Endocrine system of man: Biology unit 6 notes (FBISE class 12)

Chemical coordination

The cellular functions of the body need to be continuously regulated. The nerve fibres do not innervate all the cells of the body, so a special kind of coordination system is required. The endocrine system serves the role of coordinating most body cells.

The hormonal system is concerned with the control of the different metabolic functions of the body: the rate of chemical reactions, the transport of substances through cell membranes, growth, and secretions. This coordination is called chemical coordination.

Chemical coordination: Control of the metabolic functions of the body (rate of chemical reactions, transport through cell membranes, growth and secretions) by the endocrine (hormonal) system.

Nerve fibres do not reach all the cells of the body, but hormones carried by the blood reach every cell, so a special coordination system is required.

6.1HormonesOften examined

Hormones are organic chemicals secreted by a cell. They coordinate different functions in the body by carrying messages through the blood to various organs and tissues. Scientists have identified over 50 hormones in the human body so far.

Hormones control many different bodily processes, such as:

metabolism
homeostasis (constant internal balance), such as blood pressure and blood sugar regulation, fluid (water) and electrolyte balance, and body temperature
growth and development
sexual function and reproduction
the sleep-wake cycle and mood

Hormones can stimulate changes in target cells even though they are present in extremely small concentrations. Chemically, most hormones are either amines, peptides, proteins or glycoproteins, and steroids.

6.1.1Hormone as a chemical messengerOften examined

Hormone: A small soluble organic molecule that is effective in low concentration. It is essentially a chemical messenger that transports a signal from one cell to another.
Target: The site where specific receptors for the hormone are present. The hormone has its effect only there, hence it is termed a messenger.

Hormones are chemical messengers that affect and manage hundreds of processes occurring in the body. Often, a bodily process involves a chain reaction of several different hormones.

A hormone will only act on a part of the body if the cells in the target tissue have receptors that receive the message of the hormone. Its action is like the lock and key model of enzyme action. Think of a hormone as a key and the cells of its target tissue as specially shaped locks. If the hormone fits the lock (receptor) on the cell membrane, then it will work, and the hormone will deliver a message that causes the target site to take a specific action.

The lock and key idea: a hormone (key) acts only on cells whose receptors (locks) fit it. Cells without the matching receptor ignore it.

Two types of communication

The body uses hormones for two types of communication.

Between two endocrine glands. One gland releases a hormone which stimulates another gland to change the levels of hormones it is releasing. Example: the pituitary gland releases thyroid-stimulating hormone (TSH), which triggers the thyroid gland to release its hormones, and these affect various aspects of the body.
Between an endocrine gland and a target organ. Example: the pancreas releases insulin, which then acts on the muscles and liver to help use glucose.

The two types of hormonal communication: between two endocrine glands (a) and between an endocrine gland and a target organ (b).

6.1.2Chemical nature of hormonesOften examined

All hormones are organic substances of varying structural complexity. Chemically, they belong to one of the following categories.

Category	What it includes	Examples and where they are made
(a) Steroid hormones	Hormones composed of steroids	Cortisone, aldosterone, testosterone, estrogen and progesterone, secreted by the adrenal cortex, testes, ovaries and placenta
(b) Proteinaceous hormones	Also known as polypeptide or peptide hormones	Somatotrophic, thyrotrophic and gonadotrophic hormones from the anterior lobe of the pituitary gland; insulin from the pancreas
(c) Catecholamines	Derivatives secreted by the adrenal medulla	Adrenaline and noradrenaline
(d) Amino acid derivative	Made from an amino acid	Thyroxine from the thyroid gland
(e) Peptide hormones	A group of smaller peptides	Melanocyte-stimulating hormone, oxytocin, vasopressin, adrenocorticotrophic hormone, calcitonin and parathormone

The five chemical groups of hormones, with examples from the book.

6.2Endocrine system of manOften examined

The endocrine system is the type of glandular system that consists of some 20 ductless glands lying in different parts of the body. Some of the major endocrine glands, their locations and their hormonal secretions are shown in Figure 6.1.

Figure 6.1: Major endocrine glands and their locations in the human body. The accent dot marks the place each label points to.

Feature	Endocrine gland	Exocrine gland
Ducts	Ductless: the secretion goes straight into the blood	Has ducts or openings that carry the secretion to a surface or cavity
Secretion	Hormones	Sweat, saliva, tears, breast milk, digestive juices
Where it acts	At distant target cells reached through the blood	On a body surface or in a body cavity

6.2.1Pituitary glandOften examined

The pituitary gland is located just below the hypothalamus. It is attached to the hypothalamus by a stalk called the infundibulum, which is composed of blood vessels and the fibres of neurosecretory cells. The pituitary gland is divided into three lobes: the anterior, the posterior and the median.

The pituitary gland hangs below the hypothalamus on the infundibulum and has three lobes: anterior, median (accent) and posterior.

Lobe	Hormones	Nature of the lobe
Anterior lobe	Growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinising hormone (LH), prolactin	Glandular: it makes six hormones that regulate other endocrine glands
Median lobe	Melanocyte-stimulating hormone (MSH)	A thin layer of cells between the anterior and posterior lobes, not very prominent in humans
Posterior lobe	Stores antidiuretic hormone (ADH) and oxytocin	Not glandular by itself: it is made up of axons of neurosecretory cells of the hypothalamus

6.2.1Anterior lobe of pituitaryOften examined

Classically, the anterior pituitary is considered the master gland of the endocrine system because it secretes numerous hormones, many of which regulate the activity of other endocrine glands. It secretes six hormones, all of which regulate the secretory action of other endocrine glands.

Growth hormone (GH) or somatotrophic hormone (STH)

Control: it is released under the influence of the hypothalamic growth hormone releasing factor (GHRF) and is inhibited by the hypothalamic somatostatin.
Action: GH has a direct effect on growth and development. It stimulates cell growth and cell division, stimulates the uptake of amino acids into cells and increases the rate of protein synthesis.

Condition	Cause	What happens
Dwarfism	Deficiency of GH (in childhood)	Development is much slower and the person has short stature. The body parts stay in proportion, and brain development and IQ are unaffected.
Gigantism	Oversecretion of GH during childhood	The bones are still capable of growth, so the person increases in height abnormally.
Acromegaly	Oversecretion of GH in adult life	The bones are no longer capable of increasing in length, but grow in thickness. The hands, feet, skull, nose and jawbone enlarge.

The three growth hormone disorders: the condition depends on how much GH is made and on whether the bones can still lengthen.

Worked example: reading a growth disorder from its symptoms

Patient A is 17 years old. He is much shorter than average, but his arms, legs and trunk are in proportion and his intelligence is normal. Patient B is 40 years old. He did not grow taller, but his hands, feet, nose and jawbone have become larger. Name the hormone problem and the condition of each patient and explain why they differ although both involve the same hormone.

Both patients show a problem in the growth hormone (GH) made by the anterior pituitary, since GH controls growth and development.
Patient A: slow development, short stature, normal proportions and normal brain development and IQ are the signs of deficiency of GH in childhood, which is dwarfism.
Patient B: bones that cannot lengthen any more but grow thicker, with enlarged hands, feet, skull, nose and jaw, are the signs of oversecretion of GH in adult life, which is acromegaly.
The difference is the timing. In a child the bones can still grow in length, so too much GH gives gigantism (abnormal height). In an adult the bones can no longer grow in length, so too much GH only thickens them.

Feature	Dwarfism	Gigantism	Acromegaly
GH level	Too low	Too high	Too high
Age when it starts	Childhood	Childhood	Adult
Main result	Short, proportionate body	Abnormal height	Thick bones of hands, feet, skull, nose, jaw

A: dwarfism (GH deficiency); B: acromegaly (GH excess in an adult)

Thyroid-stimulating hormone (TSH)

Control: thyrotrophin releasing factor (TRF) from the hypothalamus stimulates the synthesis and release of TSH from the anterior pituitary.
Action: TSH regulates the endocrine function of the thyroid gland. It increases the number of cells and the secretory activity of the thyroid gland.
Imbalance: oversecretion of TSH causes hyperthyroidism (excess thyroxine); undersecretion causes hypothyroidism (lack of thyroxine).

Adrenocorticotropic hormone (ACTH)

Control: corticotrophin releasing factor (CRF) from the hypothalamus causes the release of ACTH. The release of CRF is controlled by the steroid level in the blood and by direct nervous stimulation of the hypothalamus as a result of stress, for example cold, heat, pain, fright and infections.
Action: ACTH acts on the adrenal cortex and stimulates the secretion of corticosteroids (cortisone and aldosterone).

Gonadotropic hormones and prolactin

Follicle-stimulating hormone (FSH), luteinising hormone (LH) (also called interstitial cell stimulating hormone, ICSH, in the male) and prolactin or luteotrophic hormone (LTH) are collectively known as the gonadotropic hormones. These hormones act upon the reproductive system and regulate its function.

Hormone	Released by	Target	Main effect
GH	Anterior lobe (GHRF stimulates, somatostatin inhibits)	Cells, bones	Growth, cell division, amino acid uptake, protein synthesis
TSH	Anterior lobe (TRF stimulates)	Thyroid gland	Makes the thyroid secrete thyroxine
ACTH	Anterior lobe (CRF stimulates)	Adrenal cortex	Secretion of cortisone and aldosterone
FSH	Anterior lobe	Gonads	Regulates the reproductive system (ovary: estrogen from the Graafian follicle)
LH (ICSH in male)	Anterior lobe	Gonads	Regulates the reproductive system (testes: testosterone from the interstitial cells)
Prolactin (LTH)	Anterior lobe	Mammary glands	Starts and keeps up milk production (Table 6.1)

6.2.1Median lobe of pituitary

In humans, the median lobe of the pituitary is not very prominent. It is a thin layer of cells between the anterior and the posterior pituitary.

It produces melanocyte stimulating hormone (MSH). The secretion of MSH is regulated by the hypothalamic MSH inhibitory hormone. MSH increases in humans during pregnancy. It stimulates the production and release of melanin by the melanocytes in the skin and hair.

Hormone	Lobe	Controlled by	Target	Action
MSH	Median	Hypothalamic MSH inhibitory hormone	Melanocytes of skin and hair	Production and release of melanin (skin pigmentation); Table 6.1 adds appetite and sex drive

6.2.1Posterior lobe of pituitaryOften examined

The posterior pituitary is not glandular by itself. It does not synthesise any hormone. It is largely made up of the axons of the neurosecretory cells of the hypothalamus.

Figure 6.2: Role of pituitary hormones. Each arrow is labelled with the hormone that goes to that target.

The posterior pituitary stores antidiuretic hormone (ADH or vasopressin) and oxytocin. These hormones are released in response to nerve impulses from the hypothalamus.

Antidiuretic hormone (ADH)

ADH is released during the state of dehydration, decreased blood volume and low blood pressure.
It affects water retention in the kidneys and controls blood pressure (Table 6.1).
Under secretion of ADH causes diabetes insipidus, which is characterised by excessive production of diluted urine and frequent thirst. Over time it may lead to kidney problems.

