Chemical Coordination and Integration | CBSE Class 11 Biology Notes

This note covers hormones and endocrine glands, hypothalamic and pituitary control, the pineal and thymus glands, thyroid and parathyroid functions, adrenal hormones, pancreatic and gonadal hormones, hormonal disorders, hormones of other organs, and mechanisms of hormone action.

What are hormones and why is chemical coordination needed?

The neural system, the body's system of nervous coordination, provides rapid, point-to-point coordination among organs. Its effects are fast but short-lived. Nerve fibres do not supply all body cells, yet cellular functions need continuous regulation. Hormones provide chemical coordination alongside nervous coordination.

Definition: Hormones are non-nutrient chemicals produced in trace amounts that act as intercellular messengers, carrying signals between cells.

Endocrine glands lack ducts, or channels for carrying secretions, and are therefore called ductless glands. Their secretions are hormones. The classical description involves a chemical released into blood and transported to a distant target organ. The broader definition also includes messengers produced outside organised endocrine glands.

What makes up the endocrine system?

The endocrine system includes endocrine glands and scattered hormone-producing tissues or cells. The pituitary, pineal, thyroid, parathyroid, adrenal glands, pancreas, thymus and gonads are organised endocrine bodies. Gonads are the reproductive organs: testes in males and ovaries in females.

The gastrointestinal tract, meaning the digestive tract, and organs including the liver, kidneys and heart also produce hormones. Thus, an organ can have hormonal functions alongside its other activities. Hormonal coordination is not restricted to structures whose main identity is an endocrine gland.

Invertebrates have simple endocrine systems with few hormones, whereas many chemicals act as hormones in vertebrates. In humans, the neural and endocrine systems jointly coordinate physiological functions, meaning the activities through which the body works. Hormones regulate metabolism, growth and development; metabolism comprises the body's chemical reactions.

The human outline labels hypothalamus, pituitary and pineal in the head, thyroid and parathyroid in the neck, thymus in the chest, and pancreas and adrenal glands in the abdomen. Ovary and testis are separately identified for female and male bodies.

See Fig. 19.1 in your NCERT textbook

How do the hypothalamus and pituitary coordinate hormone secretion?

The hypothalamus forms the basal part of the diencephalon, a region of the forebrain. It contains groups of neurosecretory cells, meaning nerve cells that produce hormones. These groups are called nuclei. Their hormones regulate the synthesis and secretion of pituitary hormones.

How does hypothalamic control reach the pituitary?

Releasing hormones stimulate pituitary hormone secretion, while inhibiting hormones inhibit it. Gonadotrophin releasing hormone (GnRH) stimulates pituitary synthesis and release of gonadotrophins, hormones that stimulate gonadal activity. Somatostatin inhibits release of growth hormone from the pituitary.

1. Hypothalamic neurosecretory cells produce releasing or inhibiting hormones.
2. These hormones pass along axons, the long processes of nerve cells.
3. The hormones are released from the nerve endings.
4. A portal circulatory system, a connecting blood circulation, carries them to the pituitary, where they regulate the anterior pituitary.

The posterior pituitary is under direct neural regulation by the hypothalamus. This differs from the portal circulation that carries hypothalamic regulatory hormones to the anterior pituitary.

Which parts and hormones belong to the pituitary?

The pituitary gland lies in a bony cavity called the sella tursica and joins the hypothalamus by a stalk. Its two anatomical divisions are the adenohypophysis and neurohypophysis. The adenohypophysis contains pars distalis and pars intermedia; the neurohypophysis is also called pars nervosa or posterior pituitary.

Pars distalis, commonly called the anterior pituitary, produces six hormones. Pars intermedia secretes melanocyte stimulating hormone (MSH). In humans, pars intermedia is almost merged with pars distalis. MSH regulates skin pigmentation by acting on melanocytes, the cells containing the pigment melanin.

