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.

UPSC Prelims · Mains GS-III Hormones · Blood Groups ~40 min read Deficiency Diseases Very High Prelims Weight

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).

Fig 3.1 — The alimentary canal. Chemical digestion peaks in the stomach & small intestine; the small intestine is the chief site of absorption.

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

Key facts: the liver is the largest gland/internal organ; the pancreas is both exocrine (enzymes) and endocrine (insulin). Bile is stored in the gall bladder. The small intestine (with villi) is the main site of absorption; the large intestine absorbs water.

GS-III / GS-II hook: malnutrition, the "hidden hunger" of micronutrient deficiency, and India's fortification & POSHAN Abhiyaan schemes all rest on digestion & nutrition fundamentals.

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

Health links: asthma, COPD, TB, silicosis and air-pollution effects (PM2.5) all target the respiratory tract — recurring GS-III & environment cross-links. Carbon monoxide is dangerous because it binds haemoglobin ~200× more strongly than O₂.

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).

Fig 3.2 — The four-chambered heart. Right side handles deoxygenated blood (to lungs); left side handles oxygenated blood (to body). Valves prevent backflow.

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.

Prelims trap: the pulmonary artery is the only artery carrying deoxygenated blood; the pulmonary vein is the only vein carrying oxygenated blood.

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

Universal donor = O negative (no antigens). Universal recipient = AB positive (no antibodies). Rh factor: Rh+ has the D antigen; Rh− lacks it. An Rh− mother carrying an Rh+ fetus can develop erythroblastosis fetalis in later pregnancies — prevented by anti-D injection.

GS-II health hook: voluntary blood donation, e-Rakt Kosh blood banks, and thalassaemia/haemophilia (clotting-factor) programmes all build on this.

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).

Fig 3.3 — The nephron. The glomerulus filters blood; the tubule reabsorbs useful substances; waste passes as urine via ureter → bladder.

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).

GS-II health hook: rising CKD (chronic kidney disease) & diabetes burden, dialysis under PMNDP, and organ-donation (Transplantation of Human Organs Act, NOTTO) are recurring themes.

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").

Prelims facts: myelin sheath speeds impulses; the medulla controls life-support reflexes; the reflex action is faster because it bypasses the brain.

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)

High-frequency traps: pituitary = master gland (but hypothalamus controls it); iodine deficiency → goitre (basis of iodised-salt policy); insulin is made in the pancreas (islets of Langerhans), not the liver; adrenaline is the "emergency hormone".

GS-II/III hook: India's huge diabetes & thyroid-disorder burden, the National Iodine Deficiency Disorders Control Programme (NIDDCP), and lifestyle-disease NCD strategy all build on this.

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)

Prelims facts: vitamin A deficiency → night blindness; cataract = clouding of lens; the smallest bone (stapes) and largest bone (femur) are common one-liners.

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

GS-II hook: "hidden hunger" — iron, iodine & vitamin-A deficiency drive India's food-fortification, POSHAN Abhiyaan, mid-day-meal and anaemia-mukt-Bharat programmes. Vitamin D deficiency is now widespread even in sunny India.

Memory hook: vitamin D is the only one the body makes (in skin, via sunlight); vitamin C deficiency (scurvy) once devastated sailors — hence "limey".

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

Diabetes & NCD burden: India is a "diabetes capital" — ICMR-INDIAB data show a large diabetic & pre-diabetic population, keeping the insulin/pancreas facts and the NCD strategy topical. check for latest update or data

Organ donation & transplant: NOTTO reforms, "one nation one policy" for organ transplants, and rising kidney transplants keep excretory-system & ethics angles live. check for latest update or data

Anaemia & nutrition: Anaemia Mukt Bharat and food fortification target iron/iodine/vitamin deficiencies — direct applications of Section 9. 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.

UPSC Prelims — Endocrine

Q: Insulin, which regulates blood-sugar level, is secreted by the —

(a) liver
(b) pancreas
(c) thyroid
(d) adrenal gland

Answer: (b) Insulin is secreted by the beta cells of the islets of Langerhans in the pancreas; its deficiency causes diabetes mellitus.

UPSC Prelims — Vitamins

Q: Deficiency of which vitamin causes scurvy?