Oxytocin

Childbirth: oxytocin is released during childbirth in waves, and this results in labour contractions. Over secretion causes rupturing of the uterine wall, while under secretion inhibits the normal labour process.
Nursing women: in lactating women, suckling causes the release of oxytocin. During feeding it causes the dilation of the milk ducts of the mother's mammary glands and so promotes milk ejection.

Feature	ADH (vasopressin)	Oxytocin
Stored in	Posterior pituitary	Posterior pituitary
Released when	Dehydration, decreased blood volume, low blood pressure	Childbirth and suckling by the baby
Target	Kidneys	Uterus and milk ducts of the breast
Effect	Water retention in the kidneys, rise in blood pressure	Uterine contractions and milk ejection
Imbalance	Under secretion: diabetes insipidus	Over secretion: rupture of the uterine wall; under secretion: normal labour is inhibited

6.2.2Thyroid glandOften examined

The thyroid gland is composed of two lobes which are located on either side of the trachea, inferior to the larynx. It produces three active hormones: tri-iodothyronine (T3), tetra-iodothyronine (T4) or thyroxine, and calcitonin.

Figure 6.3: Thyroid and parathyroid gland. (a) Front view. (b) Back view, showing the four parathyroid glands (accent) on the posterior surface of the thyroid.

T3 and T4

These are iodine containing hormones. Tri-iodothyronine contains three iodine atoms in its structure and thyroxine contains four, hence the names T3 and T4. TSH from the anterior pituitary stimulates the production and release of these hormones.

These hormones show a variety of physiological effects:

They promote the basal metabolic rate of the body.
They enhance glucose catabolism and the synthesis of cholesterol in the liver.
They promote the development of the nervous system in the foetus and infants.
They act on muscles for their development and functioning.
They promote growth and maturation of the skeleton.
They promote normal motility of the gastrointestinal tract.

Hyperthyroidism

Hyperthyroidism is the term applied to excess of these hormones. Over secretion of T3 and T4 causes Graves' disease.

Worked example: identify the thyroid condition from the symptoms

A woman has a fast and irregular heartbeat, a high body temperature, sweating and weight loss although she eats well. Her eyeballs protrude. Tests show abnormal antibodies in her blood that act like TSH. Name the condition and the hormone involved, and explain how each symptom is related to the hormone.

The antibodies mimic TSH, so the thyroid gland is stimulated continuously. This is an autoimmune cause.
The continuous stimulation makes the thyroid secrete too much T3 and T4. Over secretion of T3 and T4 is hyperthyroidism, and the condition is Graves' disease.
T3 and T4 promote the basal metabolic rate, so the excess raises the metabolic rate. The symptoms follow from this high metabolic rate (table below).

Symptom	Link to the excess thyroid hormones
Weight loss despite adequate food	The raised basal metabolic rate burns the food faster
High body temperature and sweating	More heat is released by the raised metabolism
Rapid, irregular heartbeat and faster breathing	Typical symptoms the book lists for Graves' disease with the high metabolic rate
Protruding eyeballs	Exophthalmia, a classic symptom of hyperthyroidism

Graves' disease (hyperthyroidism): too much T3 and T4 because antibodies mimic TSH

Hypothyroidism

Hypothyroidism is the under secretion of thyroxine.

In adults, the full-blown hypothyroid syndrome is called myxedema. It is characterised by a low metabolic rate, feeling chilled, puffy eyes, thick and dry skin with hair lost from the scalp and eyebrows, oedema, tongue swelling, constipation, and an enlarged thyroid gland, the goitre. Myxedema may result from a deficiency of iodine in the diet.
In infants, congenital under secretion results in a severe hypothyroidism called cretinism. It is characterised by mental retardation with poor physical growth and disproportionate body size. Bone maturation and puberty are severely delayed and infertility is common.

Feature	Hyperthyroidism (Graves' disease)	Hypothyroidism (myxedema, cretinism)
Thyroxine	Over secretion	Under secretion
Metabolic rate	High	Low
Body temperature	Increased, with sweating	Feeling chilled
Other signs	Rapid, irregular heartbeat, exophthalmia	Puffy eyes, dry skin, hair loss, goitre, constipation

Calcitonin

It is produced by the C-cells of the thyroid gland.
Excessive calcium level in the blood stimulates the release of calcitonin; declining blood calcium inhibits its secretion.
It increases the deposition of calcium in the bone matrix.
It inhibits the absorption of calcium by the intestine and decreases its reabsorption by the kidney tubules, allowing its excretion in the urine.
It appears more important in childhood, when the skeleton grows quickly and the bones are changing dramatically in mass, size and shape.

If calcitonin is deficient, calcium is not deposited in the bones. The high blood calcium level then disturbs the functioning of muscles and the nervous system and may lead to kidney stones.

6.2.3Parathyroid glandsOften examined

In humans there are four parathyroid glands. All four glands are located on the thyroid gland. They are small, light coloured masses that stick out from the posterior surface of the thyroid gland (Figure 6.3 b).

Parathormone (parathyroid hormone, PTH) is the single most important hormone controlling the calcium balance of the blood. Its release is triggered by low blood calcium levels and inhibited by high blood calcium levels. Parathormone works antagonistically to calcitonin.

Calcitonin and parathormone work against each other (antagonistically) to keep the blood calcium level steady. The hormone-releasing steps are drawn in accent.

Imbalance	Cause	Effects
Over secretion of parathormone	Usually a parathyroid gland tumour	Calcium is released from the bones, so the bones deform, soften and tend to fracture spontaneously. Blood calcium rises (hypercalcemia), which depresses the nervous system and causes weakness of muscles. Excess calcium salts precipitate in the kidneys and cause stone formation.
Under secretion of parathormone	Too little hormone	Blood calcium falls (hypocalcemia). This increases the excitability of neurons and can lead to tetany, in which the muscles remain in a contracted state. If untreated, it can be fatal.

Worked example: predict the effects of losing the parathyroid glands

During an operation on the neck, the four parathyroid glands of a patient are damaged. Predict what happens to the blood calcium, the nerves and the muscles of the patient, and compare it with a patient whose parathyroid glands over secrete.

The glands that release parathormone are lost. Parathormone is the most important hormone that controls calcium balance and is released when blood calcium is low.
Without it, blood calcium cannot be raised. The blood calcium level falls: hypocalcemia.
Low blood calcium increases the excitability of neurons.
The muscles remain in a contracted state: tetany. If it is not treated it can be fatal.

Feature	Parathormone under secretion	Parathormone over secretion
Blood calcium	Falls (hypocalcemia)	Rises (hypercalcemia)
Bones	Not mentioned in the book	Release calcium, soften, deform and fracture
Nerves and muscles	Neurons more excitable; tetany	Nervous system depressed; muscle weakness
Kidneys	Not mentioned	Calcium salts precipitate: stones

Hypocalcemia and tetany (opposite of the over secretion picture)

6.2.4PancreasOften examined

The pancreas is composed of two types of tissue.

Exocrine tissue produces and secretes digestive juice.
Endocrine tissue is distributed in the form of patches in the pancreas. These patches are called the islets of Langerhans. They secrete two hormones, insulin and glucagon.

Figure 6.4: Pancreas and islets of Langerhans. The islet cells (alpha in accent, beta larger and paler) release insulin and glucagon directly into the blood capillary.

	Insulin	Glucagon
Secreted by	Beta (B) cells, which are larger in number	Alpha (a) cells, which are fewer in number
Released when	Blood glucose level rises, for example right after a meal	Blood glucose level is low; the sympathetic nervous system also stimulates its release
Suppressed by	Not given in the book	High blood glucose, insulin and somatostatin
Effect on blood glucose	Reduces it to the normal level	Increases it
How	Increases glucose uptake by most body cells, especially skeletal muscle and fat cells; promotes glycogenesis (glucose to glycogen); increases the use of glucose in cellular respiration; promotes conversion of excess glucose to fats; inhibits gluconeogenesis (glucose synthesis)	Acts antagonistically to insulin and so reverses the activities performed by insulin
Imbalance	Under secretion: diabetes mellitus. Over secretion: glucose utilisation is too great and blood glucose falls, which upsets nerve and muscle functioning	Not given in the book

Both kinds of cell respond directly to the level of blood glucose.

Under secretion of insulin leads to the metabolic disease known as diabetes mellitus, which is characterised by a high glucose level in the blood and urine. If excess insulin is produced, the utilisation of glucose is too great and its level falls in the blood, which upsets nerve and muscle functioning.

Worked example: identify the parts of an islet of Langerhans

The figure shows an islet of Langerhans and the tissue beside it. Name the parts A to E and say which hormone each of the cells A and B releases.

A points to a large cell in the islet. The beta cells are the larger and more numerous cells, so A is a beta cell. It releases insulin.
B points to a smaller cell. The alpha cells are fewer and smaller, so B is an alpha cell. It releases glucagon.
C is the tube that runs through the islet. The islet cells release their hormones straight into it, so it is a blood capillary.
D is the group of rings beside the islet. These produce digestive juice and belong to the exocrine tissue: acinar cells (acini).
E is the narrow tube that leaves the acinar cells and carries the digestive juice away: the pancreatic duct.

A b e t a ce l l (in s u l in); B a l p ha ce l l (g l u c a g o n); C b l oo d c a p i l l a r y; D a c ina r ce l l s; E p an cr e a t i c d u c t

6.2.5Adrenal glandsOften examined

Each of the two adrenal glands rests on a kidney. Each adrenal gland is composed of an inner portion called the medulla and an outer portion, the cortex.

Figure 6.5: Adrenal gland. The gland rests on the kidney. In section it has an outer capsule, a cortex and an inner medulla (accent).

Adrenal medulla: epinephrine and norepinephrine

Epinephrine (adrenaline) and norepinephrine are produced by the adrenal medulla. Both are released during the state of emergency under the influence of the sympathetic nervous system. Both are involved in the body's immediate response to stress. The two hormones exert the same effects in different ways, that is, a synergistic effect.

	Epinephrine	Norepinephrine
Strength	The more potent stimulator of metabolic activities	Greater influence on peripheral vasoconstriction
Effects	Stimulates metabolic activities, bronchial dilation, and increased blood flow to the skeletal muscles and the heart	Narrowing (constriction) of the peripheral blood vessels
Net effect of both	Rise in blood pressure	Rise in blood pressure

Over secretion of these hormones may cause hypertension and aggressive behaviour during routine life.
Under secretion causes failure to combat an emergency situation.

Adrenal cortex

The two major types of hormones produced by the adrenal cortex are the glucocorticoids and the mineralocorticoids. Both are produced under the influence of ACTH.

Type	Example	Function
Glucocorticoids	Cortisone	Help to regulate the blood glucose level
Mineralocorticoids	Aldosterone	Help to regulate the level of minerals in the blood (salt and water balance)

Disease	Cause	Features
Addison's disease	Under secretion of cortical hormones	General metabolic disturbance, in particular weakness of muscle action and loss of salts. A stress situation such as cold may lead to collapse and death.
Cushing's disease	Over secretion of cortical hormones	Excessive protein breakdown resulting in muscular and bone weakness.

