Topic 3: Human Physiology
Physiology is the study of how the body's systems work — built from the cells & tissues of Topic 2. This single-source file covers the digestive, respiratory, circulatory (heart, blood, blood groups, Rh factor), excretory (kidney, nephron, dialysis), nervous and endocrine (all hormones & disorders) systems, the skeletal-muscular system and sense organs, and the vitamins, minerals & deficiency diseases that UPSC loves — with labelled diagrams, tables and real dated PYQs with full model answers.
On this page
- 1.Digestive System & Nutrition
- 2.Respiratory System
- 3.Circulatory System — Heart
- 4.Blood & Blood Groups
- 5.Excretory System — Kidney
- 6.Nervous System
- 7.Endocrine System & Hormones
- 8.Skeletal, Muscular & Sense Organs
- 9.Vitamins, Minerals & Deficiency Diseases
- 10.Current Affairs Link
- 11.Prelims PYQs
- 12.Mains PYQs + Model Answers
- ★15-Minute Revision Box
Conceptual Clarity — How UPSC Tests Human Physiology
Human physiology is among the highest-yield Prelims areas in S&T — almost every year sees a question on hormones, vitamins, or blood. Sort your prep into three question-types:
- Definitional / static — "which gland secretes insulin?", "deficiency of vitamin C causes?". Pure recall; the commonest style and easiest marks.
- Statement-elimination — matched pairs (hormone–gland, vitamin–disease, organ–function) where one wrong match decides the answer. Needs the tables below memorised exact.
- Applied / current — a health news hook (diabetes burden, dialysis, organ transplant, universal donor) traced back to a physiology fundamental — the GS-III / GS-II health bridge.
Highest-frequency themes: hormone–gland pairs & disorders · vitamin & mineral deficiency diseases · blood groups & universal donor/recipient · kidney & dialysis · digestive enzymes · human heart & double circulation · neuron & reflex arc.
1. Digestive System & Nutrition
Digestion is the breakdown of complex food into simple, absorbable molecules. The human digestive system is a ~9 m alimentary canal (mouth → oesophagus → stomach → small intestine → large intestine → anus) plus associated glands (salivary glands, liver, pancreas).
Digestive enzymes & where they act
| Region | Secretion / enzyme | Acts on |
|---|---|---|
| Mouth (saliva) | Salivary amylase (ptyalin) | Starch → maltose |
| Stomach (gastric juice) | HCl + pepsin; rennin (infants) | Proteins → peptides; kills microbes |
| Small intestine (pancreas) | Trypsin, lipase, amylase | Proteins, fats, carbs |
| Liver | Bile (no enzyme) | Emulsifies fats |
| Small intestine (intestinal juice) | Maltase, lactase, peptidase | Final breakdown → glucose, amino acids |
2. Respiratory System
Respiration supplies O₂ to cells and removes CO₂. Air path: nostrils → pharynx → larynx (voice box) → trachea → bronchi → bronchioles → alveoli. Gas exchange happens across the thin-walled alveoli of the lungs.
- Breathing mechanism: the diaphragm and intercostal muscles change chest volume — inhalation (diaphragm flattens) and exhalation (relaxes).
- Transport: O₂ is carried by haemoglobin in RBCs (as oxyhaemoglobin); most CO₂ travels as bicarbonate in plasma.
- Cellular respiration: glucose + O₂ → CO₂ + H₂O + ATP (in mitochondria — link to Topic 2).
| Term | Meaning |
|---|---|
| Aerobic respiration | With O₂ → 38 ATP (net ~36); complete breakdown |
| Anaerobic respiration | Without O₂ → lactic acid (muscle) / ethanol (yeast); little ATP |
| Vital capacity | Max air exhaled after deepest inhalation |
3. Circulatory System — The Heart
The human heart is a four-chambered muscular pump (two atria + two ventricles) that drives double circulation — blood passes through the heart twice per full body circuit (pulmonary + systemic).
- Pulmonary circulation: right ventricle → lungs → left atrium (picks up O₂).
- Systemic circulation: left ventricle → aorta → body → right atrium (returns CO₂).
- Arteries carry blood away from heart (thick, no valves; O₂-rich except pulmonary artery). Veins return blood to heart (valves; O₂-poor except pulmonary vein).
- Pacemaker: the SA node (sino-atrial) sets the heartbeat. Normal rate ~72/min. BP normal ~120/80 mmHg.
