Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Chapter Introduction
Chapter 22 — Chemical Coordination and Integration belongs to the Human Physiology unit of NCERT Class 11 Biology. While the nervous system provides point-to-point rapid coordination, the endocrine system provides slower, diffuse, and long-lasting chemical coordination through hormones. This chapter covers all the major endocrine glands of the human body — hypothalamus, pituitary, thyroid, parathyroid, thymus, adrenal, pancreas, pineal, and gonads — along with their hormones, mechanisms of action, feedback regulation, and the disorders that arise from hormonal imbalances. Understanding chemical coordination is essential for NEET and board exams, as questions frequently test gland-hormone-disorder linkages.
Key Definitions
1. Hormone — A non-nutrient chemical messenger produced in trace amounts by endocrine glands; it is transported by blood and acts on specific target cells at a distance from its source. 2. Endocrine gland — A ductless gland that secretes hormones directly into the surrounding interstitial fluid and then into the bloodstream for transport to distant targets. 3. Exocrine gland — A gland with ducts that carries its secretion (sweat, saliva, gastric juice, digestive enzymes) to the target site or body surface. 4. Target cell — A cell that possesses specific receptors (protein molecules) on its surface or inside it that can bind a particular hormone, enabling that hormone to exert its effect. 5. Second messenger — An intracellular signalling molecule (cAMP, IP3, Ca2+, DAG, cGMP) that is produced or activated when a peptide hormone binds to a cell-surface receptor; it relays and amplifies the signal inside the cell. 6. Genomic mechanism — The mechanism of hormone action where the hormone (steroid/thyroid) enters the cell, binds to an intracellular receptor, and the hormone-receptor complex directly interacts with DNA to alter gene expression and produce new proteins. 7. Negative feedback — A regulatory mechanism in which the output of a process decreases when the regulated variable reaches its set point, maintaining homeostasis; this is the most common feedback loop in endocrine regulation. 8. Positive feedback — A regulatory mechanism in which the output increases, amplifying the response until a specific event terminates it; rare in the body (e.g., oxytocin during labour). 9. Goitre — Abnormal enlargement of the thyroid gland, commonly caused by iodine deficiency (simple goitre) or autoimmune stimulation (Graves' disease). 10. Diabetes mellitus — A metabolic disorder characterised by chronic hyperglycaemia due to either insulin deficiency (Type I) or insulin resistance (Type II), leading to glycosuria, polyuria, and polydipsia. 11. Tropic hormone — A hormone whose primary function is to stimulate another endocrine gland to secrete its own hormone (e.g., TSH stimulates the thyroid, ACTH stimulates the adrenal cortex, FSH and LH stimulate the gonads).
Detailed Concepts
Hypothalamus
The hypothalamus forms the basal part of the diencephalon and is the master regulator of the endocrine system. It controls pituitary secretion through two mechanisms:
Releasing hormones (stimulate adenohypophysis):
- GnRH (Gonadotropin-Releasing Hormone) — stimulates FSH and LH secretion
- TRH (Thyrotropin-Releasing Hormone) — stimulates TSH secretion
- CRH (Corticotropin-Releasing Hormone) — stimulates ACTH secretion
- GHRH (Growth Hormone-Releasing Hormone) — stimulates GH secretion
Inhibiting hormones (suppress adenohypophysis):
- Somatostatin (GHIH) — inhibits GH secretion
- Dopamine / Prolactin-Inhibiting Factor (PIF) — inhibits prolactin secretion
The hypothalamus also produces oxytocin and ADH (Vasopressin), which are transported down the axons of neurosecretory cells and stored in the posterior pituitary for release.