Another hormone, androgen (testosterone), is also produced from the adrenal cortex in small amounts in both male and female bodies. Its major site of secretion is the testes, which are the male gonads.

6.2.6GonadsOften examined

Gonads are a special type of endocrine glands which, besides hormone secretion, also produce gametes. The female gonads are the ovaries; the male gonads are the testes.

Hormonal control of the gonads as the book describes it. Dashed arrows are the inhibiting (negative feedback) effects on the pituitary.

Ovaries: estrogen and progesterone

The ovaries secrete the female sex hormones estrogen and progesterone.

	Estrogen	Progesterone
Made by	Graafian follicle under the stimulation of FSH	Corpus luteum in response to LH during the normal menstrual cycle; placenta during pregnancy
When / control	Secreted at puberty. Estrogen has negative feedback upon FSH	Inhibits further FSH secretion from the pituitary, so no more follicles ripen
Functions	Responsible for the secondary sex characteristics in females; aids in the healing and repair of the uterine wall after menstruation	Causes further thickening and vascularisation of the uterus wall to maintain the state of pregnancy; suppresses ovulation
Under secretion	In young females they fail to mature sexually; in adults it leads to sterility	During the menstrual cycle it decreases the chance of pregnancy and may cause early menstruation; during pregnancy it may lead to miscarriage
Over secretion	May lead to fibroids (abnormal growth) in the uterus and polycystic ovaries	Not given in the book

Testes: testosterone

The testes produce sperm and the male sex hormone testosterone, which is secreted from the interstitial cells among the seminiferous tubules under the influence of ICSH (LH).

During puberty, testosterone initiates the maturation of the male reproductive organs and the appearance of secondary sex characteristics and sex drive.
It is necessary for normal sperm production and maintains the reproductive organs in their mature functional state in adult males.
Under secretion of this hormone causes the development of feminine characteristics and male sterility.

Table 6.1Endocrine glands and their hormones (Table 6.1)Often examined

Table 6.1 of the book gathers the endocrine glands, the hormones they secrete and what each hormone does. Use it as a revision table after learning each gland.

Table 6.1: Endocrine glands and their hormones
Site of production	Hormone(s) secreted	Hormone function
Pituitary gland	Antidiuretic hormone (vasopressin)	Affects water retention in kidneys; controls blood pressure
Pituitary gland	Adrenocorticotropic hormone (ACTH)	Controls production of sex hormones (estrogen in women and testosterone in men) and the production of eggs in women and sperm in men (as printed in the book; see the note below)
Pituitary gland	Growth hormone (GH)	Affects growth and development; stimulates protein production; affects fat distribution
Pituitary gland	Luteinizing hormone (LH) and follicle-stimulating hormone (FSH)	Controls production of sex hormones (estrogen in women and testosterone in men) and the production of eggs in women and sperm in men
Pituitary gland	Oxytocin	Stimulates contraction of uterus and milk ducts in the breast
Pituitary gland	Prolactin	Initiates and maintains milk production in breasts; impacts sex hormone levels
Pituitary gland	Thyroid-stimulating hormone (TSH)	Stimulates the production and secretion of thyroid hormones
Median lobe of the pituitary gland	Melanocyte-stimulating hormone (MSH)	MSH plays a role in melanin production in the skin, influencing skin pigmentation. It also influences appetite and sex drive.
Kidneys	Renin and angiotensin	Controls blood pressure, both directly and also by regulating aldosterone production from the adrenal glands
Kidneys	Erythropoietin	Affects red blood cell (RBC) production
Pancreas	Glucagon	Raises blood sugar levels
Pancreas	Insulin	Lowers blood sugar levels; stimulates metabolism of glucose, protein, and fat
Ovaries	Estrogen	Affects development of female sexual characteristics and reproductive development, important for functioning of uterus and breasts; also protects bone health
Ovaries	Progesterone	Stimulates the lining of the uterus for fertilization; prepares the breasts for milk production
Parathyroid glands	Parathyroid hormone (PTH)	Most important regulator of blood calcium levels
Thyroid gland	Thyroid hormone	Controls metabolism; also affects growth, maturation, nervous system activity, and metabolism
Adrenal glands	Aldosterone	Regulates salt, water balance, and blood pressure
Adrenal glands	Corticosteroid	Controls key functions in the body; acts as an anti-inflammatory; maintains blood sugar levels, blood pressure, and muscle strength; regulates salt and water balance
Adrenal glands	Epinephrine	Increases heart rate, oxygen intake, and blood flow
Adrenal glands	Norepinephrine	Maintains blood pressure
Testes (testicles)	Testosterone	Develop and maintain male sexual characteristics and maturation
Pineal gland	Melatonin	Releases melatonin during night hours to help with sleep
Hypothalamus	Growth hormone releasing hormone (GHRH)	Regulates growth hormone release in the pituitary gland
Hypothalamus	Thyrotropin releasing hormone (TRH)	Regulates thyroid stimulating hormone release from the pituitary
Hypothalamus	Gonadotropin releasing hormone (GnRH)	Regulates LH/FSH production in the pituitary gland
Hypothalamus	Corticotropin releasing hormone (CRH)	Regulates adrenocorticotropin release in the pituitary gland
Thymus	Humoral factors	Helps develop the lymphoid system

Skills: interpreting and communication

Note

Skills box. Explain the role of artificially synthesised steroids in sports and their long term effects on its users. Steroids are artificial substances. They are developed in order to do the job of testosterone. A steroid can be classified as either anabolic or androgenic. Anabolic functions include those that promote formation of muscles, vertical growth and regulation of weight gain or loss. Androgenic refers to masculine attributes such as agility, strength and endurance. With the help of these drugs, sportsmen can become bigger, stronger, more agile and hence more competitive. Artificial steroid users carry many severe health risks. Major medical problems associated with steroids include a weakened immune system, liver disease, kidney disease, high blood pressure, high cholesterol, increased risk for heart disease, blood clots, strokes, tissue damage and cancer.

Question	Key points from the book
Why might GH make short people grow taller?	GH stimulates growth of the long bones, but only if the bones are not yet fully grown, and it has to be given over a long time
What do artificial steroids do?	They do the job of testosterone: anabolic (muscle formation, growth, weight) and androgenic (masculine attributes) effects
What are the risks?	Weakened immune system, liver and kidney disease, high blood pressure and cholesterol, heart disease, blood clots, strokes, tissue damage and cancer

6.3Neurosecretory role of hypothalamusOften examined

The hypothalamus is part of the forebrain. It regulates a wide spectrum of physiological functions such as hunger, thirst, sleep and temperature. It also monitors metabolites and hormone levels in the blood. The hypothalamus is the master control centre of the endocrine system: its endocrine signals directly control the pituitary gland.

Neurosecretory cells

The hypothalamus contains special groups of neurosecretory cells. These cells conduct impulses and have also developed secretory capacity to a high level. They produce regulatory hormones which regulate the synthesis and secretion of pituitary hormones.

The hormones produced by the hypothalamus are either releasing factors, which stimulate the secretion of pituitary hormones, or inhibiting factors, which inhibit the secretion of pituitary hormones.
They are produced in the cell bodies of the neurosecretory cells and packed into granules.
The granules are transported down the axon by cytoplasmic streaming.
The axon endings of the neurosecretory cells synapse with blood capillaries and release their hormones into the blood when stimulated. The blood carries them to the target area.

How the hypothalamus controls the pituitary: releasing and inhibiting factors reach the anterior lobe through the blood, while ADH and oxytocin are made in neurosecretory cells (accent) and released from the endings of their axons in the posterior lobe.

In addition, the neurosecretory cells that arise from the hypothalamus also produce two primary hormones, antidiuretic hormone (ADH) and oxytocin. These are stored in the posterior lobe of the pituitary gland and are released from there when needed.

Table 6.2: Hypothalamic hormones and their effect on pituitary gland
Hormone from the hypothalamus	Anterior pituitary response
Growth hormone releasing factor (GHRF)	Secretion of growth hormone (GH)
Somatostatin	Inhibition of GH
Thyrotrophin releasing factor (TRF)	Secretion of thyroid stimulating hormone (TSH)
Adrenocorticotrophin releasing factor (CRF)	Secretion of adrenocorticotrophic hormone (ACTH)
Prolactin inhibiting factor (PIF)	Inhibits secretion of prolactin
Gonadotrophin releasing hormone (GnRH)	Secretion of FSH and LH

Chains of control from the hypothalamus to the pituitary to a target gland. The pituitary hormones (accent) are the middle step of each chain.

6.2Pineal gland, thymus and other endocrine tissues

The pineal gland (accent) is a small gland attached to the hypothalamus. The pituitary gland hangs below the hypothalamus.

Gland	Hormone	Function
Pineal gland	Melatonin	Influences daily (circadian) rhythms; releases melatonin during night hours to help with sleep
Thymus	Thymosin (humoral factors)	Helps develop the lymphoid system; turns lymphocytes from the bone marrow into T lymphocytes. It undergoes atrophy during adulthood.

Tissue	Hormone	What the book says it does
Stomach wall	Gastrin	Travels in the blood stream but exerts its effect locally, stimulating the production of pepsinogen and hydrochloric acid
Cells of the duodenal wall	Secretin and cholecystokinin	Control pancreatic and liver secretions
Placenta	Progesterone	Maintains pregnancy
Various tissues	Prostaglandins	A group of localised hormones that provide protection during infections
Brain	Endorphins	Bind to pain receptors and so block the sensation of pain
Kidney	Renin	Monitors blood pressure and takes corrective action if it drops
Kidney	Erythropoietin (EPO)	Acts on the bone marrow to increase the production of red blood cells. Bleeding or moving to high altitudes (where oxygen is scarcer) trigger its release
Kidney	Calcitriol	Acts on the cells of the intestine to promote the absorption of calcium from the diet
Heart	Natriuretic peptides (ANP and BNP)	Named in Figure 6.1
Adipose tissue	Leptin and resistin	Named in Figure 6.1

6.4Feedback mechanismOften examined

A feedback mechanism is a type of interaction in which a controlling mechanism is itself controlled by the product of the reactions it is controlling. After receiving the signal, a change occurs to correct the deviation, either by depressing it, which is negative feedback, or by enhancing it, which is positive feedback.

The idea of feedback in four steps

A level in the body (for example blood glucose) moves away from its normal value.
The gland or control centre senses the change.
It releases (or stops releasing) a hormone.
The hormone changes the level. The new level is then sensed, and that in turn adjusts the hormone again.

The two kinds of feedback loop. In negative feedback the hormone reverses the change (accent) and the level returns to the set point. In positive feedback the hormone makes the change grow until an endpoint is reached.

Worked example: reading the graph of feedback

The graph shows how a controlled level behaves after a small disturbance. State which curve shows negative feedback and which shows positive feedback, and give one body example of each from this unit.