4. Blood & Blood Groups
Blood is a connective tissue (Topic 2!) with a fluid matrix, plasma (~55%), suspending the formed elements (~45%).
| Component | Function |
|---|---|
| Plasma | Transports nutrients, hormones, wastes; ~90% water |
| RBC (erythrocytes) | Carry O₂ (haemoglobin); no nucleus in mammals; made in bone marrow |
| WBC (leucocytes) | Immunity & defence (link Topic 5) |
| Platelets (thrombocytes) | Blood clotting |
ABO & Rh blood groups
| Group | Antigen (RBC) | Antibody (plasma) | Can receive from |
|---|---|---|---|
| A | A | anti-B | A, O |
| B | B | anti-A | B, O |
| AB | A & B | none | ALL — universal recipient |
| O | none | anti-A & anti-B | O only |
5. Excretory System — The Kidney
Excretion removes nitrogenous wastes (mainly urea). The two kidneys filter blood; the functional unit is the nephron (~1 million per kidney).
- Steps: glomerular filtration → tubular reabsorption (glucose, water, salts returned) → secretion → urine.
- Dialysis = artificial filtering of blood when kidneys fail (haemodialysis). Chronic failure needs a kidney transplant.
- Other excretory organs: lungs (CO₂), skin (sweat), liver (urea formation, bile pigments).
6. Nervous System
The nervous system is the body's fast electrical control network. It has the CNS (brain + spinal cord) and the PNS (nerves). Its unit is the neuron (longest cell in the body — Topic 2).
Parts of the brain
| Part | Controls |
|---|---|
| Cerebrum | Thinking, memory, voluntary action, intelligence |
| Cerebellum | Balance, posture, coordination |
| Medulla oblongata | Involuntary vitals — heartbeat, breathing, BP |
| Hypothalamus | Temperature, hunger, thirst; links to endocrine |
- Neuron parts: dendrites (receive) → cell body → axon (transmit); signal crosses the synapse via neurotransmitters.
- Reflex arc: a rapid involuntary response (e.g. hand off a hot object) processed by the spinal cord, not the brain — receptor → sensory neuron → spinal cord → motor neuron → effector.
- Autonomic: sympathetic ("fight or flight") vs parasympathetic ("rest and digest").
7. Endocrine System & Hormones
The endocrine system is the body's slow chemical control network — ductless glands secrete hormones directly into the blood. This is the single most-tested table in physiology.
| Gland | Hormone | Function / disorder |
|---|---|---|
| Pituitary ("master gland") | Growth hormone (GH) | Growth; excess = gigantism/acromegaly, deficiency = dwarfism |
| Thyroid | Thyroxine (needs iodine) | Metabolism; deficiency = goitre/cretinism, excess = Graves' |
| Parathyroid | Parathormone | Calcium balance |
| Pancreas (islets) | Insulin & glucagon | Blood sugar; insulin deficiency = diabetes mellitus |
| Adrenal | Adrenaline; cortisol | "Fight or flight"; stress response |
| Testis / Ovary | Testosterone / Oestrogen & progesterone | Secondary sexual characters; reproduction (Topic 4) |
8. Skeletal, Muscular & Sense Organs
- Skeletal system: the adult human body has 206 bones (a newborn has ~300). Functions: support, protection, movement, blood-cell formation (marrow), mineral storage (Ca, P). The femur (thigh) is the longest bone; the stapes (ear) is the smallest.
- Joints: ball-and-socket (shoulder, hip), hinge (elbow, knee), pivot (neck).
- Muscles: skeletal (voluntary, striated), smooth (involuntary), cardiac (heart) — from Topic 2 tissues.
Sense organs
| Organ | Key facts |
|---|---|
| Eye | Retina has rods (dim light) & cones (colour); image forms on retina; myopia = short-sight (concave lens), hypermetropia = long-sight (convex lens) |
| Ear | Hearing + balance; ear ossicles = malleus, incus, stapes |
| Skin | Largest organ; touch, temperature, pain |
| Tongue | Taste (sweet, sour, salt, bitter, umami) |
9. Vitamins, Minerals & Deficiency Diseases
Vitamins are micronutrients needed in tiny amounts. Fat-soluble = A, D, E, K (stored in body); water-soluble = B-complex & C (not stored, needed daily). This table is a UPSC perennial.