Neurosecretory cells link the hypothalamus to the pituitary via:
- The hypothalamo-hypophyseal portal system (blood vessel link to anterior pituitary)
- The hypothalamo-hypophyseal nerve tract (axon link to posterior pituitary)
Pituitary Gland (Hypophysis)
The pituitary is located in the sella turcica of the sphenoid bone and is attached to the hypothalamus by a stalk called the infundibulum. It has two main divisions:
| Hormone | Part | Target | Function |
|---|---|---|---|
| GH (Growth Hormone / Somatotropin) | Adenohypophysis | All body cells | Growth, protein synthesis, cell division; excess in childhood → gigantism; excess in adults → acromegaly; deficiency in childhood → dwarfism |
| PRL (Prolactin) | Adenohypophysis | Mammary glands | Stimulates lactation (milk production) |
| TSH (Thyroid-Stimulating Hormone) | Adenohypophysis | Thyroid gland | Stimulates synthesis and secretion of T3 and T4 |
| ACTH (Adrenocorticotropic Hormone) | Adenohypophysis | Adrenal cortex | Stimulates glucocorticoid (cortisol) secretion |
| LH (Luteinising Hormone) | Adenohypophysis | Gonads | Males: stimulates Leydig cells → testosterone; Females: ovulation, corpus luteum formation |
| FSH (Follicle-Stimulating Hormone) | Adenohypophysis | Gonads | Males: stimulates spermatogenesis via Sertoli cells; Females: follicular development and estrogen synthesis |
| MSH (Melanocyte-Stimulating Hormone) | Adenohypophysis (pars intermedia) | Melanocytes | Regulates skin pigmentation and melanin synthesis |
| Oxytocin | Neurohypophysis (stored and released) | Uterus, mammary glands | Uterine contraction during labour (positive feedback loop); milk ejection reflex |
| Vasopressin / ADH | Neurohypophysis (stored and released) | Kidneys (DCT, collecting duct) | Promotes water reabsorption; deficiency → diabetes insipidus (large volumes of dilute urine) |
Key point: Oxytocin and ADH are produced in the hypothalamus but stored and released from the posterior pituitary. The posterior pituitary does not synthesise any hormones of its own.
Thyroid Gland
The thyroid gland is located in the neck, on either side of the trachea, with two lobes connected by a thin strip of tissue called the isthmus.
Follicular cells produce:
- T4 (Thyroxine / Tetraiodothyronine) — 4 iodine atoms
- T3 (Triiodothyronine) — 3 iodine atoms; 3–5 times more active than T4
Iodine is actively taken up by the thyroid follicular cells. T4 is converted to T3 in target tissues.
Functions of thyroid hormones:
- Regulation of basal metabolic rate (BMR)
- Promotion of protein synthesis and normal growth
- Essential for CNS development in the foetus and newborn
- Stimulation of red blood cell production
- Support of skeletal development
- Regulation of carbohydrate, fat, and protein metabolism
Calcitonin is secreted by parafollicular C cells (between the thyroid follicles). It lowers blood calcium levels by promoting calcium deposition in bones and reducing Ca2+ reabsorption in kidneys. It is antagonistic to PTH.
Thyroid Disorders:
| Disorder | Cause | Symptoms |
|---|---|---|
| Goitre (simple) | Iodine deficiency → low T3/T4 → increased TSH (negative feedback removed) → thyroid enlargement | Swollen neck |
| Cretinism | Hypothyroidism during pregnancy or childhood | Intellectual disability, stunted growth, low BMR, coarse skin |
| Myxoedema | Hypothyroidism in adults | Low BMR, weight gain, lethargy, puffy face, cold intolerance, dry skin |
| Graves' disease / Exophthalmic goitre | Hyperthyroidism (autoimmune — antibodies mimic TSH and overstimulate thyroid) | High BMR, weight loss, bulging eyes (exophthalmos), tachycardia, anxiety, tremors |
Parathyroid Gland
There are four parathyroid glands embedded on the posterior surface of the thyroid gland.
PTH (Parathyroid Hormone) is a hypercalcaemic hormone that increases blood Ca2+ levels through three mechanisms:
1. Stimulates osteoclasts — increases bone resorption, releasing Ca2+ into the blood 2. Increases Ca2+ reabsorption in the renal tubules — reduces calcium loss in urine 3. Stimulates calcitriol synthesis (active vitamin D) in the kidneys — enhances Ca2+ absorption from the intestine
PTH and calcitonin form an antagonistic pair: PTH raises blood Ca2+, calcitonin lowers it. Together they maintain calcium homeostasis.
Thymus
- Located in the upper chest, behind the sternum (anterior mediastinum)
- Largest and most active at puberty, then gradually regresses with age (involution) and is replaced by fat
- Secretes thymosins, a group of hormones that promote the maturation and differentiation of T-lymphocytes
- Essential for cell-mediated immunity
- Decreased thymosin production in old age leads to a weakened immune response, making the elderly more susceptible to infections
Adrenal Gland
The adrenal glands sit superior to each kidney (hence "suprarenal"). Each gland has two distinct regions: an outer cortex and an inner medulla, which differ in embryonic origin, structure, and function.
