Chapter in a nutshell: The endocrine system is the body's chemical control network — a set of ductless glands that pour secretions called hormones directly into the blood to regulate distant target organs. The four glands prescribed by the syllabus are the adrenal, pancreas, thyroid and pituitary; their hormones govern emergencies, blood-sugar, metabolism and growth, and a feedback mechanism keeps every level normal.
1. Why the Body Needs Chemical Regulation
Body activities are highly complex and must occur at the proper time and in the correct sequence (e.g. digestive juices should pour into the food canal only when food is present). This is achieved partly by the nervous system and partly by chemical regulators called hormones (horma = to stir up / excite into action).
Hormones are secretions from specific cells or glands, carried by the blood to all parts of the body, but their effect is produced only in one or more specific parts called the target organ (or target cells).
| Hormonal control | Nervous control |
|---|---|
| Usually slow | Immediate / rapid |
| Transmitted chemically through blood | Transmitted electro-chemically through nerve fibres |
| Affects different organs (widespread) | Affects only particular muscles/glands (local) |
| Effect is short-term or long-lasting | Effect only short-lived |
| Can affect growth | Cannot affect growth |
| Can bring about chemical changes and regulate metabolism | Cannot regulate metabolism |
| Cannot be modified by previous experience | Can be modified to some extent by learning |
2. General Properties of Hormones
- Secreted directly into the blood (not into lymph).
- Regulate physiological processes by chemical means, affecting the body's enzyme systems.
- Act on target organs/cells, usually away from their source.
- A hormone of one species usually shows similar influence in other species.
- Produced in very small quantities and are biologically very active (adrenaline is active even at 1 part in 300,000,000).
- Chemically may be peptides/proteins (insulin — water-soluble), amines (adrenaline — water-soluble), or steroids (testosterone — lipid-soluble).
- Their excess (hypersecretion) or deficiency (hyposecretion) can both lead to serious consequences.
- They are not stored in the body and are excreted from the system.
Endocrine glands (endo = inside, crine = to secrete) are also called ductless glands because their secretions are poured directly into the blood, not through any duct.
3. The Adrenal Glands
The adrenal glands (ad = near, renal = kidney) are cap-like structures above the kidneys. Each gland has two parts: a central medulla and a peripheral cortex — both secrete different hormones (described in Fig. 10.2 as a cross-section showing the inner medulla surrounded by the outer cortex).
(i) Adrenal Medulla — Adrenaline ("the emergency hormone")
The medulla secretes adrenaline (also called epinephrine/noradrenaline). It prepares the body to meet emergencies — for "fight" (face danger) or "flight" (run away). It is released during emotional stress, excitement or anger, and the gland is stimulated by the autonomic nervous system.| Body part | Effect of adrenaline | Sensation |
|---|---|---|
| Heart | Beats faster, BP rises | Thumping heart |
| Breathing centre | Faster, deeper breathing | Panting |
| Skin arterioles | Constricts them | Turns pale |
| Digestive arterioles | Constricts them | Dry mouth |
| Body muscles | Tenses them | Shivering |
| Liver | Glycogen → glucose | No sensation |
(ii) Adrenal Cortex
Best-known hormone: cortisone (suppresses inflammation). Cortical hormones fall into:- Mineralocorticoids (e.g. aldosterone): regulate mineral metabolism — Na⁺ and K⁺ ions.
- Glucocorticoids (e.g. cortisol): regulate carbohydrate, protein and fat metabolism; raise blood glucose; help adapt to stresses (heat, cold, burns, infections).
- Certain cortical hormones act like sex hormones.
| Disorder | Cause | Symptoms |
|---|---|---|
| Addison's disease | Hyposecretion of cortex | Loss of energy/weight, skin pigmentation, nausea, hypoglycemia |
| Cushing's syndrome | Hypersecretion of cortex | Obesity, hyperglycemia, osteoporosis, salt/water retention |
| Adrenal virilism | Overgrowth of cortex in a mature woman | Beard, moustache, deep voice (maleness) |
4. The Pancreas
The pancreas is both a duct gland (exocrine) — pouring pancreatic juice into the duodenum for digestion — and a ductless gland (endocrine). Its endocrine cells form scattered groups called the Islets of Langerhans (islet = little island; Fig. 10.3 shows an islet of hormone-secreting cells amid enzyme-secreting tissue). These produce hormones from three cell types:
| Cell type | Hormone | Action |
|---|---|---|
| Beta (β) cells | Insulin | Lowers blood sugar |
| Alpha (α) cells | Glucagon | Raises blood sugar |
| Delta (δ) cells | Somatostatin (not in syllabus) | Inhibits insulin & glucagon |
Glucagon (from alpha cells) stimulates the breakdown of glycogen → glucose in the liver, raising blood sugar. Insulin and glucagon work as antagonists to maintain blood sugar (Fig. 10.4).
