Overview

Human Body Systems is a high-yield area for UPSC Prelims — questions on organ functions, hormones, blood groups, diseases, and the immune system appear frequently. This topic covers the major organ systems with exam-relevant facts, clinical significance, and UPSC-specific pointers.

Circulatory System (Cardiovascular)

Feature Detail
Heart Muscular organ with 4 chambers — 2 atria (upper) and 2 ventricles (lower); weighs approximately 250–350 g
SA Node Located in the right atrium; the "natural pacemaker" — generates electrical impulses that set heart rate
Heart rate Normal resting rate: 60–100 bpm (beats per minute)
Cardiac output Heart pumps approximately 5 litres/minute at rest (~7,200–7,500 litres/day)
Circulation Pulmonary (heart → lungs → heart) and Systemic (heart → body → heart); double circulation

Blood

Component Detail
RBCs (Erythrocytes) Carry oxygen via haemoglobin; biconcave disc shape; no nucleus in mature RBCs; lifespan ~120 days; produced in bone marrow
WBCs (Leucocytes) Defence against infection; 5 types: Neutrophils (most abundant, ~60–70%), Lymphocytes (T cells, B cells), Monocytes, Eosinophils, Basophils
Platelets (Thrombocytes) Essential for blood clotting; fragments of megakaryocytes; lifespan ~8–10 days
Plasma ~55% of blood volume; contains water (92%), proteins (albumin, globulin, fibrinogen), electrolytes, hormones

Blood Groups

System Detail
ABO System Discovered by Karl Landsteiner in 1901 (Nobel Prize in Physiology or Medicine, 1930); 4 groups — A, B, AB (universal recipient), O (universal donor)
Rh Factor Discovered in 1940 by Landsteiner and Alexander Wiener; named after the Rhesus monkey; Rh+ (antigen present) or Rh- (absent)
Rh incompatibility Rh- mother carrying Rh+ foetus can develop antibodies → Erythroblastosis foetalis (haemolytic disease of the newborn) in subsequent pregnancies

Respiratory System

Feature Detail
Organs Nose → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli
Alveoli Approximately 480 million in human lungs; total surface area ~70 sq metres (roughly the size of half a tennis court) — maximises gas exchange
Gas exchange O₂ diffuses from alveoli into blood; CO₂ diffuses from blood into alveoli — occurs across a thin respiratory membrane (~0.5 µm thick)
Haemoglobin Each molecule can carry 4 oxygen molecules; oxyhaemoglobin (bright red) vs deoxyhaemoglobin (dark red)
Breathing rate Normal adult: 12–20 breaths per minute at rest
Vital capacity Maximum air that can be exhaled after maximum inhalation: ~3,500–4,500 mL

Digestive System

Organ Key Facts
Mouth Mechanical digestion (chewing) + chemical digestion by salivary amylase (breaks down starch); pH ~6.8
Stomach Secretes HCl (hydrochloric acid) — pH 1.5–3.5; pepsin (protein digestion); churning action; food stays ~2–4 hours
Small intestine Length: ~6 metres (textbook figure); three parts: duodenum (receives bile + pancreatic juice), jejunum, ileum; lined with villi and microvilli that increase absorptive surface area ~600-fold
Liver Largest internal organ (~1.2–1.5 kg); produces bile (800–1,000 mL/day); detoxification; glycogen storage; can regenerate from as little as 25% of original tissue
Pancreas Dual function: exocrine (digestive enzymes — lipase, trypsinogen, amylase) and endocrine (insulin, glucagon from Islets of Langerhans)
Large intestine Length: ~1.5 metres; absorbs water and electrolytes; houses gut microbiome (~38 trillion bacteria); forms faeces

Nervous System

Central Nervous System (CNS)

Structure Detail
Brain Weight: ~1.4 kg; contains approximately 86 billion neurons (the older claim of 100 billion is now considered an overestimate)
Cerebrum Largest part (~85% of brain weight); divided into left and right hemispheres; responsible for thought, memory, speech, voluntary actions
Cerebellum "Little brain" — controls coordination, balance, posture, and fine motor skills
Medulla oblongata Controls involuntary functions — breathing, heart rate, blood pressure, swallowing, vomiting
Hypothalamus Regulates body temperature, hunger, thirst, sleep, and controls the pituitary gland
Spinal cord ~45 cm long; carries nerve signals between brain and body; controls reflex actions