Androgens are male sex hormones. Ovarian follicles are structures in the ovary that develop towards ovulation, the release of an ovum, or egg cell. Luteinising hormone induces ovulation of fully mature, or graafian, follicles. The corpus luteum is the structure formed from follicular remnants after ovulation.

Mammary glands are the milk-producing glands. The following hormones connect the anterior pituitary with growth, milk formation, thyroid and adrenal activity, and reproductive functions.

Excess GH causes abnormal growth called gigantism; low secretion produces stunted growth called pituitary dwarfism. Excess GH in adults, especially in middle age, can cause acromegaly, severe disfigurement especially of the face. It may cause serious complications and premature death if unchecked, and often remains undetected for years.

What does the posterior pituitary release?

The posterior pituitary stores and releases oxytocin and vasopressin. Both are actually synthesised in the hypothalamus and transported along axons. Oxytocin stimulates smooth muscle contraction, including vigorous uterine contractions during childbirth and milk ejection from mammary glands, the milk-producing glands.

Vasopressin acts mainly at the kidney, increasing reabsorption (return to the blood) of water and electrolytes by distal tubules, parts of the kidney's tubular system. Electrolytes are dissolved substances forming ions, electrically charged atoms or groups of atoms. Vasopressin reduces water loss through urine (diuresis), explaining its alternative name, antidiuretic hormone (ADH).

Impaired ADH synthesis or release reduces the kidney's ability to conserve water, causing water loss and dehydration, the loss of body water. This condition is diabetes insipidus.

The drawing labels hypothalamic neurons above the anterior and posterior pituitary. It shows the portal circulation leading towards the anterior region and long neuronal processes extending towards the posterior region.

See Fig. 19.2 in your NCERT textbook

How do the pineal gland and thymus influence body rhythms and immunity?

What is the role of melatonin?

The pineal gland lies on the dorsal, or back, side of the forebrain. It secretes melatonin, a hormone with a very important role in regulating the body's 24-hour, or diurnal, rhythm. This rhythm includes the recurring pattern of sleep and wakefulness.

Melatonin helps maintain normal sleep-wake and body-temperature rhythms. It also influences metabolism, pigmentation, the menstrual cycle and defence capability. The menstrual cycle is the recurring reproductive cycle in females. These actions extend beyond regulating the timing of sleep alone.

How do thymosins support immune responses?

The thymus is a lobular gland, consisting of lobes, between the lungs and behind the sternum, or breastbone. It lies on the ventral, or front, side of the aorta, the major artery leaving the heart. It has a major role in immune-system development.

The thymus secretes thymosins, peptide hormones. Peptides are chains of amino acids, the building units of proteins. Thymosins support differentiation, or specialised development, of T-lymphocytes. These immune cells provide cell-mediated immunity, defence carried out through cells.

Thymosins also promote the production of antibodies, defensive proteins, providing humoral immunity, or antibody-mediated defence. Thus, the hormone group supports both cell-mediated and humoral immune responses. In old individuals, degeneration of the thymus decreases thymosin production, and immune responses become weak.

The two glands therefore have distinct characteristic roles: pineal melatonin helps regulate diurnal rhythms, while thymic thymosins support immune development. Melatonin also influences defence capability, but the differentiation of T-lymphocytes is specifically associated with thymosins.

How do thyroid and parathyroid hormones regulate metabolism and calcium?

What does the thyroid produce?

The thyroid gland has two lobes, one on either side of the trachea, or windpipe. A thin connective-tissue flap called the isthmus joins them. Thyroid follicles consist of follicular cells enclosing a cavity; stromal tissues form the supporting tissue between these structures.

Follicular cells synthesise tetraiodothyronine, also called thyroxine (T₄), and triiodothyronine (T₃). Iodine is essential for their normal synthesis. These hormones regulate the basal metabolic rate, the body's basic rate of energy use, and control carbohydrate, protein and fat metabolism.