| Vitamin | Chemical name | Deficiency disease |
|---|---|---|
| A | Retinol | Night blindness, xerophthalmia |
| B1 | Thiamine | Beriberi |
| B3 | Niacin | Pellagra |
| B12 | Cobalamin | Pernicious anaemia |
| C | Ascorbic acid | Scurvy (bleeding gums) |
| D | Calciferol | Rickets (children), osteomalacia (adults) |
| K | Phylloquinone | Poor blood clotting |
Minerals
| Mineral | Role | Deficiency |
|---|---|---|
| Iron | Haemoglobin | Anaemia |
| Iodine | Thyroxine | Goitre |
| Calcium | Bones, teeth, clotting | Weak bones, rickets |
| Fluorine | Teeth | Dental caries; excess = fluorosis |
10. Current Affairs Link (2024–2026)
Physiology stays topical through health policy, disease burden and medical technology. Verify the latest before the exam. check for latest update or data
| Recent theme | Fundamental it tests | Why it matters for UPSC |
|---|---|---|
| Diabetes / NCD rise | Insulin, endocrine | GS-II health; lifestyle diseases. |
| Organ transplant reform | Kidney, excretory | GS-II policy + GS-IV ethics. |
| Anaemia Mukt Bharat | Iron, blood | GS-II nutrition & women/child health. |
| Millet / fortification push | Vitamins & minerals | GS-III food security. |
- Recurring exam hooks: master gland · iodine & goitre · universal donor/recipient · vitamin C = scurvy · insulin & pancreas · pulmonary artery paradox · reflex arc bypasses brain.
11. Prelims PYQs
Objective questions anchored to genuinely tested UPSC themes on human physiology. Each carries a worked rationale.
Q: Insulin, which regulates blood-sugar level, is secreted by the —
Answer: (b) Insulin is secreted by the beta cells of the islets of Langerhans in the pancreas; its deficiency causes diabetes mellitus.
Q: Deficiency of which vitamin causes scurvy?
Answer: (c) Vitamin C (ascorbic acid) deficiency causes scurvy — bleeding gums & poor wound healing.
Q: A person with which blood group is the universal donor?
Answer: (c) O negative has no A/B antigens and no Rh(D) antigen, so it can be given to anyone — the universal donor. AB positive is the universal recipient.
Q: Which blood vessel carries deoxygenated blood?
Answer: (c) The pulmonary artery is the only artery carrying deoxygenated blood (heart → lungs); the pulmonary vein is the only vein carrying oxygenated blood.
Q: Which part of the brain controls involuntary actions such as heartbeat and breathing?
Answer: (c) The medulla oblongata controls vital involuntary functions — heartbeat, breathing and blood pressure.
Q: Goitre is caused by the deficiency of —
Answer: (b) Iodine deficiency impairs thyroxine synthesis, enlarging the thyroid (goitre) — the basis of the iodised-salt policy.
Q: The main site of absorption of digested food in humans is the —
Answer: (b) The small intestine, with its villi, is the chief site of absorption; the large intestine mainly absorbs water.
Q: How many bones are there in the adult human body?
Answer: (c) An adult has 206 bones; a newborn has ~300, many of which fuse with growth.
Likely: hormone–gland matching · vitamin–deficiency matching · universal donor/recipient · pulmonary artery/vein · SA node pacemaker · reflex arc & spinal cord · enzymes & their substrates · nephron & dialysis · smallest/largest bone. check for latest update or data
12. Mains PYQs + Model Answers
Physiology anchors applied GS-II/III questions on public health, nutrition and medical technology. The frameworks below show how to deploy it analytically.
Q: "The rising burden of non-communicable diseases (NCDs) like diabetes reflects a nutrition and lifestyle transition in India." Discuss and suggest measures.
Model Answer
- Introduction — define: NCDs are chronic non-infectious diseases (diabetes, hypertension, heart disease) now causing the majority of deaths in India; diabetes reflects failure of insulin regulation (Section 7).
- Drivers:
- Dietary transition — more sugar, refined carbs & processed food; sedentary urban lifestyles; stress; rising obesity.
- Ageing population and genetic predisposition of Indians to central obesity & insulin resistance.
- Consequences: productivity & economic loss; catastrophic health spending; strain on health system; complications (CKD needing dialysis — Section 5).
- Measures: National NCD programme & population screening; sugar/fat labelling & taxation; millet & balanced-diet promotion (Eat Right India); physical-activity & Fit India; strengthen primary care (Ayushman Bharat HWCs).