Neuron Structure

Component Detail
Cell body (Soma) Contains the nucleus and cytoplasm; site of protein synthesis
Dendrites Short, branching extensions that receive signals from other neurons and transmit them towards the cell body
Axon Long, slender projection that carries electrical impulses (action potentials) away from the cell body to the axon terminal
Myelin sheath Insulating layer of fat and protein around the axon; produced by Schwann cells (PNS) and oligodendrocytes (CNS); enables saltatory conduction — impulses jump between Nodes of Ranvier
Synapse Junction between two neurons; signal transmitted via neurotransmitters (e.g., acetylcholine, dopamine, serotonin) across the synaptic cleft
Conduction speed Myelinated axons: up to 150 m/s; unmyelinated axons: 0.5–10 m/s — myelin increases speed by a factor of 10 or more

Reflex Arc

A reflex arc is the neural pathway that mediates a reflex action — an involuntary, rapid response to a stimulus (e.g., pulling hand away from a hot object). It bypasses the brain, with integration occurring in the spinal cord, enabling faster response.

Five components: Receptor → Sensory (afferent) neuron → Interneuron (integration centre in spinal cord) → Motor (efferent) neuron → Effector (muscle/gland)

Peripheral Nervous System (PNS)

Division Detail
Somatic Voluntary control of skeletal muscles; 12 pairs of cranial nerves + 31 pairs of spinal nerves
Autonomic Involuntary control of internal organs; subdivided into Sympathetic (fight-or-flight) and Parasympathetic (rest-and-digest)

Endocrine System

Gland Hormones Key Facts
Pituitary Growth hormone (GH), TSH, ACTH, FSH, LH, Prolactin, ADH, Oxytocin "Master gland" — located at the base of the brain; controls other endocrine glands; has anterior and posterior lobes
Thyroid T3 (triiodothyronine), T4 (thyroxine), Calcitonin Regulates metabolism; iodine deficiency → goitre; hypothyroidism in infants → cretinism; butterfly-shaped gland in the neck
Parathyroid Parathormone (PTH) 4 small glands behind thyroid; regulates calcium levels; hypoparathyroidism → tetany (muscle spasms)
Adrenal Cortisol (cortex), Aldosterone (cortex), Adrenaline/Epinephrine (medulla) Sit atop each kidney; cortisol = stress response; adrenaline = fight-or-flight; Addison's disease = cortisol deficiency; Cushing's syndrome = cortisol excess
Pancreas Insulin (beta cells), Glucagon (alpha cells) Insulin lowers blood sugar; glucagon raises it; diabetes mellitus = insulin deficiency/resistance
Pineal Melatonin Regulates sleep-wake cycle (circadian rhythm)
Thymus Thymosin Active in childhood; site of T-cell maturation; shrinks with age

Insulin Discovery

Feature Detail
Discovered by Frederick Banting and Charles Best at the University of Toronto in 1921
Nobel Prize 1923 — awarded to Banting and J.J.R. Macleod (not Best); Banting shared his prize money with Best; Macleod shared with J.B. Collip

Excretory System (Urinary)

Feature Detail
Kidneys Two bean-shaped organs; filter blood; produce urine; each ~10–12 cm long, ~150 g
Nephron Functional unit of the kidney; approximately 1 million nephrons per kidney
Nephron parts Bowman's capsule → Proximal tubule → Loop of Henle → Distal tubule → Collecting duct
GFR Glomerular Filtration Rate: ~125 mL/min (~180 litres/day filtered; ~1.5 litres excreted as urine)
ADH Antidiuretic Hormone (from posterior pituitary) — promotes water reabsorption in collecting ducts; deficiency → diabetes insipidus
Aldosterone From adrenal cortex — promotes sodium reabsorption and potassium secretion in distal tubule
Dialysis Artificial filtration of blood when kidneys fail; haemodialysis (external machine) and peritoneal dialysis (uses abdominal lining)