Thyroid hormones support red blood cell formation and influence water and electrolyte balance. They also contribute to development and maturation of the central neural system. The thyroid additionally produces the protein hormone thyrocalcitonin (TCT), which lowers blood calcium.

What happens when thyroid activity changes?

Iodine deficiency causes hypothyroidism, reduced thyroid hormone activity, and enlargement of the thyroid called goitre. During pregnancy, hypothyroidism causes defective development and maturation of the growing baby. Effects include stunted growth called cretinism, intellectual impairment, low intelligence quotient, abnormal skin and deaf-mutism.

Intelligence quotient is a measure of intellectual ability; deaf-mutism involves hearing and speech impairment. In adult women, hypothyroidism may make the menstrual cycle irregular. This disturbance of the menstrual cycle is an additional possible effect of reduced thyroid activity.

Hyperthyroidism is abnormally high thyroid hormone synthesis and secretion, which can arise through thyroid cancer or thyroid nodules, localised growths in the gland. Exophthalmic goitre, also called Graves' disease, is a form characterised by thyroid enlargement, protruding eyeballs, increased basal metabolic rate and weight loss.

How does parathyroid hormone affect calcium balance?

Humans have four parathyroid glands on the back of the thyroid, one pair in each lobe. They secrete parathyroid hormone (PTH), a peptide whose secretion is regulated by circulating calcium ions. Ca²⁺ denotes a calcium ion with two positive charges.

PTH increases blood Ca²⁺ and is therefore hypercalcaemic, meaning blood-calcium-raising. It stimulates bone resorption, the dissolution or demineralisation of bone. It also increases calcium reabsorption by renal tubules, the kidney tubules, and calcium absorption from digested food. PTH and TCT together contribute to calcium balance.

The ventral view labels the thyroid, trachea and vocal cord. The dorsal view marks four parathyroid glands as small red structures on the back of the thyroid lobes.

See Fig. 19.3 in your NCERT textbook

How do adrenal hormones coordinate stress responses and internal balance?

One adrenal gland lies above each kidney. Each has an outer adrenal cortex and a central adrenal medulla. The cortex has three layers: zona glomerulosa outside, zona fasciculata in the middle and zona reticularis inside. Learn them in this order, from outside to inside.

What does the adrenal medulla secrete?

The medulla secretes adrenaline, also called epinephrine, and noradrenaline, also called norepinephrine. These hormones are collectively called catecholamines. Their rapid secretion during stress of any kind and emergency situations gives them the names emergency hormones or hormones of fight or flight.

Catecholamines increase alertness, pupil dilation, sweating and piloerection, the raising of hairs. They increase heartbeat, the strength of heart contraction and respiration rate. They stimulate breakdown of glycogen, a stored carbohydrate, raising blood glucose, the sugar circulating in blood, and also stimulate the breakdown of lipids, or fats, and proteins.

How do cortical hormones differ?

Hormones of the adrenal cortex are called corticoids. Glucocorticoids are involved in carbohydrate metabolism; cortisol is the main glucocorticoid in humans. Mineralocorticoids regulate water and electrolyte balance; aldosterone is the main mineralocorticoid.

Glucocorticoids stimulate gluconeogenesis, formation of glucose from non-carbohydrate materials; lipolysis, fat breakdown; and proteolysis, protein breakdown. They inhibit cellular uptake and use of amino acids. Cortisol helps maintain cardiovascular, meaning heart and blood vessel, functions and kidney functions.

Glucocorticoids, particularly cortisol, produce anti-inflammatory reactions, reducing inflammation, and suppress immune responses. Cortisol also stimulates red blood cell production. Underproduction of adrenal cortical hormones alters carbohydrate metabolism, causing acute weakness and fatigue associated with Addison's disease.

Aldosterone acts mainly at renal tubules. It stimulates reabsorption of Na⁺, the positively charged sodium ion, and water, while increasing excretion of K⁺, the positively charged potassium ion, and phosphate ions. It helps maintain electrolytes, body-fluid volume, osmotic pressure and blood pressure.