Schematic: how the controlled level behaves

1

2

3

4

5

6

7

2

4

6

8

10

12

14

O

time

level

set point

endpoint

negative feedback

positive feedback

Schematic only (arbitrary units). With negative feedback a disturbed level is pushed back to the set point. With positive feedback the change grows faster and faster until an endpoint is reached.

The dashed line is the set point, the normal level the body tries to keep.
The curve that falls back towards the dashed line is negative feedback: the response opposes the change and brings the system back to its set point. Example: insulin lowering a high blood glucose level.
The curve that climbs away from the set point faster and faster is positive feedback: the response reinforces the change. It stops only when an endpoint is reached. Example: oxytocin and uterine contractions during childbirth, which end with the delivery of the baby and placenta.
Negative feedback keeps the level steady (homeostasis); positive feedback is not homeostatic.

Falling curve: negative feedback (insulin and glucose). Rising curve: positive feedback (oxytocin in labour)

6.4.1Positive feedbackOften examined

In positive feedback an end product speeds up its own production. These responses are not homeostatic and are rare in healthy individuals.

An example of positive feedback is childbirth. The early contractions of labour begin to force the baby's head against the cervix to dilate (open) it. Stretch-receptive neurons in the cervix respond to this extension by signalling the hypothalamus, which responds by triggering the release of the hormone oxytocin. Oxytocin stimulates more and stronger uterine contractions. Stronger contractions create further pressure on the cervix, which in turn prompts the release of more hormones. The feedback cycle is finally terminated by the expulsion of the baby and its placenta.

Figure 6.6: Positive feedback mechanism of oxytocin during childbirth. Each pass of the cycle makes the next stronger.

The oxytocin positive feedback loop

The head of the baby pushes against the cervix and stretches it.
Stretch-receptive neurons send nerve impulses from the cervix to the brain (hypothalamus).
The brain stimulates the pituitary gland to secrete oxytocin.
Oxytocin is carried in the bloodstream to the uterus.
Oxytocin stimulates stronger uterine contractions, which push the baby towards the cervix and increase the stretch. The cycle returns to step 1.

6.4.2Negative feedbackOften examined

In negative feedback, the end product results in the reversal of the direction of change and tends to maintain homeostasis. In this system an endocrine gland is sensitive either to the concentration of a substance it regulates or to the concentration of a product from a process it controls.

Insulin: when blood glucose is too high

If blood glucose becomes too high, the beta (B) cells of the islets of Langerhans respond by releasing insulin. Insulin lowers blood glucose by making the cell surface membranes more permeable to glucose. It activates transport proteins in the membranes, allowing glucose to pass into the cells. Insulin also activates enzymes inside the cells. Some of these enzymes convert glucose to glycogen.

Figure 6.7: Negative feedback mechanism of insulin. The response (falling glucose) switches off the stimulus (rising glucose).

Glucagon: when blood glucose is too low

If the levels of blood glucose get too low, the alpha (a) cells in the islets of Langerhans secrete glucagon. This hormone fits into the receptor sites on the cell surface membranes and activates the enzymes inside the cells that convert glycogen to glucose. The glucose then passes out of the cells and into the blood, raising the blood glucose level.

Negative feedback mechanism of glucagon, in the same form as Figure 6.7. A falling glucose level is corrected by glucagon.

In this way, the negative feedback mechanism controls blood glucose.

Step	Blood glucose too high	Blood glucose too low
Detected by	Beta cells of the islets	Alpha cells of the islets
Hormone	Insulin	Glucagon
Action on cells	Makes membranes more permeable to glucose and activates transport proteins and enzymes	Fits receptor sites on the cell surface membranes and activates enzymes
Inside the cells	Glucose is converted to glycogen	Glycogen is converted to glucose
Result	Blood glucose falls	Blood glucose rises

Worked example: blood glucose after a meal and after a long fast

Explain with the help of negative feedback what the body does (a) just after eating a sugary meal and (b) many hours after the last meal. Say what is detected, which cells respond, which hormone is released and what the result is.

(a) After the meal. The stimulus is a rising blood glucose level. The beta cells of the islets of Langerhans detect it and release insulin. Insulin makes the cell membranes more permeable to glucose and activates enzymes, so liver and other body cells take up glucose, and the liver stores it as glycogen. Blood glucose falls back to the normal level and the beta cells stop releasing insulin.
(b) After a long fast. The stimulus is a falling blood glucose level. The alpha cells detect it and release glucagon (the sympathetic nervous system also stimulates its release). Glucagon activates the enzymes that convert glycogen to glucose, and the glucose passes out of the cells into the blood. Blood glucose rises back to normal.
In both cases the response reverses the change, so this is negative feedback, and the two hormones act antagonistically.

	(a) After a meal	(b) After a fast
Stimulus	Blood glucose rises	Blood glucose falls
Cells that respond	Beta cells	Alpha cells
Hormone	Insulin	Glucagon
Main response	Glucose into cells; glucose to glycogen	Glycogen to glucose; glucose into blood
Result	Blood glucose falls to normal	Blood glucose rises to normal

Insulin lowers high blood glucose; glucagon raises low blood glucose

Table 6.3: Difference between positive feedback mechanism and negative feedback mechanism
Positive feedback mechanism	Negative feedback mechanism
It amplifies or increases the change, pushing the system further away from its set point	It counteracts the change, bringing the system back to its set point
It leads to a rapid, escalating response until a specific endpoint is reached	It maintains stability and homeostasis within a system
Examples are uterine contractions during childbirth and blood clotting	Body temperature regulation and blood glucose control
The response reinforces the initial stimulus	The response opposes the initial stimulus
The overall function is to rapidly complete a process or reach a specific outcome	The overall function is to maintain a stable internal environment

STEAM activity 6.1: hormone role-play

The book gives a classroom activity in which students act out the working of the endocrine system.

STEAM activity 6.1: steps

Hormone assignment and role-playing. Divide the class into groups, each representing a specific endocrine gland and its hormones (for example the pituitary gland with growth hormone, LH and FSH). Each student researches the function of an assigned hormone and how it interacts with other body systems (students representing the thyroid would research the role of thyroxine in metabolism). Students then act out how the hormone travels in the bloodstream, interacts with target cells (other students or props) and affects the body.
Simulating the endocrine system. Use a large space, a hallway or an open classroom as the bloodstream; students carrying a hormone "message" on a card or prop move through it. Assign areas or objects to represent target organs (for example a table as the liver, a chair as the heart). As the hormones travel, students describe how their hormone affects the organ, using their research as a guide.
Assessment. Observation of student engagement and understanding during the role-play; written reports summarising the hormone represented, its function and how it interacts with different body systems; group presentations to the class.

Example scenario

A student representing growth hormone (GH) from the pituitary gland "enters the bloodstream" and travels towards a student representing the bones. The GH student would then act out how GH promotes bone growth, possibly by using hands to simulate bone development, or explain how GH influences protein production.

Benefit	What it means
Hands-on learning	Students actively participate and learn through movement and interaction
Visual representation	The simulation makes a complex process easier to understand
Role-playing	Students "become" the hormones and so understand how they work
Collaborative learning	Students work together in groups, which fosters collaboration and communication

Key terms

Chemical coordination: Control of the metabolic functions of the body (rate of reactions, transport through membranes, growth and secretions) by hormones.
Hormone: A small soluble organic molecule, effective in low concentration, secreted by a cell and carried by the blood to coordinate functions of other cells.
Chemical messenger: A substance, such as a hormone, that transports a signal from one cell to another.
Target: The site, cells or tissue, that has specific receptors for a hormone and therefore responds to it.
Receptor: A specially shaped site on a target cell that fits one hormone like a lock fits a key.
Endocrine gland: A ductless gland that releases its hormones directly into the blood.
Exocrine gland: A gland that releases its substances through ducts or openings onto body surfaces or into body cavities, for example sweat, saliva and digestive juices.
Steroid hormones: Hormones composed of steroids, such as cortisone, aldosterone, testosterone, estrogen and progesterone.
Proteinaceous hormones: Polypeptide or peptide hormones, such as the anterior pituitary hormones and insulin.
Catecholamines: Adrenaline and noradrenaline, secreted by the adrenal medulla.
Pituitary gland: The gland just below the hypothalamus with anterior, median and posterior lobes.
Infundibulum: The stalk, made of blood vessels and neurosecretory cell fibres, that attaches the pituitary to the hypothalamus.
Master gland: The anterior pituitary, because many of its hormones regulate the activity of other endocrine glands.
Neurosecretory cells: Cells of the hypothalamus that conduct impulses and secrete regulatory hormones.
Growth hormone (GH): Anterior pituitary hormone that stimulates cell growth and division, amino acid uptake and protein synthesis.
Dwarfism: Short stature with body parts in proportion and normal IQ, caused by deficiency of GH in childhood.
Gigantism: Abnormal increase in height caused by oversecretion of GH in childhood.
Acromegaly: Thickening of bones and enlargement of hands, feet, skull, nose and jawbone caused by oversecretion of GH in adult life.
Thyroid-stimulating hormone (TSH): Anterior pituitary hormone that stimulates the thyroid gland to make and release its hormones.
Adrenocorticotropic hormone (ACTH): Anterior pituitary hormone that acts on the adrenal cortex and stimulates secretion of corticosteroids.
Gonadotropic hormones: FSH, LH (ICSH in the male) and prolactin, which act on and regulate the reproductive system.
Melanocyte-stimulating hormone (MSH): Median lobe hormone that stimulates the production and release of melanin by melanocytes in skin and hair.
Antidiuretic hormone (ADH): Hormone (vasopressin) stored in the posterior pituitary and released during dehydration, low blood volume and low blood pressure.
Diabetes insipidus: Disease from under secretion of ADH, with excessive diluted urine and frequent thirst.
Oxytocin: Posterior pituitary hormone that causes labour contractions and milk ejection.
T3 and T4: Iodine-containing thyroid hormones (tri-iodothyronine and thyroxine) that raise the basal metabolic rate.
Hyperthyroidism: Excess of thyroid hormones; over secretion of T3 and T4 causes Graves' disease.
Graves' disease: An autoimmune hyperthyroidism in which antibodies mimic TSH; shows high metabolic rate, rapid heartbeat, weight loss and exophthalmia.
Hypothyroidism: Under secretion of thyroxine.
Myxedema: The adult hypothyroid syndrome with low metabolic rate, puffy eyes, dry skin, hair loss and goitre.
Cretinism: Severe congenital hypothyroidism in infants with mental retardation, poor growth and delayed puberty.
Goitre: Enlarged thyroid gland.
Calcitonin: Hormone of the thyroid C-cells that increases deposition of calcium in bone and lowers blood calcium.
Parathormone: Parathyroid hormone (PTH), the most important regulator of blood calcium; antagonistic to calcitonin.
Hypercalcemia: High blood calcium, which depresses the nervous system and weakens muscles.
Hypocalcemia: Low blood calcium, which increases neuron excitability and can lead to tetany.
Tetany: A condition in which muscles remain in a contracted state.
Islets of Langerhans: Patches of endocrine tissue in the pancreas that secrete insulin and glucagon.
Insulin: Hormone of the beta cells that lowers blood glucose.
Glucagon: Hormone of the alpha cells that raises blood glucose; antagonistic to insulin.
Diabetes mellitus: Metabolic disease caused by under secretion of insulin, with high glucose in blood and urine.
Glycogenesis: Conversion of glucose to glycogen.
Gluconeogenesis: Synthesis of glucose (inhibited by insulin).
Adrenal medulla: Inner part of the adrenal gland that makes epinephrine and norepinephrine.
Adrenal cortex: Outer part of the adrenal gland that makes glucocorticoids, mineralocorticoids and a little androgen under ACTH.
Synergistic effect: Two hormones, such as epinephrine and norepinephrine, producing the same overall effect in different ways.
Glucocorticoids: Adrenal cortex hormones, for example cortisone, that help regulate blood glucose.
Mineralocorticoids: Adrenal cortex hormones, for example aldosterone, that help regulate the level of minerals in the blood.
Addison's disease: Under secretion of cortical hormones: metabolic disturbance, muscle weakness and loss of salts.
Cushing's disease: Over secretion of cortical hormones: excessive protein breakdown with muscle and bone weakness.
Gonads: Endocrine glands that also produce gametes: ovaries in females and testes in males.
Estrogen: Ovarian hormone from the Graafian follicle under FSH; responsible for secondary sex characteristics in females.
Progesterone: Hormone from the corpus luteum (and placenta) that thickens the uterine wall, maintains pregnancy and inhibits FSH.
Testosterone: Male sex hormone from the interstitial cells of the testes under ICSH.
Releasing factors: Hypothalamic hormones that stimulate the secretion of pituitary hormones.
Inhibiting factors: Hypothalamic hormones that inhibit the secretion of pituitary hormones.
Feedback mechanism: Interaction in which a controlling mechanism is itself controlled by the product of the reactions it controls.
Positive feedback: Feedback in which an end product speeds up its own production, for example oxytocin in childbirth.
Negative feedback: Feedback in which the end product reverses the direction of change and maintains homeostasis, for example insulin and glucagon.
Melatonin: Hormone of the pineal gland that influences circadian rhythms and helps sleep.