Skeletal System

Feature Detail
Total bones 206 in the adult human body (infants have ~270; many fuse during growth)
Largest bone Femur (thigh bone) — ~50 cm in adults; strongest bone
Smallest bone Stapes (stirrup bone in the middle ear) — ~2.6–3.4 mm
Types of joints Immovable (skull sutures), Slightly movable (vertebrae), Freely movable — Ball-and-socket (hip), Hinge (knee), Pivot (neck)
Bone composition ~65% minerals (calcium phosphate/hydroxyapatite), ~35% organic matter (collagen); bone marrow produces blood cells
Osteoporosis Loss of bone density; common in post-menopausal women due to estrogen decline; risk factors: calcium/vitamin D deficiency

Immune System

Types of Immunity

Type Detail
Innate immunity Non-specific, immediate; includes physical barriers (skin, mucous membranes), phagocytes (neutrophils, macrophages), inflammation, complement system
Adaptive immunity Specific, develops over time; involves T cells (cell-mediated) and B cells (antibody-mediated); has memory — faster response on re-exposure

Key Cells

Cell Detail
T cells Mature in the thymus; types: Helper T cells (CD4+), Cytotoxic T cells (CD8+), Regulatory T cells
B cells Mature in bone marrow; differentiate into plasma cells that produce antibodies (immunoglobulins — IgG, IgM, IgA, IgE, IgD)
NK cells Natural Killer cells — innate immunity; destroy virus-infected and tumour cells

Vaccine Types

Type Mechanism Examples
Live attenuated Weakened pathogen; strong immune response BCG (TB), MMR, Oral Polio (Sabin)
Inactivated/Killed Dead pathogen; weaker response, needs boosters Injectable Polio (Salk), Rabies, Hepatitis A
Subunit/Recombinant Specific protein/antigen from pathogen Hepatitis B, HPV, Pertussis (acellular)
mRNA mRNA instructs cells to produce antigen Pfizer-BioNTech, Moderna (COVID-19)
Viral vector Harmless virus delivers genetic code of antigen Oxford-AstraZeneca, Sputnik V (COVID-19)
Toxoid Inactivated toxin Tetanus, Diphtheria

Vitamins and Deficiency Diseases

Vitamin Chemical Name Deficiency Disease Key Sources
A Retinol Night blindness, xerophthalmia Carrots, liver, dairy, fish oil
B₁ Thiamine Beriberi (nervous/cardiovascular) Whole grains, legumes, pork
B₂ Riboflavin Cheilosis (cracked lips), glossitis Milk, eggs, green vegetables
B₃ Niacin Pellagra (3 D's — diarrhoea, dermatitis, dementia) Meat, fish, groundnuts
B₉ Folic acid Megaloblastic anaemia, neural tube defects in foetus Leafy greens, pulses, liver
B₁₂ Cobalamin Pernicious anaemia (megaloblastic), neurological damage Meat, fish, dairy (absent in plant foods)
C Ascorbic acid Scurvy (bleeding gums, fatigue, poor wound healing) Citrus fruits, amla, guava
D Calciferol Rickets (children), osteomalacia (adults) Sunlight, fish liver oil, fortified milk
E Tocopherol Haemolytic anaemia (rare), neurological problems Nuts, seeds, vegetable oils
K Phylloquinone Impaired blood clotting, haemorrhage Green leafy vegetables, soybean oil

Reproductive System (Brief UPSC-Relevant Facts)

Feature Detail
Male Testes produce sperm and testosterone; ~300 million sperm per ejaculation; spermatogenesis takes ~74 days
Female Ovaries produce ova and estrogen/progesterone; one ovum released per ~28-day menstrual cycle; menopause typically between ages 45–55
Fertilisation Occurs in the fallopian tube (oviduct); zygote implants in the uterus
IVF In Vitro Fertilisation — fertilisation outside the body; first IVF baby: Louise Brown (25 July 1978, UK); first Indian IVF baby: Kanupriya Agarwal (3 October 1978)
Surrogacy law Surrogacy (Regulation) Act, 2021 — bans commercial surrogacy; allows only altruistic surrogacy; surrogate must be a close relative, married, aged 25–35, with at least one child; intended couple must be Indian, married for 5+ years, with no living biological/adopted child; penalty for violations: up to 10 years imprisonment and fine up to ₹10 lakh