Osmotic pressure is the pressure needed to prevent net solvent movement across a selectively permeable membrane due to a concentration difference. Steroids are compounds with a characteristic four-ring chemical structure. Small amounts of androgenic steroids from the cortex also contribute to hair growth during puberty, the period of sexual maturation, including pubic and facial hair.

Part (a) shows an adrenal gland above a kidney. Part (b) is a section of the gland, with the outer adrenal cortex and inner adrenal medulla labelled.

See Fig. 19.4 in your NCERT textbook

How do insulin and glucagon maintain blood glucose?

The pancreas is a composite gland with endocrine and exocrine functions. Exocrine glands deliver secretions through ducts. Its endocrine component consists of clusters called the Islets of Langerhans. A normal human pancreas contains about 1 to 2 million islets, representing only 1 to 2 per cent of pancreatic tissue.

The two main cell types are α-cells, read as alpha cells, and β-cells, read as beta cells. Alpha cells secrete glucagon; beta cells secrete insulin. Both hormones are peptides and participate in glucose homeostasis, the maintenance of blood glucose balance.

Why is glucagon a glucose-raising hormone?

Glucagon acts mainly on hepatocytes, or liver cells. It stimulates glycogenolysis, the breakdown of glycogen, a stored form of carbohydrate. This increases blood glucose, a condition called hyperglycaemia. Glucagon also stimulates gluconeogenesis and reduces cellular glucose uptake and utilisation.

These actions explain why glucagon is called hyperglycaemic: it raises blood glucose. Its effects include both processes that increase glucose availability and reduced removal and use of glucose by cells.

How does insulin produce the opposite effect?

Insulin acts mainly on hepatocytes and adipocytes, the cells of adipose or fat tissue. It increases cellular glucose uptake and utilisation. Glucose rapidly moves from blood into these cells, reducing blood glucose. This fall is called hypoglycaemia.

Insulin also stimulates glycogenesis, conversion of glucose into glycogen in target cells. A target cell is a cell on which a hormone acts through an appropriate receptor, a specific hormone-binding protein. Blood glucose homeostasis is maintained jointly by insulin and glucagon.

Prolonged hyperglycaemia leads to diabetes mellitus, associated with glucose loss through urine and formation of harmful compounds called ketone bodies. Insulin deficiency and/or insulin resistance, reduced responsiveness to insulin, result in this disorder. Insulin therapy is used successfully to treat diabetic patients.

Note: Diabetes mellitus involves disturbed glucose regulation. Diabetes insipidus involves impaired ADH synthesis or release and reduced kidney water conservation. The shared word diabetes does not make their hormonal causes identical.

What hormonal functions do testes and ovaries perform?

How do androgens act in males?

A pair of testes lies in the scrotal sac outside the abdomen. Each testis functions as a primary sex organ and an endocrine gland. It contains seminiferous tubules, the sperm-forming tubules, and stromal or interstitial tissue between them.

Leydig cells, also called interstitial cells, occupy the spaces between tubules. They produce androgens, mainly testosterone. These hormones regulate development, maturation and function of male accessory sex organs, the associated reproductive organs and ducts.

Androgens stimulate muscular growth, facial and axillary, or armpit, hair growth, aggressiveness and a low-pitched voice. They strongly stimulate spermatogenesis, the formation of spermatozoa or sperm cells. FSH and androgens together regulate this process.

Androgens act on the central neural system and influence libido, male sexual behaviour. They also produce anabolic, meaning synthetic or building, effects on protein and carbohydrate metabolism. Thus, their effects include reproductive development, behaviour and metabolic activity.

How do ovarian hormones act in females?

The paired ovaries lie in the abdomen. An ovary is a primary female sex organ, producing one ovum, or egg cell, during each menstrual cycle. It also produces two groups of steroid hormones, hormones belonging to the steroid chemical class: oestrogens and progesterone.