Adrenal Cortex (3 zones — mnemonic: GFR)
| Zone | Hormone Type | Example | Function |
|---|---|---|---|
| Zona glomerulosa | Mineralocorticoids | Aldosterone | Na+ reabsorption and K+ excretion in kidneys; maintains blood pressure and electrolyte balance |
| Zona fasciculata | Glucocorticoids | Cortisol | Gluconeogenesis, anti-inflammatory, immunosuppressive, stress response; maintains blood glucose |
| Zona reticularis | Sex corticoids (androgens) | Dehydroepiandrosterone (DHEA) | Supplementary role in developing secondary sex characteristics; precursors to gonadal steroids |
Adrenal Medulla
- Chromaffin cells secrete catecholamines: adrenaline (epinephrine) — ~80% and noradrenaline (norepinephrine) — ~20%
- Called emergency hormones or hormones of "fight or flight"
- Effects: increased heart rate, blood pressure, cardiac output, sweating, pupil dilation, glycogenolysis (in liver and muscle), lipolysis, bronchodilation
- These hormones rapidly prepare the body for emergency situations by mobilising energy reserves
Adrenal Disorders
| Disorder | Cause | Symptoms |
|---|---|---|
| Cushing's syndrome | Excess cortisol (often from tumour or prolonged steroid therapy) | Moon face, buffalo hump, central obesity, thin limbs, high BP, hyperglycaemia, easy bruising |
| Addison's disease | Deficient cortisol and often aldosterone (autoimmune destruction of adrenal cortex) | Bronzing of skin, low BP, weakness, weight loss, salt craving, nausea, hypoglycaemia |
| Adrenal virilism | Excess sex corticoids from adrenal cortex | Masculinisation in females (facial hair, deep voice, menstrual irregularity) |
Pancreas
The pancreas is a dual gland — it has both exocrine and endocrine functions.
- Exocrine part: Acinar cells secrete digestive enzymes (amylase, trypsinogen, lipase) into the duodenum via the pancreatic duct
- Endocrine part: Islets of Langerhans — 1 to 2 million scattered clusters of endocrine cells
| Cell Type | Proportion | Hormone | Function |
|---|---|---|---|
| alpha cells (a) | ~20% | Glucagon | Hyperglycaemic — promotes glycogenolysis and gluconeogenesis; increases blood glucose |
| beta cells (b) | ~70% | Insulin | Hypoglycaemic — promotes glucose uptake by cells, glycogenesis, lipogenesis, protein synthesis; decreases blood glucose |
Insulin and glucagon form an antagonistic pair that maintains blood glucose homeostasis.
Diabetes Mellitus
| Type | Cause | Features |
|---|---|---|
| Type I (IDDM — Insulin-Dependent Diabetes Mellitus) | Autoimmune destruction of beta cells; absolute insulin deficiency | Early onset (usually childhood/adolescent); requires insulin injections; prone to ketosis and ketoacidosis |
| Type II (NIDDM — Non-Insulin-Dependent Diabetes Mellitus) | Insulin resistance (cells fail to respond to insulin); relative insulin deficiency | Adult onset; managed by diet, exercise, oral hypoglycaemic drugs; less prone to ketosis |
Common symptoms of diabetes mellitus:
- Hyperglycaemia (high blood glucose)
- Glycosuria (glucose in urine)
- Polyuria (excessive urination)
- Polydipsia (excessive thirst)
- Polyphagia (excessive hunger)
- Weight loss, fatigue
- Delayed wound healing
Pineal Gland
- Located on the dorsal side of the forebrain, attached to the roof of the third ventricle
- Secretes melatonin, which regulates:
- The 24-hour (circadian) rhythm of the body — sleep-wake cycle
- Sleep induction — melatonin levels rise at night
- Skin colour in some animals (aggregation of melanin pigment)
- Onset of puberty — melatonin inhibits reproductive maturation
- Acts as an antioxidant
- Decreased melatonin may lead to insomnia and early onset of puberty
- Increased melatonin may delay puberty and cause seasonal affective disorder (SAD)
Testis and Ovary (Gonads)
| Gland | Hormone | Function |
|---|---|---|
| Testis (Leydig / interstitial cells) | Testosterone (androgen) | Male secondary sex characteristics (facial hair, deep voice, muscle mass), spermatogenesis, muscle and bone growth |
| Ovary (growing follicles) | Estrogen | Female secondary sex characteristics (breast development, broad hips), endometrial proliferation, mammary gland development |
| Ovary (corpus luteum) | Progesterone | Supports pregnancy, maintains endometrium, inhibits uterine contractions, prepares mammary glands for lactation |
Hormone Mechanisms of Action
Hormones exert their effects through two fundamentally different mechanisms depending on their chemical nature:
| Feature | Steroid Hormones | Peptide / Protein Hormones |
|---|---|---|
| Chemical nature | Lipid-soluble (derived from cholesterol) | Water-soluble (amino acid-based) |
| Membrane permeability | Can cross the cell membrane freely | Cannot cross the cell membrane |
| Receptor location | Intracellular (cytoplasm or nucleus) | Cell surface (plasma membrane) |
| Mechanism | Genomic — hormone enters cell, binds intracellular receptor → hormone-receptor complex enters nucleus → binds to specific DNA sequences (hormone response elements) → alters gene expression → new mRNA → new protein synthesis | Non-genomic — hormone binds surface receptor → activates G protein → activates second messenger (cAMP, IP3, Ca2+) → enzyme cascade → phosphorylation of target proteins → cellular response |