UPSC Relevance

Prelims Focus Areas

  • Blood groups: ABO (Landsteiner 1901, Nobel 1930); Rh factor (1940); universal donor (O-), universal recipient (AB+)
  • Hormones: insulin (beta cells), glucagon (alpha cells); insulin discovered by Banting & Best (1921), Nobel 1923 (Banting & Macleod)
  • Bones: 206 in adults; femur (largest), stapes (smallest)
  • Brain: ~1.4 kg, 86 billion neurons; cerebrum largest part
  • Nephron: functional unit of kidney; GFR ~125 mL/min; ADH controls water reabsorption
  • Alveoli: ~480 million; surface area ~70 sq m
  • Liver: largest internal organ; produces bile; can regenerate
  • Vaccine types: live attenuated, inactivated, mRNA, subunit, viral vector, toxoid
  • RBC lifespan: ~120 days; 5 types of WBC
  • SA node: natural pacemaker in right atrium
  • Vitamins: A → night blindness, B₁ → beriberi, B₃ → pellagra, C → scurvy, D → rickets
  • Neuron: dendrites receive, axon transmits; myelin sheath enables saltatory conduction (up to 150 m/s)
  • Reflex arc: 5 components — receptor, sensory neuron, interneuron, motor neuron, effector
  • Surrogacy (Regulation) Act 2021: commercial surrogacy banned; only altruistic surrogacy allowed

Mains Focus Areas

  • How does the immune system distinguish between self and non-self? Relevance to autoimmune diseases
  • Organ donation and transplantation in India — ethical, legal, and medical dimensions
  • Impact of lifestyle diseases (diabetes, hypertension, cardiovascular disease) on India's public health
  • mRNA vaccine technology — implications for future pandemic preparedness
  • Endocrine disruptors in the environment — impact on human health

Vocabulary

Haemoglobin

  • Pronunciation: /ˌhiːməˈɡloʊbɪn/
  • Definition: The iron-containing oxygen-transport protein in red blood cells that binds oxygen in the lungs and releases it to tissues throughout the body, giving blood its red colour.
  • Origin: From Greek haima ("blood") + Latin globulus ("little ball," referring to the globulin protein); coined as hæmatoglobin in 1845, shortened to hæmoglobin by 1862.

Synapse

  • Pronunciation: /ˈsɪnæps/
  • Definition: The junction between two nerve cells, consisting of a minute gap across which electrical impulses are transmitted by chemical neurotransmitters such as acetylcholine and dopamine.
  • Origin: From Greek sunapsis ("conjunction"), from sun- ("together") + haptein ("to clasp, fasten"); introduced into neuroscience by Charles Sherrington in 1897.

Peristalsis

  • Pronunciation: /ˌpɛrɪˈstælsɪs/
  • Definition: The involuntary wavelike contraction and relaxation of muscles in the walls of hollow organs such as the oesophagus and intestines, which propels food and other contents forward through the digestive tract.
  • Origin: From Greek peristaltikos ("contracting around"), from peri- ("around") + stellein ("to place, set in order"); first used in a medical context by the physician Galen in the 2nd century AD.