Oestrogens are synthesised and secreted mainly by growing ovarian follicles. They stimulate growth and activity of female secondary sex organs, follicular development, mammary gland development and secondary sex characters, meaning bodily features associated with sexual maturation, such as a high-pitched voice. They also regulate female sexual behaviour.

After ovulation, the ruptured follicle becomes the corpus luteum, which secretes mainly progesterone. Progesterone supports pregnancy and acts on mammary glands. It stimulates formation of alveoli, the sac-like structures that store milk, and milk secretion.

The sites and actions are therefore linked: growing follicles mainly secrete oestrogens, while the corpus luteum mainly secretes progesterone. These secretions connect ovarian changes with hormonal support of reproductive functions. Prolactin regulates milk formation, whereas oxytocin stimulates milk ejection.

Which hormones are produced by the heart, kidneys and digestive tract?

Hormone secretion also occurs in tissues that are not organised endocrine glands. The heart, kidneys and gastrointestinal tract provide examples. Their hormones influence blood pressure, blood cell production and digestive activity, showing how chemical coordination connects different body functions.

How are blood pressure and blood cell formation affected?

The atrial wall, the wall of an upper heart chamber, produces atrial natriuretic factor (ANF), a peptide that decreases blood pressure. Its sequence of action is:

1. Blood pressure increases.
2. The atrial wall secretes ANF in response.
3. ANF causes dilation, or widening, of blood vessels.
4. This vessel dilation reduces blood pressure.

Juxtaglomerular cells of the kidney secrete erythropoietin. This peptide stimulates erythropoiesis, meaning formation of red blood cells. Its role differs from the direct blood-pressure-lowering action described for ANF.

What do the four major digestive hormones do?

Endocrine cells in different gastrointestinal regions secrete four major peptide hormones. Their targets and effects differ, so the hormone name should be connected with both the organ acted upon and the secretion or activity it changes.

Growth factors are hormones from several other non-endocrine tissues, essential for normal tissue growth, repair and regeneration.

How do hormones act through receptors and second messengers?

Hormone receptors are specific proteins located in target tissues. Each receptor is specific to one hormone only. Hormone binding forms a hormone-receptor complex, which produces biochemical changes, regulating the target tissue's metabolism and physiological functions.

How are hormones grouped by chemical nature?

The chemical groups are peptide, polypeptide and protein hormones; steroids; iodothyronines; and amino-acid derivatives. Polypeptides are amino-acid chains; steroids are a chemically distinct hormone group; iodothyronines are thyroid hormones; amino-acid derivatives are compounds formed from amino acids.

• Peptide, polypeptide and protein hormones: insulin, glucagon, pituitary hormones and hypothalamic hormones.
• Steroids: cortisol, testosterone, oestradiol and progesterone. Oestradiol is an oestrogen.
• Iodothyronines: the thyroid hormones.
• Amino-acid derivatives: epinephrine is an example.

What happens at membrane-bound receptors?

Membrane-bound receptors are situated on the target cell's membrane. Hormones interacting with them normally do not enter the target cell. They generate second messengers, intracellular signals that regulate cellular metabolism. Examples are cyclic adenosine monophosphate (cyclic AMP), inositol trisphosphate (IP₃) and Ca²⁺.

1. A hormone, such as FSH, reaches its specific membrane-bound receptor.
2. The hormone binds to the receptor on the target cell membrane.
3. This interaction generates a second messenger inside the cell.
4. The messenger brings about biochemical responses that regulate cellular metabolism.
5. Physiological responses follow, such as ovarian growth in the FSH example.

FSH is shown binding at a receptor in an ovarian cell membrane. Downward arrows connect this interaction with second-messenger generation, biochemical responses and physiological responses, with ovarian growth given as the example.

See Fig. 19.5a in your NCERT textbook

How do intracellular receptors differ?

