is formed by three processes that take place along the nephron: glomerular filtration, selective reabsorption and tubular secretion.

Figure 4.4: Urine formation along the nephron

Glomerular filtration: the high pressure in the glomerulus forces water and small solutes into Bowman's capsule. Plasma proteins and blood cells stay in the blood.
Selective reabsorption: useful substances such as glucose, amino acids, vitamins, salts and most of the water are taken back into the peritubular capillaries.
Tubular secretion: hydrogen and ammonium ions, drugs and toxins pass from the blood into the tubule. This helps keep the pH of urine between 4.8 and 7.5.

Mechanism of urine concentration

Water is reabsorbed along the whole nephron, but a hypertonic (concentrated) urine can only form if water is drawn out of the collecting duct. This is achieved by the countercurrent multiplier mechanism, which depends on the loop of Henle and the vasa recta.

Figure 4.5: The countercurrent multiplier mechanism

Countercurrent multiplier, step by step

Filtrate flows down the thin descending limb. The surrounding medulla is salty, so water leaves the limb by osmosis and the filtrate becomes more concentrated.
At the bottom of the loop, the filtrate is at its most concentrated. It then flows up the thick ascending limb.
The ascending limb actively pumps sodium chloride out into the medulla. This movement is controlled by aldosterone. The surrounding medulla becomes more concentrated, and water leaves the descending limb more easily.
The vasa recta carries blood in the opposite direction. It brings out the water and salt slowly, so the concentration gradient is kept instead of being washed out.
In the collecting duct, ADH increases the permeability of the walls to water. Water leaves the duct down the gradient, and the urine becomes hypertonic (concentrated).

Summary

The countercurrent system builds a concentration gradient in the medulla, and ADH makes the collecting duct permeable to water. Water therefore leaves the duct, and the urine that reaches the renal pelvis is hypertonic.

58 marks

What are kidney stones? Discuss the causes and treatment of kidney stones?

Solution

Definition

Urinary stones are hard, crystalline mineral masses that stick together to form small pebbles in the kidney or urinary tract. They can be as small as grains of sand or as large as golf balls. The condition of having stones in the kidney is called nephrolithiasis. Branching stones are called staghorn stones.

Types of stone

Calcium oxalate
Calcium phosphate
Magnesium ammonium phosphate
Uric acid
Cystine

Causes

Cause	Effect in the urine or blood	Type of stone formed
Hypercalcemia (high calcium in the blood)	Hypercalciuria (high calcium in the urine)	Calcium oxalate, the most common type (about 70%)
Hyperoxaluria (high oxalate in the urine)	Oxalate combines with calcium	Calcium oxalate
Hyperuricemia (high uric acid in the blood)	Uric acid is excreted in excess	Uric acid (about 10%)
High concentration of cysteine or phosphates	Crystals form in the urine	Cystine or calcium phosphate
Continuous dehydration	Urine is concentrated, so crystals form more easily	Any type

The urine test helps to predict the type. Acidic urine (low pH) suggests calcium oxalate, alkaline urine (high pH) suggests calcium phosphate, and persistently acidic urine suggests the uric acid type. Common symptoms include severe pain in the lower back, blood in the urine, nausea, vomiting, fever and chills, or foul-smelling or cloudy urine.

Treatment

Two methods are common. In extracorporeal shock wave lithotripsy (ESWL), shock waves are generated outside the body by a lithotripter and break the stone into small pieces, which mostly pass out through the urethra. For a larger stone, percutaneous nephrolithotripsy (PCNL) is used: a tube is inserted through the back into the kidney, a scope shows the stone, and ultrasound breaks it up. The fragments are then removed through the tract. Open surgery is now almost never needed, except for a large bladder stone.

Treatment of kidney stones by size of stone

ESWL: no cut is made; the fragments pass out in the urine.
PCNL: a tube is made into the kidney, so the stone is seen directly and removed in pieces.

68 marks

What is renal dialysis? Describe the two types of renal dialysis.

Solution

Definition

Renal dialysis is an artificial method of filtering toxins, excess water and waste from the blood when the kidneys can no longer do this. It works on the principle that the kidneys filter blood, but dialysis is less effective, efficient and thorough than the natural process. Substances move across a semipermeable membrane by diffusion, from the blood into a fluid called dialysate.