Diabetes mellitus (hyperglycemia) — from insufficient insulin (mellitus = honey, sugar passes in urine): high blood sugar, sugar-loaded urine, constant thirst, loss of weight/strength, possible loss of eyesight. Treated by administering insulin (not a cure).
Over-secretion of insulin lowers blood sugar (hypoglycemia); the brain may enter coma. An insulin overdose causes insulin shock — relieved by sweet biscuits/candy.
5. The Thyroid Gland
The thyroid is a bilobed (butterfly-shaped) gland in front of the neck, just below the larynx, its two lobes joined by a narrow isthmus (Fig. 10.5 shows it astride the trachea, with parathyroids behind). It secretes thyroxine (and calcitonin — not in syllabus).
Thyroxine regulates basal metabolism (rate of cellular oxidation → heat production at rest) and influences general body growth, ossification of bones, body temperature and mental development. Its key ingredient is iodine.
| Condition | Description | |
|---|---|---|
| Hypothyroidism (under) | Simple goitre | Swelling of neck from iodine deficiency (common in hilly regions) |
| Cretinism | In children — dwarfism + mental retardation | |
| Myxoedema | In adults — sluggishness, swelling of face and hands | |
| Hyperthyroidism (over) | Exophthalmic goitre | Protruding eyes, raised metabolism, rapid heartbeat, shortness of breath, restlessness |
6. The Pituitary Gland — "the Master Gland"
The pituitary is a pea-sized projection hanging from the base of the mid-brain, below the hypothalamus. It is the master gland because it controls almost all other endocrine glands. It has two distinct lobes — anterior and posterior — plus a tiny intermediate lobe (almost absent in humans); the whole gland weighs only about half a gram (Fig. 10.7).
A. Anterior Pituitary
| Hormone | Action / Disorder |
|---|---|
| Growth hormone (GH / somatotropin) | Promotes body growth (esp. skeleton). Deficiency in childhood → dwarfism; excess in childhood → gigantism (giants up to 2.7 m); excess in adult → acromegaly (large jaws, nose, hands, feet) |
| Thyroid-stimulating hormone (TSH) | Activates thyroid to secrete thyroxine |
| Adrenocorticotropic hormone (ACTH) | Regulates adrenal cortex |
| Gonadotropic hormones | FSH (egg/sperm formation), LH (corpus luteum → progesterone; testosterone), Prolactin (milk secretion) |
B. Posterior Pituitary
- Antidiuretic hormone (ADH / vasopressin): increases reabsorption of water from kidney tubules and constricts blood vessels (raises BP). Deficiency → diabetes insipidus (water diabetes) — frequent, copious urination, thirst.
- Oxytocin (oxys = quick, tokos = childbirth): stimulates vigorous uterine contractions during birth and milk ejection.
7. Control of Hormonal Secretions — Feedback Mechanism
The body keeps every level normal. When a level changes, messages are sent to increase (if it falls below normal) or decrease (if it rises above normal). Ordering the opposite to restore normal is negative feedback — the commonest type ("Too much → slow down"; "Too little → speed up").
Example: Blood sugar rises after meals → command to lower → glucose deposited as glycogen; falls during exercise → command to raise → glucose released from liver → normal restored.
Positive feedback is rare: e.g. uterine contractions in childbirth — one contraction signals to continue contracting until delivery is complete.
Worked / Structured Examples
Example 1 — Trace insulin action after a sugary meal. Blood glucose rises → β-cells of Islets of Langerhans secrete insulin → insulin promotes glucose uptake/utilisation by body cells and deposition as glycogen in liver/muscle → blood sugar falls to normal (negative feedback).