Intracellular receptors lie inside target cells and are mostly nuclear receptors, located in the nucleus. Steroid hormones and iodothyronines interact with intracellular receptors. They mostly regulate gene expression, the use of genetic information, or chromosome function through interaction of the hormone-receptor complex with the genome, the cell's genetic material.

The cumulative biochemical actions produce physiological and developmental effects. In the illustrated steroid mechanism, oestrogen enters a uterine cell, a cell of the uterus, and the hormone-receptor complex interacts with the genome. The sequence includes messenger ribonucleic acid (mRNA), which carries genetic instructions, and proteins.

Arrows show oestrogen crossing the uterine cell membrane towards a hormone-receptor complex in the nucleus. The labelled sequence runs from genome to mRNA, proteins and physiological responses, identified as tissue growth and differentiation.

See Fig. 19.5b in your NCERT textbook

Receptor location therefore separates two mechanisms: membrane interactions generate second messengers, while intracellular complexes mostly influence gene expression or chromosome function. Both mechanisms ultimately connect hormone recognition with biochemical changes and physiological responses in the target tissue.

Glossary

• Hormone — A non-nutrient chemical produced in trace amounts that carries messages between cells.
• Endocrine gland — A ductless gland whose secretions are hormones involved in chemical coordination.
• Releasing hormone — A hypothalamic hormone that stimulates the secretion of pituitary hormones.
• Gonadotrophins — Pituitary hormones, LH and FSH, which stimulate activity of the gonads.
• Diurnal rhythm — A 24-hour body rhythm, including normal sleep-wake and body-temperature rhythms.
• Goitre — Enlargement of the thyroid gland, associated with iodine deficiency in hypothyroidism.
• Hypercalcaemic hormone — A hormone that increases blood calcium levels, as parathyroid hormone does.
• Catecholamines — Adrenaline and noradrenaline, hormones rapidly secreted by the adrenal medulla during stress.
• Glycogenolysis — Breakdown of glycogen, stimulated by glucagon and contributing to increased blood glucose.
• Glycogenesis — Conversion of glucose into glycogen in target cells, stimulated by insulin.
• Spermatogenesis — Formation of spermatozoa, regulated by FSH and androgens in males.
• Corpus luteum — Structure formed from the ruptured ovarian follicle after ovulation, secreting mainly progesterone.
• Erythropoiesis — Formation of red blood cells, stimulated by the kidney hormone erythropoietin.
• Hormone receptor — A specific protein in target tissue that binds a hormone to initiate biochemical changes.
• Second messenger — An intracellular signal generated after hormone interaction with a membrane-bound receptor, regulating cellular metabolism.

Common errors and misconceptions

• Misconception: Every hormone comes from an organised endocrine gland. Correct: Hormone-producing cells and tissues also occur in organs such as the heart, kidneys and gastrointestinal tract.
• Misconception: The posterior pituitary synthesises oxytocin and vasopressin. Correct: It stores and releases these hormones, which are synthesised in the hypothalamus and transported along axons.
• Misconception: Prolactin and oxytocin have identical roles in milk release. Correct: Prolactin regulates milk formation, while oxytocin stimulates milk ejection from mammary glands.
• Misconception: PTH lowers blood calcium. Correct: PTH raises blood calcium through effects on bone, kidney tubules and absorption from digested food; TCT lowers blood calcium.
• Misconception: Insulin and glucagon both increase blood glucose. Correct: Glucagon increases blood glucose, while insulin enhances glucose uptake and glycogenesis, lowering blood glucose.
• Misconception: Diabetes insipidus and diabetes mellitus have the same hormonal basis. Correct: The former involves impaired ADH synthesis or release; the latter involves insulin deficiency and/or insulin resistance.
• Misconception: The adrenal medulla secretes cortisol and aldosterone. Correct: These are adrenal cortical hormones. The medulla secretes adrenaline and noradrenaline, the emergency hormones.
• Misconception: Every hormone must enter a cell to act. Correct: Hormones using membrane-bound receptors normally do not enter target cells; they generate second messengers inside them.