Multiple choice questionsp. 130–131

Select the correct answer.

1

Steroid hormones are secreted by

Athe adrenal cortex
Bthe gonads
Cthe thyroid
Dboth A and BAnswer

Why

Steroid hormones come from the adrenal cortex, the testes and the ovaries (and the placenta in pregnancy), so both A and B are correct (p.118, p.123). The thyroid secretes thyroxine, an amino acid derivative, not a steroid, so C is wrong.

2

Examples of posterior pituitary hormones are

AFSH and LH
Bprolactin and parathormone
Cmelatonin and prostaglandin
DADH and oxytocinAnswer

Why

ADH (antidiuretic hormone, vasopressin) and oxytocin are stored and released by the posterior lobe (p.120). FSH, LH and prolactin come from the anterior lobe, parathormone from the parathyroid glands, and melatonin from the pineal gland. Prostaglandins are local hormones, not pituitary hormones.

3

The primary targets for FSH are cells in the

Ahypothalamus
BovaryAnswer
Cthyroid
Dpituitary

Why

FSH controls egg production in women and sperm production in men (Table 6.1), so its targets are the reproductive organs. Of the options only the ovary is a target. The pituitary is where FSH is made, and the hypothalamus releases GnRH, which acts on the pituitary, not on FSH's target.

4

Which of the following controls the activity of all others?

Athyroid
BpituitaryAnswer
Cadrenal cortex
Dgonads

Why

The anterior pituitary is called the master gland because its hormones (TSH, ACTH, FSH and LH) regulate the thyroid, the adrenal cortex and the gonads (p.119). The thyroid, adrenal cortex and gonads are all controlled by it, so none of them can be the answer.

Note. The book calls the hypothalamus the master control centre of the endocrine system (p.126), and it controls the pituitary. The hypothalamus is not one of the options, so the answer is the pituitary, the master gland named on p.119.

5

Which of the following have antagonistic (opposing) effects?

Aparathyroid hormone and calcitoninAnswer
Bglucagons and thyroxine
Cgrowth hormone and epinephrine
Dcortisone and ACTH

Why

Parathormone raises blood calcium and calcitonin lowers it. The book says parathormone works antagonistically to calcitonin (p.122). Growth hormone and epinephrine both raise blood glucose, so they act in the same direction, and ACTH stimulates the secretion of cortisone rather than opposing it.

6

Which of the following hormones has broadest range of targets?

AADH
Boxytocin
CTSH
DepinephrineAnswer

Why

Epinephrine acts on several organs at once: it speeds up the heart, dilates the bronchi, increases blood flow to the heart and skeletal muscles, and stimulates metabolic activity in general (p.123, Table 6.1). ADH acts mainly on the kidney tubules, oxytocin on the uterus and milk ducts, and TSH only on the thyroid gland.

7

The pancreas increases its output of insulin in response to

Aan increase in body temperature
Bchanging cycle of dark and light
Ca decrease in blood glucose
Dan increase in blood glucoseAnswer

Why

Beta cells respond directly to the level of blood glucose, and insulin is secreted when blood sugar rises, for example right after a meal (p.122). A fall in blood glucose makes the alpha cells release glucagon, which is the opposite response, so C is wrong.

8

Name the gland that is located at the base of the throat, just inferior to the laryngeal prominence (Adam's apple).

Apituitary
Bpineal gland
Chypothalamus
DthyroidAnswer

Why

The thyroid gland has two lobes on either side of the trachea, just below the larynx (p.120). The pituitary, the pineal gland and the hypothalamus all lie in the brain (p.126), so none of A, B or C is in the neck.

9

Action of parathormone in the human body

Adecreases blood sodium level
Bincreases blood sodium level
Cdecreases blood calcium level
Dincreases blood calcium levelAnswer

Why

Parathormone is released when blood calcium is low, and it raises blood calcium by releasing calcium from the bones (p.122). It does not act on blood sodium, so A and B are wrong, and C is the opposite of its action.

10

Which of the following endocrine glands secretes growth hormones?

Athyroid gland
Bpituitary glandAnswer
Cadrenal gland
Dtestes

Why

Growth hormone (somatotrophic hormone, STH) is secreted by the anterior lobe of the pituitary gland (p.119). The thyroid, adrenal and testes secrete other hormones.

11

Which of the following pair of endocrine glands is located in the brain?

Ahypothalamus and thymus
Bpituitary and parathyroid
Cthyroid and pineal
Dhypothalamus and pinealAnswer

Why

The hypothalamus (part of the forebrain) and the pineal gland, which is attached to it, both lie in the brain (p.126). The thymus is in the chest, the thyroid and parathyroid glands are in the neck, so every option except D contains a gland outside the brain.

12

What gland is located just superior to the kidneys?

Apituitary
BadrenalAnswer
Cpancreas
Dovaries

Why

Each adrenal gland rests on top of a kidney (p.123, Fig. 6.5). The pituitary is in the brain and the ovaries are in the pelvis, far below the kidneys.

13

Which of the following glands can be classified as an endocrine and an exocrine gland?

Athyroid
Bthymus
CpancreasAnswer
Dpituitary

Why

The pancreas has exocrine tissue (acinar cells, which secrete digestive juice through the pancreatic duct) and endocrine tissue (the islets of Langerhans, which secrete insulin and glucagon) (p.122). The other three glands are purely endocrine.

Note. The book prints this question as 'Which if the following gland which can be classified as an endocrine and an exocrine gland?'. The wording is cleaned here; the meaning is unchanged.

14

Which of these hormones is made by the posterior pituitary?

AFSH
BLH
CACTH
DADHAnswer

Why

ADH is the only option that the posterior pituitary releases (it is stored and released there, p.120). FSH, LH and ACTH are all made in the anterior lobe.

Note. The book also states that the posterior pituitary does not synthesize any hormone (p.120); ADH is made by neurosecretory cells of the hypothalamus. The question's wording is loose, but D is the only option that fits, so it is the key.

15

Which one of the following is the smallest in the human body?

Apineal gland
Bpituitary gland
Cadrenal gland
Dparathyroid glandsAnswer

Why

The parathyroid glands are the smallest of these: there are four tiny glands, each only a few millimetres across and weighing a few tens of milligrams. The pituitary is roughly pea-sized, and each adrenal gland is much larger.

Note. The book gives no measurements for these glands, so this key relies on standard anatomy and the book's description of the parathyroid glands as small masses (p.122).

16

Which of the following hormones are responsible for the 'fight-or-flight' response?

Aepinephrine and norepinephrineAnswer
Binsulin and glucagon
Cestrogen and progesterone
Dthyroxin and melatonin

Why

Adrenaline (epinephrine) and noradrenaline (norepinephrine) from the adrenal medulla are released in emergencies under the influence of the sympathetic nervous system, and they prepare the body for immediate response to stress (p.123). Insulin and glucagon control blood glucose, estrogen and progesterone are sex hormones, and thyroxine and melatonin control metabolism and the sleep-wake cycle.

17

Calcitonin is a hormone of which of the following:

Aadrenal cortex
Bthyroid glandAnswer
Cpituitary gland
Dthymus gland

Why

The C-cells of the thyroid gland produce calcitonin (p.121). The adrenal cortex makes steroids, the pituitary makes the six anterior hormones and ADH and oxytocin, and the thymus makes thymosins.

18

All of the following are hormones of the anterior pituitary except:

Ahuman growth hormone (GH)
Bfollicle-stimulating hormone (FSH)
Cparathyroid hormone (PTH)Answer
Dthyroid-stimulating hormone (TSH)

Why

GH, FSH and TSH are all anterior lobe hormones (p.119). Parathyroid hormone is secreted by the parathyroid glands, which lie on the thyroid gland, not by the pituitary.

19

The secretions from which of these glands differs between males and females?

Aadrenal
Bparathyroid
CgonadalAnswer
Dpancreas

Why

The gonads differ between the sexes: the ovaries secrete estrogen and progesterone, and the testes secrete testosterone (p.123, p.125). The adrenal cortex makes small amounts of androgens in both sexes, and the parathyroid glands and pancreas are the same in males and females.

20

Calcium level in the blood is regulated by the:

Athyroid
Bparathyroid
Cposterior pituitary
DA and BAnswer

Why

The C-cells of the thyroid release calcitonin, which lowers blood calcium, and the parathyroid glands release parathormone, which raises it (p.121, p.122). Both A and B are therefore correct. The posterior pituitary is not involved in calcium control.

Short answer questionsp. 131–132

13 marks

Why are hormones called chemical messengers?

Solution

Hormones are chemicals secreted into the blood, which carries them to other cells, so they transmit a signal from one cell to another.
Each hormone acts at a target site with specific receptors, so it changes only the cells that have the matching receptor.
Hormones work in very low concentration, yet they can change the activity of target cells (p.117).

23 marks

What are neurosecretory cells?

Solution

Neurosecretory cells are specialised nerve cells in the hypothalamus that conduct nerve impulses and also secrete hormones.
They produce the releasing and inhibiting factors that control the anterior pituitary, and they make ADH and oxytocin, which are packed into granules and carried down the axon.
When stimulated, the axon endings release the hormones into the blood capillaries (p.126).