(a) Vitamin A
(b) Vitamin B12
(c) Vitamin C
(d) Vitamin D

Answer: (c) Vitamin C (ascorbic acid) deficiency causes scurvy — bleeding gums & poor wound healing.

UPSC Prelims — Blood groups

Q: A person with which blood group is the universal donor?

(a) AB positive
(b) A negative
(c) O negative
(d) B positive

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.

UPSC Prelims — Heart

Q: Which blood vessel carries deoxygenated blood?

(a) Aorta
(b) Pulmonary vein
(c) Pulmonary artery
(d) Carotid artery

Answer: (c) The pulmonary artery is the only artery carrying deoxygenated blood (heart → lungs); the pulmonary vein is the only vein carrying oxygenated blood.

UPSC Prelims — Nervous

Q: Which part of the brain controls involuntary actions such as heartbeat and breathing?

(a) Cerebrum
(b) Cerebellum
(c) Medulla oblongata
(d) Hypothalamus

Answer: (c) The medulla oblongata controls vital involuntary functions — heartbeat, breathing and blood pressure.

UPSC Prelims — Thyroid

Q: Goitre is caused by the deficiency of —

(a) iron
(b) iodine
(c) calcium
(d) vitamin C

Answer: (b) Iodine deficiency impairs thyroxine synthesis, enlarging the thyroid (goitre) — the basis of the iodised-salt policy.

UPSC Prelims — Digestion

Q: The main site of absorption of digested food in humans is the —

(a) stomach
(b) small intestine
(c) large intestine
(d) liver

Answer: (b) The small intestine, with its villi, is the chief site of absorption; the large intestine mainly absorbs water.

UPSC Prelims — Skeleton

Q: How many bones are there in the adult human body?

(a) 300
(b) 256
(c) 206
(d) 180

Answer: (c) An adult has 206 bones; a newborn has ~300, many of which fuse with growth.

Prelims — anticipated themes

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.

Mains GS-II 15 marks · 250 words

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.

Mains GS-II 15 marks · 250 words

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.

Mains GS-IV / GS-II 10 marks · 150 words

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.

Mains GS-III 15 marks · 250 words

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.

Mains GS-III — anticipated themes

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

Digestion & respiration:

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

Heart & blood:

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)

Excretion & nervous:

Nephron = kidney unit; urea = main waste; dialysis when kidneys fail
Cerebrum = thinking, cerebellum = balance, medulla = heartbeat/breathing
Reflex arc = spinal cord (bypasses brain)

Hormones (gland → disorder):

Pituitary = master gland → dwarfism/gigantism
Thyroid (iodine) → goitre/cretinism
Pancreas (insulin) → diabetes
Adrenaline = emergency hormone

Vitamins (deficiency):

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

Minerals & bones:

Iron → anaemia; iodine → goitre; calcium → bones
206 bones; femur = longest, stapes = smallest
Vitamin A → night blindness; eye: rods (dim) + cones (colour)

Highest-frequency themes: hormone–gland pairs · vitamin–disease pairs · universal donor/recipient · pulmonary artery paradox · master gland · iodine & goitre · reflex arc bypasses brain · nephron & dialysis.

System	Key organ
Digestive	Small intestine
Respiratory	Lungs (alveoli)
Circulatory	Heart
Excretory	Kidney
Nervous	Brain
Endocrine	Pituitary

Hormone	Gland
Insulin	Pancreas
Thyroxine	Thyroid
Adrenaline	Adrenal
Growth hormone	Pituitary

Topic 3: Human Physiology

On this page

Conceptual Clarity — How UPSC Tests Human Physiology

1. Digestive System & Nutrition

Digestive enzymes & where they act

2. Respiratory System

3. Circulatory System — The Heart

4. Blood & Blood Groups

ABO & Rh blood groups

5. Excretory System — The Kidney

6. Nervous System

Parts of the brain

7. Endocrine System & Hormones

8. Skeletal, Muscular & Sense Organs

Sense organs

9. Vitamins, Minerals & Deficiency Diseases

Minerals

10. Current Affairs Link (2024–2026)

11. Prelims PYQs

12. Mains PYQs + Model Answers

Model Answer

Model Answer

Model Answer

Model Answer

15-Minute Revision Box

Must-Remember Facts — Human Physiology