Exam-style questions with model answers

Q1. Define a hormone and state why endocrine glands are called ductless glands. Give one point for each. [2 marks]

1. A hormone is a non-nutrient chemical produced in trace amounts that acts as an intercellular messenger.
2. Endocrine glands are called ductless because they lack ducts; the secretions of these glands are hormones.

Q2. Distinguish hormone synthesis from storage and release for oxytocin and vasopressin. Then state one action of each hormone. [3 marks]

1. Oxytocin and vasopressin are synthesised in the hypothalamus and transported along axons to the posterior pituitary, which stores and releases them.
2. Oxytocin stimulates smooth muscle contraction, including vigorous contraction of the uterus during childbirth.
3. Vasopressin acts mainly at the kidney, stimulating reabsorption of water and electrolytes by distal tubules and reducing water loss through urine.

Q3. Explain why parathyroid hormone is called hypercalcaemic. State its overall effect and its actions on bone, renal tubules and calcium absorption from food. [4 marks]

1. Parathyroid hormone is hypercalcaemic because it increases the concentration of calcium ions in blood, contributing to calcium balance.
2. It acts on bones and stimulates bone resorption, meaning dissolution or demineralisation of bone.
3. It stimulates reabsorption of calcium ions by renal tubules, the tubular structures of the kidneys.
4. It increases absorption of calcium from digested food, also contributing to its overall blood-calcium-raising action.

Q4. Compare glucagon and insulin in five points: cell of origin, chemical nature, main target cells, cellular glucose uptake and utilisation, and metabolic actions with their effect on blood glucose. [5 marks]

1. Glucagon is secreted by alpha cells of the pancreatic Islets of Langerhans, whereas insulin is secreted by beta cells of those islets.
2. Both glucagon and insulin are peptide hormones, although their effects on blood glucose differ and jointly maintain blood glucose homeostasis.
3. Glucagon acts mainly on hepatocytes, or liver cells. Insulin acts mainly on hepatocytes and adipocytes, the cells of adipose tissue.
4. Glucagon reduces cellular glucose uptake and utilisation. Insulin enhances these processes, causing rapid movement of glucose from blood into its main target cells.
5. Glucagon stimulates glycogenolysis and gluconeogenesis, raising blood glucose. Insulin stimulates glycogenesis, the conversion of glucose into glycogen, and lowers blood glucose.

Q5. Explain membrane-bound hormone action in five points, using follicle stimulating hormone (FSH) and ovarian growth as the example. Include receptor binding, cell entry, second messengers, biochemical changes and the physiological response. [5 marks]

1. FSH interacts with a specific hormone receptor situated on the ovarian target cell membrane, forming a hormone-receptor complex at the membrane.
2. Hormones that interact with membrane-bound receptors normally do not enter the target cell. Their membrane interaction initiates the response inside the cell.
3. The interaction generates second messengers inside the target cell. Cyclic adenosine monophosphate, called cyclic AMP, and calcium ions are examples in this mechanism.
4. Second messengers bring about biochemical responses and regulate cellular metabolism, linking the hormone's binding outside the cell with changes within it.
5. The biochemical responses produce physiological effects. In the FSH example, ovarian growth represents the resulting physiological response of the target tissue.

Q6. State one principal action for each of these gastrointestinal hormones: gastrin, secretin, cholecystokinin and gastric inhibitory peptide. Include the target organ or tissue for gastrin, secretin and cholecystokinin. [4 marks]

1. Gastrin acts on gastric glands and stimulates secretion of hydrochloric acid and pepsinogen, the inactive precursor of pepsin.
2. Secretin acts on the exocrine pancreas and stimulates secretion of water and bicarbonate ions.
3. Cholecystokinin acts on the pancreas and gall bladder, stimulating secretion of pancreatic enzymes and bile juice, respectively.
4. Gastric inhibitory peptide inhibits gastric secretion and motility, that is, the secretory and movement activities of the stomach.