| Speed of action | Slow (hours) — requires transcription, translation, and new protein synthesis | Fast (seconds to minutes) — uses pre-existing enzymes and proteins |
| Examples | Cortisol, testosterone, estrogen, progesterone, aldosterone, thyroxine | Insulin, glucagon, ADH, ACTH, FSH, LH, TSH, adrenaline, oxytocin |
Second messengers (activated by peptide hormones via G proteins):
- cAMP (cyclic AMP) — the most common; activates Protein Kinase A (PKA), which phosphorylates target proteins
- IP3 (Inositol triphosphate) — triggers release of Ca2+ from the endoplasmic reticulum
- DAG (Diacylglycerol) — activates Protein Kinase C (PKC)
- Ca2+ — acts as a second messenger itself; binds to calmodulin and other proteins
- cGMP (cyclic GMP) — less common; activates Protein Kinase G
The hormone-receptor complex is highly specific — a receptor binds only one type of hormone. However, one hormone may produce multiple effects on different target cells depending on the receptors present.
Feedback Mechanisms
Feedback mechanisms regulate hormone secretion to maintain homeostasis in the body.
| Type | Mechanism | Examples |
|---|---|---|
| Negative feedback (most common) | Output DECREASES when the regulated variable reaches the set point; maintains homeostasis | TSH–T3/T4 axis: high T3/T4 → inhibits TRH from hypothalamus and TSH from pituitary → thyroid reduces T3/T4 output; Insulin–glucose: high blood glucose → insulin release → glucose falls → insulin secretion decreases; Cortisol–ACTH–CRH: high cortisol → inhibits ACTH and CRH |
| Positive feedback (rare) | Output INCREASES, amplifying the response until a specific event terminates the loop | Oxytocin during labour: uterine contractions → stimulate more oxytocin release → stronger contractions → even more oxytocin → continues until the baby is delivered; LH surge before ovulation: rising estrogen → positive feedback on LH → LH surge → ovulation |
Negative feedback pathway (TSH–T3/T4 example): 1. Hypothalamus releases TRH 2. TRH stimulates anterior pituitary to release TSH 3. TSH stimulates thyroid gland to produce T3 and T4 4. Rising T3/T4 levels inhibit both TRH (hypothalamus) and TSH (pituitary) 5. T3/T4 production decreases — homeostasis restored
Positive feedback pathway (Oxytocin during labour): 1. Baby's head presses on the cervix 2. Nerve impulses reach the hypothalamus 3. Hypothalamus triggers posterior pituitary to release oxytocin 4. Oxytocin causes stronger uterine contractions 5. Stronger contractions push baby further → more cervical pressure → more oxytocin 6. Loop continues until delivery, which ends the stimulus
Important Diagrams
1. Human endocrine system — All glands labelled: hypothalamus, pituitary, pineal, thyroid, parathyroid, thymus, adrenal, pancreas, testis/ovary 2. Hypothalamus–pituitary axis — Releasing hormones, inhibiting hormones, hypothalamo-hypophyseal portal system, nerve tract 3. Feedback mechanism — Negative feedback (TSH–T3/T4 pathway with arrows) and positive feedback (oxytocin–labour loop) 4. Steroid vs peptide hormone mechanism — Genomic pathway (nucleus, gene expression) vs second messenger pathway (surface receptor, G protein, cAMP) 5. Adrenal gland cross-section — Capsule, cortex zones (glomerulosa, fasciculata, reticularis), medulla with chromaffin cells 6. Islets of Langerhans — Alpha cells and beta cells within pancreatic tissue, showing glucagon and insulin secretion
Common Mistakes
1. Confusing T3 and T4 — T3 (triiodothyronine, 3 iodine atoms) is MORE active than T4 (thyroxine, 4 iodine atoms), even though T4 is produced in larger quantities. T4 is converted to T3 in target tissues. 2. Saying oxytocin and ADH are produced by the posterior pituitary — They are PRODUCED by neurosecretory cells in the hypothalamus and only STORED and RELEASED by the posterior pituitary. The posterior pituitary does not synthesise hormones. 3. Confusing Cushing's syndrome with Addison's disease — Cushing's is caused by EXCESS cortisol (moon face, obesity); Addison's is caused by DEFICIENT cortisol (bronzing, weakness, low BP). They are opposite disorders. 4. Forgetting which hormone raises vs lowers blood glucose — Insulin is hypoglycaemic (lowers blood glucose); glucagon is hyperglycaemic (raises blood glucose). 5. Writing that all steroid hormones act via second messengers — Steroid hormones use the GENOMIC mechanism (intracellular receptors, direct gene expression changes). Only peptide/protein hormones use second messengers. 6. Confusing the adrenal cortex with the medulla — The cortex (outer) produces steroid hormones — aldosterone, cortisol, sex corticoids. The medulla (inner) produces catecholamines — adrenaline and noradrenaline. 7. Confusing diabetes insipidus with diabetes mellitus — Diabetes insipidus involves ADH deficiency (excessive dilute urine, excessive thirst, but NO glucose in urine). Diabetes mellitus involves insulin deficiency or resistance (glucose in urine, high blood sugar).