Example 2 — The "maiden speech" reaction. Facing a large audience triggers the adrenal medulla to release adrenaline → heart beats faster (thumping), digestive arterioles constrict → dry mouth, skin arterioles constrict → pallor. This is the fight-or-flight response.
Example 4 — Why is the pancreas an "exo-endocrine" gland? As an exocrine gland it secretes pancreatic juice via a duct into the duodenum; as an endocrine gland its Islets of Langerhans secrete insulin/glucagon directly into blood.
Example 5 — Distinguish the two diabetes. Diabetes mellitus: low insulin → sugar in urine. Diabetes insipidus: low ADH → no sugar in urine ("insipid" = tasteless), excessive watery urine.
Key Terms — Quick Glossary
| Term | Meaning |
|---|---|
| Endocrine gland | Ductless gland secreting hormones directly into blood |
| Hormone | Chemical messenger acting on a distant target organ |
| Target organ | Specific organ/cells where a hormone acts |
| Tropic hormone | Hormone that stimulates another endocrine gland |
| Insulin | β-cell hormone; lowers blood sugar |
| Glucagon | α-cell hormone; raises blood sugar |
| Thyroxine | Thyroid hormone regulating basal metabolism (needs iodine) |
| Adrenaline | Adrenal-medulla "emergency" (fight-or-flight) hormone |
| Growth hormone | Anterior-pituitary hormone for body growth |
| ADH / vasopressin | Posterior-pituitary hormone; water reabsorption in kidney |
| Negative feedback | Self-correcting control restoring normal level |
| Islets of Langerhans | Endocrine cell-clusters of the pancreas |
Common Mistakes to Avoid
- Don't say "insulin converts glucose to glycogen." Say insulin enables cells to absorb and use glucose or store it as glycogen.
- Don't confuse diabetes mellitus (insulin deficiency, sugar in urine) with diabetes insipidus (ADH deficiency, no sugar in urine).
- Don't mix the thyroid disorders: cretinism is in children (dwarfism + mental retardation); myxoedema is in adults.
- Don't call the thyroid the master gland — the pituitary is the master gland.
- Don't swap medulla and cortex: the medulla (inner) makes adrenaline; the cortex (outer) makes corticoids.
- Don't write that excess GH in an adult causes gigantism — in an adult it causes acromegaly; gigantism is in childhood.
Likely Exam Questions (with crisp answers)
- Why are endocrine glands called ductless glands? Because they pour their secretions (hormones) directly into the blood, not through any duct.
- Name the gland with both endocrine and exocrine functions. The pancreas.
- Which hormone is the "emergency hormone" and from where? Adrenaline, from the adrenal medulla.
- State two functions of insulin. Promotes glucose utilisation by body cells; stores extra glucose as glycogen in liver/muscle.
- What causes diabetes mellitus? Insufficient secretion of insulin (from β-cells).
- Why is iodine important as a nutrient? It is the active ingredient in thyroxine; deficiency causes simple goitre.
- What does thyroxine regulate? Basal metabolism, growth, ossification, body temperature and mental development.
- Differentiate cretinism and myxoedema. Cretinism — children, dwarfism + mental retardation; Myxoedema — adults, sluggishness + facial/hand swelling.
- Why is the pituitary called the master gland? It controls almost all other endocrine glands via tropic hormones.
- Define tropic hormones with an example. Hormones that stimulate other endocrine glands, e.g. TSH stimulates the thyroid.
- What does deficiency of ADH cause? Diabetes insipidus (water diabetes) — frequent, copious urination.
- Give one function each of oxytocin. Stimulates uterine contractions during childbirth and milk ejection.
- What is negative feedback? Give an example. Self-correction that opposes a change to restore normal — e.g. blood-sugar regulation by insulin/glucagon.
- Effect of GH oversecretion in childhood vs adulthood. Childhood → gigantism; adulthood → acromegaly.
- Why is insulin not given orally? It is a protein and would be digested in the gut, so it must be injected.
- Name the hormones of the islets of Langerhans. Insulin (β), glucagon (α), somatostatin (δ).
- Which gland controls Na⁺/K⁺ balance? The adrenal cortex, via mineralocorticoids (aldosterone).