32026 Annual I, Set D, Q.2 (ix)3 marks

Name the hormones of anterior pituitary gland.

Solution

Growth hormone (GH), also called somatotrophic hormone (STH).
Thyroid-stimulating hormone (TSH).
Adrenocorticotropic hormone (ACTH).
Follicle-stimulating hormone (FSH).
Luteinising hormone (LH), also called interstitial cell-stimulating hormone (ICSH) in males.
Prolactin, also called leuteotrophic hormone (LTH).

43 marks

Why is the anterior lobe of pituitary gland called the master gland?

Solution

It secretes numerous hormones, and the book states that all six of its hormones regulate the secretory action of other endocrine glands (p.119).
TSH controls the thyroid gland, ACTH controls the adrenal cortex, and FSH and LH control the gonads, so the pituitary sets the activity of several other glands.
Its own secretion is controlled by releasing and inhibiting factors from the hypothalamus, so it sits at the centre of the endocrine control chain.

53 marks

Describe the median lobe of pituitary gland.

Solution

The median lobe is a thin layer of cells between the anterior and posterior lobes, and in humans it is not very prominent.
It secretes melanocyte-stimulating hormone (MSH), which stimulates the melanocytes of the skin and hair to produce melanin.
Its secretion is controlled by MSH-inhibitory hormone from the hypothalamus, and MSH is raised during pregnancy (p.119).

63 marks

How is the secretion of ADH controlled?

Solution

ADH (antidiuretic hormone) is made by neurosecretory cells of the hypothalamus and released from the posterior pituitary in response to nerve impulses from the hypothalamus (p.120).
ADH is released during dehydration, decreased blood volume and low blood pressure, which makes the kidneys retain water and restores blood volume.
Under secretion of ADH causes diabetes insipidus, and over secretion may cause kidney problems.

7

Write the differences between:

Solution

(a) Exocrine and endocrine glands

Feature	Exocrine glands	Endocrine glands
Ducts	Secrete through ducts or openings	Ductless
Secretions	Sweat, saliva, tears, milk and digestive juices	Hormones
Destination	Onto body surfaces or into body cavities	Carried by the blood to target cells or organs

(b) Steroid hormones and proteinaceous hormones

Feature	Steroid hormones	Proteinaceous (peptide) hormones
Chemical nature	Steroids, lipid based	Chains of amino acids (polypeptides or peptides)
Examples	Cortisone, aldosterone, testosterone, estrogen, progesterone	Growth hormone, TSH, FSH, LH, insulin, ADH, oxytocin
Source	Adrenal cortex, testes, ovaries and placenta	Anterior pituitary, pancreas and posterior pituitary
Cell membrane	Passes through the cell membrane	Cannot pass through the cell membrane

(c) First messenger and second messenger

Feature	First messenger	Second messenger
What it is	The hormone, carried in the blood	A small molecule inside the cell (for example cyclic AMP)
Location	Outside the cell	Inside the cytoplasm of the cell
Role	Binds to a receptor on the cell surface and carries the message	Relays the message inside the cell and activates enzymes

(d) Receptor of proteinaceous hormones and steroid hormones

Feature	Receptor of proteinaceous hormones	Receptor of steroid hormones
Position	On the outer surface of the cell membrane	Inside the cell, in the cytoplasm or nucleus
Reason	The hormone cannot enter the cell, so it binds a surface receptor	The hormone diffuses through the membrane and binds an internal receptor
Result	Triggers second messengers or enzymes in the cell	The hormone-receptor complex enters the nucleus and affects gene activity

(e) Hypothyroidism and hyperthyroidism

Feature	Hypothyroidism	Hyperthyroidism
Cause	Under secretion of thyroxine, often from lack of iodine in the diet	Over secretion of T3 and T4, for example in Graves' disease
Metabolic rate	Low; the person feels chilled	High; rapid and irregular heartbeat, sweating
Typical signs	Puffy eyes, dry skin, hair loss, goitre (enlarged thyroid)	Exophthalmia (protruding eyeballs) and weight loss despite adequate food

(f) Calcitonin and parathormone

Feature	Calcitonin	Parathormone (PTH)
Source	C-cells of the thyroid gland	Parathyroid glands on the posterior surface of the thyroid
Stimulus	High blood calcium	Low blood calcium
Effect on blood calcium	Lowers it: calcium is deposited in bone and excreted in urine	Raises it: calcium is released from bones
Relationship	Works antagonistically to PTH	Works antagonistically to calcitonin

(g) Beta and alpha cells of the islets of Langerhans

Feature	Beta cells	Alpha cells
Hormone	Insulin	Glucagon
Number	Larger in number	Fewer in number
Stimulus	Rise in blood glucose, for example after a meal	Fall in blood glucose
Effect	Lowers blood glucose	Raises blood glucose

(h) Insulin and glucagon

Feature	Insulin	Glucagon
Source	Beta cells of the islets of Langerhans	Alpha cells of the islets of Langerhans
Trigger	High blood glucose	Low blood glucose; the sympathetic nervous system also stimulates it
Action	Increases glucose uptake by cells, promotes glycogenesis, inhibits gluconeogenesis	Converts glycogen to glucose in the cells
Effect	Lowers blood glucose	Raises blood glucose; antagonistic to insulin

(i) Diabetes insipidus and diabetes mellitus

Feature	Diabetes insipidus	Diabetes mellitus
Hormone deficiency	Under secretion of ADH	Under secretion of insulin
Main sign	Excessive production of dilute urine and frequent thirst	High glucose level in the blood and in the urine
Organ affected	Kidney tubules, which reabsorb water	Metabolism of glucose in the body cells

(j) Estrogen and progesterone

Feature	Estrogen	Progesterone
Source	Graafian follicle, under stimulation by FSH	Corpus luteum (stimulated by LH); placenta in pregnancy
Main role	Develops female sexual characteristics at puberty and repairs the uterine wall after menstruation	Thickens and vascularises the uterine lining; prepares the breasts for milk
Effect on FSH	Negative feedback on FSH	Inhibits further FSH secretion, so no more follicles ripen

(k) Positive and negative feedback

Feature	Positive feedback	Negative feedback
Effect on the change	Amplifies the change, pushing the system further from its set point	Counteracts the change, bringing the system back to its set point
Example	Uterine contractions during childbirth (oxytocin)	Blood glucose control by insulin and glucagon
Overall function	Rapidly completes a process	Maintains a stable internal environment

83 marks

What is the relationship between a hormone and a target cell?

Solution

A hormone travels in the blood to all body cells, but it acts only on target cells, which carry specific receptors for it.
The hormone fits its receptor like a key fitting a lock; once bound, the target cell changes its activity (lock and key model, p.117).
A cell without the matching receptor is unaffected, so one hormone can act on many target cells that share the same receptor.

93 marks

Locate the endocrine glands in the human body.

Solution

The main endocrine glands and their positions are shown in the figure, and the list below gives each position.

Hypothalamus and pituitary gland: at the base of the brain; the pituitary hangs from the hypothalamus by the infundibulum.
Pineal gland: in the brain, attached to the hypothalamus.
Thyroid gland: in the neck, on either side of the trachea, below the larynx; the four parathyroid glands lie on its posterior surface.
Thymus: in the upper chest.
Pancreas: in the abdomen, with its islets of Langerhans scattered through the gland.
Adrenal glands: one on top of each kidney.
Gonads: testes in the scrotum (male) and ovaries in the pelvic cavity (female).

Figure 6.1: Major endocrine glands and their locations in the human body (schematic front view).

103 marks

How does a feedback mechanism regulate the activity of the endocrine system?

Solution

In a feedback mechanism the product of a reaction controls the mechanism that produces it, so the gland's activity is adjusted to the body's needs.
In negative feedback the response reverses the change: a rise in blood glucose triggers insulin, which lowers glucose and so stops its own release, while a fall triggers glucagon (p.128).
In positive feedback the response amplifies the change, as oxytocin does in childbirth, until the process is completed (birth ends the cycle).
Most hormone levels are held within a narrow range by negative feedback, and positive feedback is used only to complete a few specific processes.

113 marks

Make a diagram of positive feedback involving the hormone oxytocin.

Solution

The diagram shows the five stages of the positive feedback loop of oxytocin during childbirth. Each stage causes the next, and the loop is closed by the baby pushing harder on the cervix.

Figure 6.6: Positive feedback loop of oxytocin in childbirth, with the five stages in order.

123 marks

Predict the effects that insufficient amounts of FSH and LH will have on the menstrual cycle.

Solution

Without enough FSH, the Graafian follicles do not ripen, so less estrogen is made and the uterine lining is not repaired and built up properly.
Without enough LH, ovulation does not occur and the corpus luteum does not form, so progesterone falls and the uterine lining is not maintained.
The menstrual cycle becomes irregular or may stop, and the woman is likely to be infertile (p.123, p.125).

Extensive answer questionsp. 132

18 marks

Describe the function of the endocrine system in the human body.

Solution

Chemical coordination

The nerve fibres do not reach every cell of the body, so the endocrine system coordinates most body cells by means of hormones. This is called chemical coordination (p.116).

Functions

Controls the rate of chemical reactions (metabolism) in the body.
Regulates the transport of substances through cell membranes.
Controls growth and development, and sexual function and reproduction.
Maintains homeostasis, such as blood pressure, blood glucose, water and electrolyte balance, and body temperature.
Controls the sleep-wake cycle and mood, and the response to stress (p.117).

Main glands and their roles

Gland	Main role
Pituitary gland	Master gland; its hormones control the thyroid, adrenal cortex and gonads, and regulate growth and milk production
Hypothalamus	Links the nervous and endocrine systems; releases regulatory hormones and makes ADH and oxytocin
Thyroid gland	Controls metabolic rate, growth and maturation; calcitonin lowers blood calcium
Parathyroid glands	Controls blood calcium through parathormone
Pancreas	Controls blood glucose through insulin and glucagon
Adrenal glands	Stress response (adrenaline and noradrenaline) and salt and water balance (corticosteroids)
Gonads	Sexual development, reproduction and secondary sexual characteristics
Pineal gland	Melatonin, which regulates the daily rhythm of sleep

The endocrine system therefore controls metabolism, water balance, growth, reproduction and the response to stress. Its hormones act more slowly than nerve impulses but their effects last longer.

26 marks

State the role of hormone as chemical messenger.

Solution

Definition

A hormone is a small, soluble organic molecule secreted by a cell into the blood. It is effective in very low concentration and carries a signal from one cell to another, which is why it is called a chemical messenger (p.117).

How a hormone delivers its message

The endocrine gland secretes the hormone directly into the blood, because it has no duct.
The blood carries the hormone to every tissue of the body.
The hormone reaches the target cells, which have specific receptors, and binds to them like a key in a lock.
Binding changes the activity of the target cell, such as increased glucose uptake or a change in gene activity; this gives the response.
When the hormone is removed from the blood its effect stops.