Exam Tips
1. Create a master table of ALL endocrine glands, their hormones, targets, and functions — this is the single most frequently tested topic in this chapter. 2. For hypothalamus, list at least 3 releasing hormones (GnRH, TRH, CRH) and 2 inhibiting hormones (somatostatin, dopamine/PIF). 3. Pituitary gland: Clearly separate adenohypophysis (7 hormones — GH, PRL, TSH, ACTH, LH, FSH, MSH) from neurohypophysis (2 hormones stored — oxytocin, ADH). 4. Thyroid disorders: Always mention iodine deficiency as the primary cause of simple goitre. Distinguish cretinism (childhood), myxoedema (adult hypothyroidism), and Graves' disease (autoimmune hyperthyroidism). 5. Adrenal gland: Remember the cortex zones with the mnemonic GFR — Glomerulosa (Mineralocorticoids), Fasciculata (Glucocorticoids), Reticularis (Sex corticoids). 6. Diabetes mellitus: Clearly distinguish Type I (autoimmune, young, insulin-dependent, ketosis-prone) from Type II (insulin resistance, adult, non-insulin-dependent, diet/exercise managed). 7. Hormone mechanism: For 5-mark questions, draw and explain both pathways — steroid hormones (genomic, intracellular, slow) and peptide hormones (second messenger, surface, fast). 8. Feedback mechanisms: Always give one example each of negative feedback (TSH–T3/T4 or insulin–glucose) and positive feedback (oxytocin in labour) with the complete pathway from stimulus to response.
Quick Revision
- Hypothalamus: Releasing hormones (GnRH, TRH, CRH, GHRH) and inhibiting hormones (somatostatin, dopamine/PIF); produces oxytocin and ADH
- Pituitary — Anterior (adenohypophysis): GH, PRL, TSH, ACTH, LH, FSH, MSH; Posterior (neurohypophysis): oxytocin, ADH (stored, not produced)
- Thyroid: T3 and T4 (iodine-dependent, regulate BMR); calcitonin (lowers Ca2+); Disorders — goitre, cretinism, myxoedema, Graves' disease
- Parathyroid: PTH (increases Ca2+ via osteoclasts, kidney reabsorption, calcitriol) — antagonistic to calcitonin
- Thymus: Thymosins — T-lymphocyte maturation; regresses with age
- Adrenal cortex: Aldosterone (Na+/K+ balance), cortisol (gluconeogenesis, anti-inflammatory), sex corticoids; Adrenal medulla: Adrenaline, noradrenaline (fight-or-flight)
- Pancreas: alpha cells → glucagon (hyperglycaemic); beta cells → insulin (hypoglycaemic); Diabetes mellitus — Type I and Type II
- Pineal: Melatonin (circadian rhythm, sleep induction, puberty onset)
- Gonads: Testosterone, estrogen, progesterone
- Steroid hormones: Intracellular receptors, genomic mechanism (slow); Peptide hormones: Surface receptors, second messengers (cAMP, IP3, Ca2+), non-genomic mechanism (fast)
- Feedback: Negative (most common — TSH–T3/T4, insulin–glucose, cortisol–ACTH); Positive (oxytocin in labour, LH surge before ovulation)