- Way forward: a preventive, "health-in-all-policies" approach linking nutrition, environment & behaviour.
- Conclusion: tackling NCDs needs physiology-informed prevention, not just treatment.
Q: Micronutrient deficiency ("hidden hunger") persists despite adequate calorie availability in India. Examine the causes and the government's response.
Model Answer
- Introduction: Hidden hunger is deficiency of essential vitamins & minerals (iron, iodine, vitamin A, D) even when calorie intake is sufficient (Section 9).
- Extent: high anaemia among women & children (NFHS), widespread vitamin-D & iron deficiency — impairs immunity, cognition & productivity.
- Causes: monotonous cereal-heavy diets; poor dietary diversity; poverty; low awareness; poor absorption due to infection/sanitation.
- Government response: POSHAN Abhiyaan; Anaemia Mukt Bharat; food fortification (iodised salt, fortified rice/wheat/oil); mid-day meal & ICDS; millet promotion.
- Way forward: dietary diversification, biofortification, behaviour-change communication, convergence across schemes.
- Conclusion: fixing hidden hunger needs nutrition-sensitive, not just calorie-focused, policy.
Q: Organ donation in India remains low despite a large need. Discuss the physiological basis and the ethical & policy challenges.
Model Answer
- Introduction: Organ transplantation replaces a failed organ (e.g. kidney — Section 5) with a healthy donor organ; India's donation rate is far below demand.
- Physiological basis: matching (blood group, tissue/HLA) prevents immune rejection; brain-death certification enables deceased donation.
- Challenges: low awareness & myths; family reluctance; weak infrastructure; commercial exploitation & organ trafficking; unequal access.
- Policy & ethics: Transplantation of Human Organs Act; NOTTO/"one nation one policy"; presumed-consent debate; equity in allocation.
- Conclusion: raising donation needs awareness, robust registries & ethical safeguards against a black market.
Q: Explain how the endocrine system maintains homeostasis, with examples of disorders relevant to public health.
Model Answer
- Introduction: Homeostasis is the maintenance of a stable internal environment; the endocrine system does this chemically via hormones from ductless glands (Section 7).
- Mechanism:
- Feedback loops — e.g. insulin & glucagon keep blood glucose steady; thyroxine sets metabolic rate; hypothalamus–pituitary axis coordinates the rest.
- Slow but widespread & sustained control, complementing the fast nervous system.
- Disorders & public health: diabetes (insulin); goitre/cretinism (iodine); dwarfism/gigantism (GH); each a major population-health issue.
- Policy links: iodised salt (NIDDCP), NCD screening, and thyroid/diabetes management build on this.
- Conclusion: endocrine balance is central to health; its failure translates directly into disease burden and policy need.
Likely: NCD & diabetes burden · hidden hunger & fortification · organ donation ethics · endocrine homeostasis · blood donation & safety · dialysis & CKD. check for latest update or data
15-Minute Revision Box
Must-Remember Facts — Human Physiology
- Liver = largest gland; pancreas = exo + endocrine; bile emulsifies fat
- Small intestine = main absorption; large intestine = water
- Pulmonary artery = deoxygenated; pulmonary vein = oxygenated
- CO binds haemoglobin ~200× stronger than O₂
- 4 chambers; left ventricle = thickest; SA node = pacemaker; ~72/min, 120/80
- Blood = connective tissue; O− = universal donor, AB+ = universal recipient
- Platelets clot; RBC = haemoglobin (no nucleus in mammals)
- Nephron = kidney unit; urea = main waste; dialysis when kidneys fail
- Cerebrum = thinking, cerebellum = balance, medulla = heartbeat/breathing
- Reflex arc = spinal cord (bypasses brain)
- Pituitary = master gland → dwarfism/gigantism
- Thyroid (iodine) → goitre/cretinism
- Pancreas (insulin) → diabetes
- Adrenaline = emergency hormone
- A → night blindness; C → scurvy; D → rickets
- B1 → beriberi; B3 → pellagra; B12 → pernicious anaemia; K → poor clotting
- Fat-soluble = A,D,E,K; water-soluble = B,C
- Iron → anaemia; iodine → goitre; calcium → bones
- 206 bones; femur = longest, stapes = smallest
- Vitamin A → night blindness; eye: rods (dim) + cones (colour)