Hormone action: the gland secretes the hormone into the blood, the blood carries it to the target cell, and the hormone binds its receptor to give a response.

Two types of communication

Type of communication	Example from the book
Between two endocrine glands	The pituitary releases TSH, which stimulates the thyroid to release thyroid hormones
Between an endocrine gland and a target organ	The pancreas releases insulin, which acts on muscles and the liver to take up and store glucose

Because a hormone acts only through its receptors, it affects only the target cells that carry the matching receptor, and the message is delivered precisely.

38 marks

Describe chemical nature of hormone and correlate it with important hormones.

Solution

Hormones are organic substances of varying structural complexity. Chemically they are steroids, peptides or proteins (including glycoproteins), amines and amino acid derivatives (p.117-118).

Chemical group	Important hormones	Gland of origin
Steroid hormones	Cortisone, aldosterone, testosterone, estrogen, progesterone	Adrenal cortex, testes, ovaries and placenta
Proteinaceous (polypeptide or peptide) hormones	Growth hormone, TSH, FSH, LH, prolactin, insulin, glucagon	Anterior pituitary and pancreas
Catecholamines (amines)	Adrenaline (epinephrine) and noradrenaline (norepinephrine)	Adrenal medulla
Amino acid derivatives	Thyroxine (T4) and triiodothyronine (T3)	Thyroid gland
Other peptide hormones	MSH, oxytocin, ADH (vasopressin), ACTH, calcitonin, parathormone	Median lobe and posterior pituitary, anterior pituitary, thyroid and parathyroid glands

Correlation with action

Steroid hormones are lipid soluble. They pass through the cell membrane and bind receptors inside the cell, so they act slowly but for a long time, for example estrogen and progesterone on the uterus.
Protein and peptide hormones are too large or too polar to pass the membrane. They bind receptors on the cell surface, for example insulin and glucagon on the liver and muscle cells.
Amines and amino acid derivatives: adrenaline acts quickly on the heart and blood vessels, and thyroxine, which contains iodine, controls the basal metabolic rate of the whole body.

The chemical nature of a hormone therefore helps to explain where it is made, how it moves in the blood, how it reaches its receptor and how quickly it acts.

48 marks

Describe the hormones secreted by the anterior lobe of pituitary gland.

Solution

The anterior lobe is the master gland. It secretes six hormones, all of which regulate the secretory action of other endocrine glands or tissues (p.119). Their release is controlled by releasing and inhibiting factors from the hypothalamus.

Figure 6.2: Anterior pituitary hormones and their target organs (simplified). The pituitary sends six hormones, shown here by the target each one acts on.

Hormone	Target and function	Controlled by (hypothalamus)
Growth hormone (GH, somatotrophic hormone, STH)	Bones and body tissues: cell growth and division, uptake of amino acids, protein synthesis	GHRF stimulates; somatostatin inhibits
Thyroid-stimulating hormone (TSH)	Thyroid gland: stimulates production and release of thyroid hormones	TRF stimulates
Adrenocorticotropic hormone (ACTH)	Adrenal cortex: stimulates secretion of cortisone and aldosterone	CRF stimulates; stress such as cold, heat, pain or infection increases its release
Follicle-stimulating hormone (FSH)	Ovaries and testes: follicle development, egg production and sperm production	GnRH stimulates
Luteinising hormone (LH, ICSH in males)	Ovaries and testes: ovulation and formation of the corpus luteum; testosterone production in males	GnRH stimulates
Prolactin (LTH, leuteotrophic hormone)	Mammary glands: initiates and maintains milk production	PIF inhibits

Growth hormone and prolactin act directly on their target tissues, while TSH, ACTH, FSH and LH act on other glands, which then release their own hormones.

56 marks

Describe the hormones secreted by the posterior lobe of pituitary gland.

Solution

The posterior lobe does not synthesize any hormone. It is made largely of axons of neurosecretory cells of the hypothalamus, and it stores and releases two hormones that these cells make (p.120, p.126).

Hormone	Target	Release and function
Antidiuretic hormone (ADH, vasopressin)	Kidney tubules and blood vessels	Released during dehydration, decreased blood volume and low blood pressure. It makes the kidney retain water; in large amounts it raises blood pressure
Oxytocin	Uterus and milk ducts of the mammary glands	Released in waves during labour to contract the uterus, and when a nursing mother suckles, to eject milk

Imbalance

Under secretion of ADH causes diabetes insipidus, with excessive production of dilute urine and frequent thirst.
Over secretion of ADH may lead to kidney problems.
Under secretion of oxytocin inhibits normal labour; over secretion can rupture the uterine wall.

68 marks

Name the hormones, their functions and imbalance of the Pituitary gland.

Solution

The pituitary gland is divided into an anterior lobe, a median lobe and a posterior lobe (p.119).

Lobe and hormone	Function	Imbalance
Anterior: growth hormone (GH)	Growth and development, protein synthesis, fat distribution	Under: dwarfism (short stature with normal body proportions and intelligence). Over in childhood: gigantism. Over in adults: acromegaly, with enlarged hands, feet, skull and jaw
Anterior: TSH	Stimulates the thyroid to make and release thyroid hormones	Over: hyperthyroidism (excess thyroxine). Under: hypothyroidism (lack of thyroxine)
Anterior: ACTH	Stimulates the adrenal cortex to secrete corticosteroids (cortisone and aldosterone)	The book names no imbalance of ACTH itself; disorders of the adrenal cortex, Addison's disease (under) and Cushing's disease (over), follow from abnormal corticosteroid levels (p.123)
Anterior: FSH and LH	Control the ovaries and testes, egg and sperm production, and the sex hormones	Under: young females fail to mature sexually; adults become sterile (estrogen). Under testosterone: male sterility and feminine characteristics
Anterior: prolactin	Initiates and maintains milk production; affects sex hormone levels	No imbalance is named in the book
Median lobe: MSH	Stimulates melanin production in the skin and hair; influences appetite and sex drive	No imbalance is named in the book
Posterior: ADH	Water retention by the kidneys; blood pressure	Under: diabetes insipidus. Over: kidney problems
Posterior: oxytocin	Contracts the uterus during labour; ejects milk	Under: inhibits normal labour. Over: rupture of the uterine wall

Note. Table 6.1 prints the sex-hormone function (control of estrogen, testosterone, eggs and sperm) against ACTH. That row is a misprint: ACTH stimulates the adrenal cortex to secrete corticosteroids (p.119).

72026 Annual I, Set B, Q.4 OR8 marks

Name the hormones, their functions and imbalance of the Thyroid gland.

Solution

The thyroid gland lies on either side of the trachea below the larynx and produces three active hormones: triiodothyronine (T3), thyroxine (T4) and calcitonin. T4 is about 90 per cent of the thyroid hormone, and T3 is about four times more potent (p.120-121).

Figure 6.3: Thyroid and parathyroid glands in front view, with the trachea and thyroid cartilage (schematic). One of the four parathyroid glands is shown on each side.

Hormone	Function	Imbalance
Thyroxine (T4) and triiodothyronine (T3)	Promote the basal metabolic rate; enhance glucose catabolism and cholesterol synthesis in the liver; promote development of the nervous system in the foetus and infants; act on muscles; promote growth and maturation of the skeleton; promote gut motility	Under (hypothyroidism): low metabolic rate, feeling chilled, puffy eyes, dry skin, hair loss, constipation, goitre; myxedema in adults; cretinism if congenital, with mental retardation, poor growth and delayed puberty. Over (hyperthyroidism): high metabolic rate, rapid irregular heartbeat, sweating, weight loss despite adequate food, exophthalmia; Graves' disease
Calcitonin (from C-cells)	Lowers blood calcium by increasing its deposition in bone and reducing its reabsorption by the kidney tubules; more important in childhood	Deficiency: calcium is not deposited in bones; high blood calcium disturbs muscles and nerves and may cause kidney stones (p.121)

86 marks

Name the hormones, their functions and imbalance of the Parathyroid glands.

Solution

There are four parathyroid glands, all on the posterior surface of the thyroid gland. Their hormone, parathormone (PTH), is the most important regulator of blood calcium, and its release is triggered by low blood calcium and inhibited by high blood calcium (p.122).

Calcium control: parathormone raises blood calcium (left) and calcitonin lowers it (right). Each hormone's release is switched off by the change it produces.

Hormone	Function	Imbalance
Parathormone (PTH)	Releases calcium from the bones and so raises blood calcium; works antagonistically to calcitonin	Over (usually a parathyroid tumour): calcium is released from bones, which soften, deform and fracture; hypercalcemia depresses the nervous system, weakens muscles and forms kidney stones. Under: hypocalcemia increases the excitability of neurons and can cause tetany, in which muscles stay contracted; untreated, it can be fatal

92026 Annual I, Set D, Q.48 marks

Name the hormones, their functions and imbalance of the Pancreas.

Solution

The pancreas has two kinds of tissue. Exocrine tissue (acinar cells) secretes digestive juice, and endocrine tissue forms the islets of Langerhans, which secrete insulin from beta cells and glucagon from alpha cells (p.122).

Figure 6.4: Islets of Langerhans among the acinar cells of the pancreas. Beta cells (insulin) form most of the islet, and alpha cells (glucagon) lie at its edge.

Hormone	Cells and function	Imbalance
Insulin	Beta cells; lowers blood glucose by increasing glucose uptake by muscle and fat cells, glycogenesis, use of glucose in respiration and conversion of glucose to fats; inhibits gluconeogenesis	Under: diabetes mellitus, with high glucose in the blood and urine (a metabolic disease). Over: glucose is used too quickly, its level falls in the blood and nerve and muscle functions are upset
Glucagon	Alpha cells; raises blood glucose by converting glycogen to glucose; released when glucose is low and also stimulated by the sympathetic nervous system; antagonistic to insulin	The book names no disorder caused by glucagon

108 marks

Name the hormones, their functions and imbalance of the Adrenal glands.

Solution

Each adrenal gland has an outer cortex and an inner medulla, and each rests on a kidney (p.123).

Figure 6.5: The adrenal gland in section. The cortex surrounds the medulla, which secretes adrenaline and noradrenaline.

Part and hormone	Function	Imbalance
Medulla: adrenaline (epinephrine) and noradrenaline (norepinephrine)	Released in emergencies under the sympathetic nervous system. Adrenaline increases heart rate, oxygen intake, bronchial dilation and blood flow to the heart and skeletal muscles; noradrenaline causes peripheral vasoconstriction. Their effects together raise blood pressure	Over: hypertension and aggressive behaviour in routine life. Under: failure to combat an emergency
Cortex: glucocorticoids (cortisone)	Regulate the blood glucose level, protein breakdown, muscle strength and bone strength; act as anti-inflammatory agents	Under: Addison's disease, with metabolic disturbance, muscle weakness and loss of salts; stress such as cold may cause collapse and death. Over: Cushing's disease, with excessive protein breakdown and muscle and bone weakness
Cortex: mineralocorticoids (aldosterone)	Regulates the balance of salts and water, and so the blood pressure	Under: loss of salts, as in Addison's disease
Cortex: androgens	Small amounts of androgen (testosterone-like) are made in both sexes	The book names no imbalance

(a) Gonads: name the hormones, their functions and imbalance.