Haemodialysis

Haemodialysis removes wastes and water by circulating blood outside the body through an external filter called a dialyser.

A catheter is inserted into a blood vessel, usually in the arm, and the blood is routed out of the body.
A blood pump moves the blood into the dialyser, which consists of tubes of cellophane (semipermeable) membrane.
Dialysate flows around the outside of the tubes in the opposite direction, so wastes, minerals and electrolytes diffuse from the blood into the dialysate.
The cleaned blood leaves the dialyser and returns to the body through a second catheter.

Figure 4.8: Haemodialysis

Peritoneal dialysis

Peritoneal dialysis uses a natural membrane in the body, the peritoneum, which encloses the abdominal cavity.

Two catheters are surgically inserted into the abdominal cavity. Dialysate enters and leaves through them.
The dialysate is left in the cavity for a time. The blood capillaries in the peritoneum are close to the dialysate.
Wastes and excess salts diffuse from the blood through the peritoneum into the dialysate, which is then drained out with the waste.

Figure 4.9: Peritoneal dialysis

Comparison

Feature	Haemodialysis	Peritoneal dialysis
Membrane	Artificial cellophane tubes in the dialyser	Natural peritoneum
Blood outside the body	Yes, pumped through the machine	No, blood stays in the body
Access	Catheter into a blood vessel	Two catheters into the abdominal cavity

78 marks

What is kidney transplantation? Describe principles of kidney transplant. What are the problems associated with kidney transplant?

Solution

Definition

Kidney transplantation is the surgical procedure of placing a fully functioning kidney into a person with chronic kidney failure.

Principles

The graft (donor kidney) is taken from a deceased donor, or from a related or unrelated living person.
ABO blood group compatibility between donor and recipient is essential.
Donor kidneys are selected on the basis of human leucocyte antigen (HLA) matching, which improves graft survival.
A person can live normally with only one kidney, so a living donor loses no health in the long term.

Steps and risks of kidney transplant

Problems

Problem	Cause	Management
Rejection	The recipient's immune system attacks the graft as foreign	Medicines such as cyclosporine suppress the immune system
Toxic effects of cyclosporine	The medicine itself harms the body	Extra doses of steroids are given at the same time

Patients must take these medicines for life. If a recipient stops taking them, rejection can occur, even ten or fifteen years after the transplant.

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 91 mark

Tubular secretion involves the movement of substances from:

AUreter to urethra
BFiltrate to blood
CBlood to filtrateAnswer
DKidney to bladder

Why

Tubular secretion is the addition of substances such as ammonium and hydrogen ions from the blood and the cells of the tubule into the filtrate, which helps keep the pH of urine at 4.8 to 7.5. The movement is therefore from blood to filtrate. Option B is the opposite process, selective reabsorption (filtrate to blood). Options A and D name organs of the urinary tract that urine passes through, not a movement of substances between blood and tubule.

2026 Annual I, Set B, Q.2 (i)3 marks

What is meant by Pyelonephritis?

Solution

Pyelonephritis is the type of urinary tract infection (UTI) in which the kidneys are infected. The book names the three types of UTI by the site of infection.

Type of UTI	Site of infection
Urethritis	Urethra
Cystitis	Urinary bladder
Pyelonephritis	Kidneys

It is caused by bacteria (for example E. coli, the commonest cause of UTI), which usually pass from the faecal matter around the anal opening to the opening of the urethra and travel upwards.
It is more common in women, because the female urethra is short (1.5 inches compared with 8 inches in men).
Because it is bacterial, it is cured by antibiotic therapy. Drinking plenty of water and personal hygiene help to prevent it.

2026 Annual I, Set B, Q.2 (ii)1x3 = 3 marks

Enlist any three problems associated with kidney transplant.

Solution

A kidney transplant places a healthy kidney into a person with chronic kidney failure. The book gives these problems (any three, 1 mark each):

Rejection. The recipient's immune system recognises the new kidney as foreign and attacks it.
Toxic effects of cyclosporine. The drug that is given to prevent rejection is itself toxic to the patient.
Lifelong medication. Drugs such as cyclosporine must be taken to suppress the immune system. If the patient stops them at any time, rejection can occur even ten or fifteen years after the transplant. Rejection and the toxic effects of the drug are treated together by adding extra doses of steroids.