Key Terms

Circulatory System

  • Pronunciation: /ˈsɜːkjʊlətəri ˈsɪstəm/
  • Definition: The organ system comprising the heart (a four-chambered muscular pump in humans), blood vessels (arteries, veins, and capillaries totalling ~100,000 km in length), and blood (~5 litres in an adult) that transports oxygen, nutrients, hormones, antibodies, and waste products throughout the body, maintaining homeostasis. Humans have a closed, double circulatory system: blood passes through the heart twice per complete circuit -- once through the pulmonary circuit (heart to lungs for oxygenation) and once through the systemic circuit (heart to body organs).
  • Context: William Harvey first accurately described the complete circulation of blood in his 1628 work Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus (An Anatomical Exercise on the Motion of the Heart and Blood in Living Beings), overturning Galen's 1,500-year-old model. Key blood facts: ABO blood group system (discovered by Karl Landsteiner, 1901, Nobel Prize 1930) classifies blood into A, B, AB (universal recipient for RBCs), and O (universal donor for RBCs) based on antigens on red blood cells. The Rh factor (Rh+/Rh-) is critical in pregnancy -- an Rh- mother carrying an Rh+ foetus can develop antibodies that attack subsequent Rh+ pregnancies (erythroblastosis fetalis), preventable with Rh immunoglobulin injection. Haemoglobin (containing iron) binds oxygen in the lungs and releases it in tissues; carbon monoxide (CO) binds haemoglobin 200 times more strongly than O2, explaining CO poisoning deaths.
  • UPSC Relevance: GS3 (General Science). Prelims tests blood groups (ABO system -- Landsteiner, 1901; AB universal recipient; O universal donor), Rh factor and pregnancy incompatibility, haemoglobin function (O2 transport, iron-containing), difference between arteries (thick-walled, carry oxygenated blood away from heart, except pulmonary artery) and veins (thin-walled, carry deoxygenated blood towards heart, except pulmonary vein), double circulation, blood pH (7.35-7.45), and platelets/thrombocytes (blood clotting). Mains connects to cardiovascular disease (India's leading cause of death, ~28% of total deaths), organ donation policy, blood bank regulation, and public health campaigns against hypertension and diabetes.

Endocrine System

  • Pronunciation: /ˈɛndəkrɪn ˈsɪstəm/
  • Definition: The network of ductless glands that secrete hormones directly into the bloodstream (unlike exocrine glands that secrete through ducts) to regulate metabolism, growth, reproduction, sleep, stress response, blood sugar, and other vital body functions. Key glands: hypothalamus (link between nervous and endocrine systems), pituitary (master gland, secretes growth hormone, TSH, ACTH, FSH, LH), thyroid (thyroxine -- regulates metabolic rate, requires iodine), parathyroid (calcium regulation), adrenal (adrenaline for fight-or-flight, cortisol for stress), pancreas (insulin and glucagon for blood sugar regulation), and gonads (oestrogen/progesterone in females, testosterone in males).
  • Context: The term endocrine was coined by British physiologist Ernest Starling in the early 20th century, who also coined the word "hormone" (from Greek horman, "to set in motion") in 1905. The pancreas is unique as both an endocrine gland (secreting insulin/glucagon into blood from Islets of Langerhans) and an exocrine gland (secreting digestive enzymes through ducts). Key disorders: diabetes mellitus (insufficient insulin production by beta cells in Type 1, or insulin resistance in Type 2 -- India has ~101 million diabetics, the world's second-highest count after China), goitre (iodine deficiency causing thyroid enlargement -- India's National Iodine Deficiency Disorders Control Programme mandates salt iodisation), dwarfism/gigantism (growth hormone imbalance), and Addison's disease (adrenal insufficiency). Endocrine disruptors (EDCs) -- chemicals like BPA, phthalates, and certain pesticides that interfere with hormonal function -- are an emerging environmental health concern.
  • UPSC Relevance: GS3 (General Science / Health). Prelims tests glands and their hormones -- pituitary (master gland, growth hormone), thyroid (thyroxine, iodine deficiency causes goitre), pancreas (insulin for lowering blood sugar, glucagon for raising it; diabetes = insulin deficiency/resistance), adrenal (adrenaline/epinephrine for fight-or-flight). Know vitamin deficiency diseases (A -- night blindness/xerophthalmia, B1 -- beriberi, B3 -- pellagra, C -- scurvy, D -- rickets in children/osteomalacia in adults, K -- poor blood clotting). Mains connects to India's diabetes epidemic (101 million cases), National Programme for Prevention and Control of Non-Communicable Diseases (NP-NCD), iodine deficiency programmes, and endocrine disruptors as environmental pollutants affecting reproductive health and child development.

Sources: Guyton & Hall — Textbook of Medical Physiology, NCBI StatPearls, Cleveland Clinic Health Library, NobelPrize.org, WHO Immunization Guidelines