The gonads are special endocrine glands that also produce gametes. The ovaries secrete estrogen and progesterone, and the testes secrete testosterone (p.123, p.125).

Hormone	Function	Imbalance
Estrogen (ovaries)	Develops female sexual characteristics at puberty; repairs the uterine wall after menstruation; protects bone health; secreted under the stimulation of FSH	Under in young females: fail to mature sexually. Under in adults: sterility. Over: fibroids in the uterus and polycystic ovaries
Progesterone (ovaries; corpus luteum and placenta)	Thickens and vascularises the uterine lining for pregnancy; inhibits further FSH secretion; prepares the breasts for milk	Under: early menstruation, lower chance of pregnancy, and miscarriage during pregnancy
Testosterone (testes)	Initiates maturation of male reproductive organs and secondary sexual characteristics; needed for sperm production and sex drive	Under: development of feminine characteristics and male sterility

Note. The book prints 'a. Gonads' after question 10 as a separate item. It is answered here as part (a) of this question.

118 marks

Describe the neurosecretory role of hypothalamus.

Solution

Location and neurosecretory cells

The hypothalamus is part of the forebrain and is the master control centre of the endocrine system. It contains neurosecretory cells, which conduct impulses and have a high secretory capacity. Their hormones are packed into granules in the cell body, carried down the axon by cytoplasmic streaming, and released from the axon endings into blood capillaries when the cell is stimulated (p.126).

Hypothalamic hormones

Releasing factors stimulate the secretion of pituitary hormones.
Inhibiting factors stop the secretion of pituitary hormones.
ADH and oxytocin are made by the neurosecretory cells, stored in the posterior lobe and released from there when needed.

Hormone from the hypothalamus	Response of the anterior pituitary
Growth hormone releasing factor (GHRF)	Secretion of growth hormone (GH)
Somatostatin	Inhibition of GH
Thyrotrophin releasing factor (TRF)	Secretion of TSH
Adrenocorticotrophin releasing factor (CRF)	Secretion of ACTH
Prolactin inhibiting factor (PIF)	Inhibits secretion of prolactin
Gonadotrophin releasing hormone (GnRH)	Secretion of FSH and LH

Pathway

A nerve impulse stimulates a neurosecretory cell in the hypothalamus.
Granules of hormone move down the axon to its endings.
The endings release the hormone into capillaries, which carry it to the anterior pituitary (releasing and inhibiting factors) or the posterior pituitary (ADH and oxytocin, released into the blood).
The pituitary hormones then act on the target glands and tissues.

Neurosecretory pathway: hypothalamic releasing and inhibiting factors reach the anterior pituitary through the blood, and ADH and oxytocin are released from the posterior pituitary.

128 marks

What is feedback mechanism? Describe the positive and negative feedback mechanism with reference to oxytocin, insulin and glucagon.

Solution

Feedback mechanism

A feedback mechanism is an interaction in which a controlling mechanism is itself controlled by the product of the reaction it controls. After a signal is received, the change is corrected either by depressing it with negative feedback or by enhancing it with positive feedback (p.127).

Positive feedback: oxytocin in childbirth

The head of the baby pushes against the cervix.
Stretch receptors in the cervix send nerve impulses to the brain.
The brain stimulates the pituitary gland to secrete oxytocin.
Oxytocin is carried in the bloodstream to the uterus.
Oxytocin stimulates stronger uterine contractions, which push the baby further towards the cervix, and the cycle repeats until the baby is born.

Figure 6.6: Positive feedback in childbirth: each stage increases the next until the baby is born.

Negative feedback: insulin and glucagon

In negative feedback the end product reverses the direction of the change and maintains homeostasis. When blood glucose rises, beta cells release insulin, which lowers it; when glucose falls, alpha cells release glucagon, which raises it (p.128).

Figure 6.7: Negative feedback in blood glucose control. Insulin (left) lowers glucose and glucagon (right) raises it; each hormone is switched off by the change it causes.

Comparison

Positive feedback	Negative feedback
Amplifies the change, pushing the system further from its set point	Counteracts the change, bringing the system back to its set point
Leads to a rapid, escalating response until an end point is reached	Maintains stability and homeostasis
Example: uterine contractions during childbirth	Example: blood glucose control

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 11 mark

Cortisol is also called:

AEpinephrine
BGlucocorticoidAnswer
CMineralocorticoid
DAndrogen

Why

Cortisol (cortisone in the book) is a glucocorticoid of the adrenal cortex: it helps to regulate the blood glucose level. Epinephrine comes from the adrenal medulla, the mineralocorticoid is aldosterone (salt and water balance), and androgen is only produced in small amounts by the cortex.

2026 Annual I, Set B, Q.2 (ix)1.5+1.5 = 3 marks

Compare Estrogen and Testosterone.

Solution

Both are sex hormones secreted by the gonads, and both are steroids. They differ as follows.

Estrogen compared with testosterone
Feature	Estrogen	Testosterone
Gland and cells	Ovary: Graafian follicle (stimulated by FSH)	Testis: interstitial cells (stimulated by ICSH, which is LH)
Sex	Female sex hormone	Male sex hormone
Functions	Develops the secondary sex characteristics in females; repairs and heals the uterine wall after menstruation	Starts maturation of male reproductive organs, secondary sex characteristics and sex drive at puberty; needed for normal sperm production
Feedback	Negative feedback on FSH	Maintains the reproductive organs in their mature state in the adult male
Under secretion	Girls fail to mature sexually; adults become sterile	Feminine characteristics develop and male sterility results

2026 Annual I, Set B, Q.4 OR2x3 = 6 marks

Describe three active hormones produced by thyroid gland.

Solution

The thyroid gland lies on either side of the trachea, below the larynx. It secretes three active hormones: tri-iodothyronine (T3), thyroxine (T4) and calcitonin (2 marks each).

Figure 6.3: Thyroid and parathyroid glands in front view, with the trachea and thyroid cartilage (schematic). One of the four parathyroid glands is shown on each side.

1. Tri-iodothyronine (T3)

Contains three atoms of iodine. About 10 per cent of the thyroid secretion; four times more potent than T4 but acts for a shorter time.
Controlled by TSH from the anterior pituitary.
Raises the basal metabolic rate, enhances glucose catabolism and cholesterol synthesis in the liver, and promotes growth of the skeleton and development of the nervous system and muscles.
Excess causes hyperthyroidism (Graves' disease); lack causes hypothyroidism.

2. Thyroxine (T4, tetra-iodothyronine)

Contains four atoms of iodine. About 90 per cent of the thyroid secretion; its action lasts four times longer than T3.
Has the same effects as T3: raises the basal metabolic rate, helps the development of the nervous system in the foetus and infants, and keeps the gut moving normally.
Under secretion: myxedema in adults, cretinism in infants, and goitre when iodine is lacking.

3. Calcitonin

Made by the C-cells of the thyroid; has no iodine.
Released when the blood calcium level is too high.
Lowers blood calcium: increases deposition of calcium in the bone matrix, reduces absorption of calcium by the intestine and its reabsorption by the kidney tubules.
If deficient, high blood calcium disturbs muscles and nerves and may cause kidney stones.

2026 Annual I, Set D, MCQ 21 mark

Which one of the following is NOT an endocrine gland?

APancreas
BParathyroid
CSalivary glandAnswer
DTestis

Why

An endocrine gland has no duct and releases hormones into the blood. The salivary gland is exocrine: it pours saliva into the mouth through a duct. The pancreas (islets of Langerhans), parathyroid and testis (testosterone) all secrete hormones into the blood; the pancreas is also exocrine, which makes it the tempting option.

2026 Annual I, Set D, Q.2 (ix)3 marks

Name the hormones secreted by anterior pituitary.

Solution

The anterior lobe of the pituitary gland secretes six hormones:

Growth hormone (GH, somatotrophic hormone, STH)
Thyroid-stimulating hormone (TSH)
Adrenocorticotropic hormone (ACTH)
Follicle-stimulating hormone (FSH)
Luteinising hormone (LH; interstitial cell stimulating hormone, ICSH, in males)
Prolactin (luteotrophic hormone, LTH)

2026 Annual I, Set D, Q.43+3 = 6 marks

Describe the location and function of pancreas as an endocrine gland.

Solution

Location (3 marks)

The pancreas is a gland lying in the upper abdomen, behind the stomach, in the curve of the duodenum.
It has two kinds of tissue: exocrine tissue (acinar cells) that makes digestive juice, and endocrine tissue, patches called the islets of Langerhans scattered through the gland.
The islets have no duct; they release their hormones directly into the blood, so the pancreas is both an exocrine and an endocrine gland.

Figure 6.4: Islets of Langerhans among the acinar cells of the pancreas. Beta cells (insulin) form most of the islet, and alpha cells (glucagon) lie at its edge.

Function as an endocrine gland (3 marks)

Hormones of the islets of Langerhans
	Insulin	Glucagon
Cells	Beta cells (more numerous)	Alpha cells (fewer)
Released when	Blood glucose is high, for example after a meal	Blood glucose is low
Effect on blood glucose	Lowers it to normal	Raises it
How	Increases glucose uptake by body cells (especially muscle and fat), promotes conversion of glucose to glycogen (glycogenesis), increases use of glucose in respiration, converts excess glucose to fat, inhibits gluconeogenesis	Acts opposite to insulin and reverses its activities
Imbalance	Under secretion: diabetes mellitus (high glucose in blood and urine); over secretion: blood glucose falls too low	None given in the book

Exam focus

Describe the hormones of the thyroid gland: T3, T4 and calcitonin, with functions and imbalance. (long)
Set B Q.4 OR (6 marks: three active hormones of the thyroid). Same as book long question 7.
Go to answer
Describe the pancreas as an endocrine gland: islets of Langerhans, insulin and glucagon, diabetes. (long)
Set D Q.4 (6 marks: location and function) and Set D MCQ 2 (pancreas is endocrine and exocrine). Same as book long question 9.
Go to answer
Compare estrogen and testosterone (gland, function, under secretion). (short)
Set B Q.2 (ix) (1.5 + 1.5 marks).
Go to answer
Name the hormones secreted by the anterior pituitary and say why it is the master gland. (short)
Set D Q.2 (ix). Same as book short questions 3 and 4.
Go to answer
Adrenal gland: medulla and cortex hormones, glucocorticoids and mineralocorticoids, Addison's and Cushing's diseases. (short)
Set B MCQ 1 (cortisol is a glucocorticoid); book long question 10 and MCQ 12 and 16.
Go to answer
Tell endocrine from exocrine glands and locate the endocrine glands in the body. (short)
Set D MCQ 2 (salivary gland is not endocrine); book short question 9 and MCQ 13.
Go to answer
Feedback mechanism: positive feedback (oxytocin) and negative feedback. (long)
Not asked in 2026 in this unit, but book long question 12 and short questions 10 and 11 ask it, and it is the link to homeostasis (unit 8).
Go to answer