2026 Annual I, Set B, Q.2 (xi)1+2 = 3 marks

What is meant by kidney failure? Enlist any four causes of chronic renal failure.

Solution

Kidney failure (1 mark)

Kidney failure (renal failure) is a decline in kidney function, particularly in the efficiency of the filtering process. Chronic renal failure is the irreversible deterioration of renal function; it is gradual and occurs over a period of years.

Four causes of chronic renal failure (2 marks)

Bacterial infection of the pelvis and the surrounding tissue of the kidney.
Nephritis, inflammation of the glomeruli.
Damage due to high blood pressure.
Diabetes mellitus.

2026 Annual I, Set B, Q.34+3 = 7 marks

Explain the structure of nephron. Make diagram as well.

Solution

Structure of the nephron (4 marks)

The nephron is the functional unit of the kidney. Each nephron has a renal corpuscle and a renal tubule, and the tubule drains into a collecting duct that receives many nephrons.

Renal corpuscle: a knot of capillaries called the glomerulus, surrounded by the thin, double-walled Bowman's capsule at the closed end of the tubule. Blood enters by the afferent arteriole and leaves by the efferent arteriole, which is half its diameter, so the pressure in the glomerulus is high.
Proximal convoluted tubule: the highly coiled part that leads away from the capsule and dips towards the renal pelvis.
Loop of Henle: a sharp loop into the medulla, with a descending limb and an ascending limb.
Distal convoluted tubule: the ascending limb returns to the region of the renal corpuscle and becomes coiled again, then joins the collecting duct.
Collecting duct: receives many nephrons; many collecting ducts join to form larger ducts that empty into the renal pelvis.

Blood supply: the afferent arteriole supplies the glomerulus; the efferent arteriole forms the peritubular capillaries around the tubules, including the vasa recta that run with the loop of Henle; these drain into the renal vein.

Labelled diagram (3 marks)

Figure 4.3: Structure of a nephron and its blood supply

2026 Annual I, Set D, MCQ 101 mark

Glomerular filtration occurs in the:

ACollecting duct
BUreter
CLoop of Henle
DBowman's capsuleAnswer

Why

Glomerular filtration takes place in the renal (Bowman's) capsule: the high pressure in the glomerular capillaries forces water and small solutes into the Bowman's capsule, where they collect as filtrate. The loop of Henle and the collecting duct are tempting because water is reabsorbed there, but that is reabsorption, not filtration. The ureter only carries urine from the kidney to the bladder.

2026 Annual I, Set D, Q.2 (i)1x3 = 3 marks

Make a comparison between functions of Glomerular capillaries and peritubular capillaries. (Three points)

Solution

Glomerular capillaries filter; peritubular capillaries reabsorb
Point (1 mark each)	Glomerular capillaries	Peritubular capillaries
Main function	Act as the filtration unit of the kidney: water and small solutes leave the blood under pressure.	Reabsorb nutrients and minerals the body needs, together with excess water, from the tubule.
What happens to proteins and blood cells	Plasma proteins are retained in the blood; blood cells and proteins stay in the capillary and are not filtered.	Nutrients and minerals are taken back into the blood; the blood then drains to the renal vein.
Position and pressure	Between the afferent and efferent arterioles inside Bowman's capsule; exceptionally high blood pressure.	Arise from the efferent arteriole and surround the proximal and distal tubules (the vasa recta run with the loops of Henle); lower pressure.

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

Identify the parts A, B, and C in the diagram and write one function of each.

Solution

The diagram is a nephron: A is at the closed end of the tubule, B is on the first part of the tubule that leads away from it, and C is at the long loop that dips down.

The nephron with the parts A, B and C marked: A is the glomerulus inside Bowman's capsule, B the proximal convoluted tubule and C the loop of Henle.

Part	Name	One function
A	Renal corpuscle (glomerulus inside Bowman's capsule)	Filtration: under high hydrostatic pressure, water and small solutes pass from the glomerular capillaries into Bowman's capsule to form the filtrate.
B	Proximal convoluted tubule	Selective reabsorption of water, ions and all organic nutrients (glucose, amino acids) back into the blood.
C	Loop of Henle	Reabsorbs water from the descending limb and sodium and chloride ions from the ascending limb, which concentrates the urine.

Note. The parts are named as in the book's nephron figure (Figure 4.3): A at the capsule, B on the proximal tubule near it and C at the loop of Henle.