Q7. Distinguish diabetes insipidus from diabetes mellitus by their hormonal basis and the associated disturbance of water or glucose balance. Give one point for each disorder. [2 marks]

1. Diabetes insipidus follows impaired ADH synthesis or release, reducing kidney water conservation and causing water loss and dehydration.
2. Diabetes mellitus involves insulin deficiency and/or insulin resistance, with prolonged high blood glucose, glucose loss in urine and harmful ketone-body formation.

Q8. Describe thyroid gland structure and hormone production in three points: arrangement of its lobes, structure of its follicles, and the two follicular hormones with their iodine requirement. [3 marks]

1. The thyroid has two lobes, one on each side of the trachea. They are connected by a thin connective-tissue flap called the isthmus.
2. The gland contains follicles and stromal tissues. Each thyroid follicle consists of follicular cells surrounding a cavity.
3. Follicular cells synthesise tetraiodothyronine, or thyroxine, and triiodothyronine. Iodine is essential for the normal rate of synthesis of these thyroid hormones.

Key takeaways

• Hormones are trace non-nutrient messengers; endocrine glands and hormone-producing tissues jointly provide chemical coordination alongside the neural system.
• Hypothalamic releasing and inhibiting hormones regulate anterior pituitary activity, while the posterior pituitary stores and releases hypothalamic oxytocin and vasopressin.
• Thyroid hormones regulate metabolism; thyrocalcitonin lowers blood calcium, while parathyroid hormone raises it through several complementary actions.
• The adrenal medulla secretes emergency catecholamines; the adrenal cortex supplies glucocorticoids, mineralocorticoids and small amounts of androgenic steroids.
• Pancreatic alpha cells secrete glucose-raising glucagon, whereas beta cells secrete glucose-lowering insulin; together they maintain glucose homeostasis.
• Testicular androgens and ovarian oestrogens and progesterone regulate reproductive development, reproductive functions and several other bodily processes.
• Heart, kidney and gastrointestinal hormones regulate blood pressure, red blood cell formation and digestive secretion or activity.
• Membrane-bound hormone receptors generate second messengers; intracellular hormone-receptor complexes mostly regulate gene expression or chromosome function.

Test yourself

Which hypothalamic hormone inhibits release of growth hormone?

Somatostatin from the hypothalamus inhibits growth hormone release from the pituitary, illustrating the action of an inhibiting hormone.

Which hormone helps maintain the normal sleep-wake rhythm?

Melatonin from the pineal gland helps maintain the body's normal 24-hour rhythms, including the sleep-wake cycle and body-temperature rhythm.

How does the thymus support both cell-mediated and humoral immunity?

Thymosins support T-lymphocyte differentiation for cell-mediated immunity and promote antibody production for humoral immunity. Thymus degeneration in old individuals reduces thymosin production.

What are the adrenal cortex layers from outside to inside?

The outer layer is zona glomerulosa, the middle layer is zona fasciculata and the inner layer is zona reticularis.

What is the difference between glycogenolysis and glycogenesis?

Glycogenolysis is glycogen breakdown, stimulated by glucagon. Glycogenesis is conversion of glucose into glycogen, stimulated by insulin in target cells.

Which ovarian structure secretes mainly progesterone?

The corpus luteum, formed from the ruptured follicle after ovulation, secretes mainly progesterone, which supports pregnancy and acts on mammary glands.

What triggers ANF secretion, and how does it lower blood pressure?

Increased blood pressure triggers ANF secretion from the atrial wall. ANF causes dilation of blood vessels, which reduces blood pressure.

Where are intracellular hormone receptors mostly located?

Intracellular receptors are mostly nuclear receptors, located in the nucleus. Hormone-receptor complexes mostly regulate gene expression or chromosome function.