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

Briefly explain the significance of selective re-absorption during urine formation.

Solution

Selective reabsorption is the process by which certain useful substances that were filtered out of the blood at the glomerulus (glucose, amino acids, vitamins, inorganic salts and some water) are taken back into the blood. Its significance (three points):

Saves useful substances. About all of the glucose and amino acids filtered are returned to the blood, so they are not lost in urine. That is why normal urine contains no glucose.
Conserves water. Most of the filtered water is reabsorbed (about 99.5% overall), which prevents dehydration and keeps the blood volume steady.
Keeps the body fluids balanced. Because only certain substances are reabsorbed, the wastes and excess salts stay in the tubule and leave in urine, so the composition of the blood is kept normal.

2026 Annual I, Set D, Q.6 OR2x3 = 6 marks

How ammonotelic, ureotelic and uricotelic animals manage the excretion of waste materials in their habitat? Elaborate.

Solution

The form of nitrogenous waste depends on how much water the animal can spare. Ammonia needs the most water, urea less, and uric acid the least.

Ammonotelic animals (2 marks)

Excrete ammonia, which is highly toxic (it raises the pH of body fluids and interferes with membrane transport) but very soluble and diffuses rapidly across cell membranes.
One gram of nitrogen needs about 500 mL of water to dilute it, so it suits aquatic animals, especially in fresh water: most fishes, protozoans, sponges, cnidaria, echinoderms and crustaceans.

Ureotelic animals (2 marks)

Convert ammonia into urea at a cost of energy. Urea is about 100,000 times less toxic than ammonia and needs only about 50 mL of water per gram of nitrogen.
They are some marine animals and all terrestrial ones with less fresh water: mammals including man, many amphibians and some marine animals such as sharks.

Uricotelic animals (2 marks)

Excrete uric acid, the least toxic. It can precipitate from solution, carries four nitrogen atoms per molecule and needs only about 1 mL of water per gram of nitrogen.
They live where water is very short: terrestrial invertebrates (land snail, many insects) and egg-laying vertebrates (reptiles such as snakes and lizards, and birds).

Feature	Ammonotelic	Ureotelic	Uricotelic
Waste	Ammonia	Urea	Uric acid
Water per g of nitrogen	500 mL	50 mL	1 mL
Habitat	Aquatic	Aquatic and terrestrial	Terrestrial

Exam focus

Explain the structure of the nephron and draw a labelled diagram (with the blood supply). (long)
Set B Q.3 was a 7-mark question on exactly this (4+3 marks); the book has it as long question 3 and short question 4. Set D Q.2 (ii) OR also asked to label a nephron.
Go to answer
Identify the labelled parts of a nephron diagram and give one function of each. (short)
Set D Q.2 (ii) OR (3 marks): the parts were the renal corpuscle, the proximal convoluted tubule and the loop of Henle.
Go to answer
How ammonotelic, ureotelic and uricotelic animals excrete their waste in their habitats. (long)
Set D Q.6 OR (2x3 marks); the book asks it as long question 1 and as differences in short question 13 (b) and (c).
Go to answer
Urine formation: glomerular filtration, selective reabsorption and tubular secretion, and the significance of each. (short)
Set D Q.2 (xi) OR asked the significance of selective reabsorption and MCQs on tubular secretion (Set B MCQ 9) and filtration (Set D MCQ 10) came in both sets.
Go to answer
Compare the glomerular capillaries and the peritubular capillaries (three points). (short)
Set D Q.2 (i) (1x3 marks); the book's Table 4.1 gives exactly this comparison.
Go to answer
Kidney failure and its causes, pyelonephritis and kidney transplant problems. (short)
Set B asked all three: Q.2 (xi) kidney failure (1+2), Q.2 (i) pyelonephritis and Q.2 (ii) problems of kidney transplant (3 marks each).
Go to answer
Discuss urine formation and the countercurrent multiplier mechanism that concentrates the urine. (long)
Not asked in 2026 as a separate question, but the book has it as long question 4 and short question 3, and the 2026 urine-formation questions depend on it.
Go to answer
Describe the two types of renal dialysis (haemodialysis and peritoneal dialysis) and the working of the dialysis machine. (long)
No 2026 paper question; the book gives it as long question 6 and short questions 9 to 11, the standard 8-mark long question after kidney failure.
Go to answer