BC02 — Human Body — Physiology
BC02 · CDS General Knowledge — Biology
★ Highest Priority — 4–5 Questions per Paper
Human physiology is the single highest-scoring chapter in CDS Biology. Questions cut across every organ system — from the nephron's filtration mechanism to the hormone that controls blood sugar. Work through each system methodically: understand its job first, then learn its parts, and finally memorise the disease that results when it fails.
CDS Focus: Largest gland = liver; largest organ = skin; functional unit of kidney = nephron; pacemaker of heart = SA node; insulin from beta cells of islets of Langerhans (pancreas); adrenaline = fight-or-flight hormone; reflex arc does NOT involve brain (involves spinal cord). These exact facts repeat across every CDS paper.
Part 1 — Digestive System
Digestion converts complex food into simple, absorbable molecules. The alimentary canal is a continuous tube from mouth to anus; associated glands add enzymes and juices at key points along the way.
1. The Alimentary Canal
Mouth (Buccal Cavity)
Secretion
Salivary amylase (ptyalin) in saliva
Key Fact
Digestion begins here
Stomach
Secretion
Gastric juice: HCl, pepsin, mucus
Action
Protein → Peptides; HCl kills bacteria
Key Fact
pH 2 (acidic); churning
Small Intestine (duodenum + jejunum)
Secretion
Bile (liver) + Pancreatic juice + Intestinal juice (lipase, amylase, protease)
Action
ALL nutrients digested and absorbed here
Key Fact
Main absorption site; villi increase surface area
Large Intestine
Secretion
No digestive enzymes
Action
Water and mineral absorption; faeces formed
Key Fact
Colon absorbs water
Associated Glands
Liver (largest gland)
Bile production; glycogen storage; detoxification
Pancreas
Exocrine (digestive enzymes) + endocrine (insulin/glucagon hormones)
Part 2 — Circulatory System
2. Circulatory System
Blood is the river that connects every cell in the body — delivering oxygen and nutrients, removing waste, and carrying the immune defenders that protect against disease. The heart is the pump; blood vessels are the channels.
RBC (Erythrocytes)
Normal Count
4.5–5.5 million per mm³
Function
Carry O₂ via haemoglobin; some CO₂ transport
Key Fact
No nucleus in mature RBC; life span 120 days
WBC (Leukocytes)
Normal Count
5,000–10,000 per mm³
Function
Immunity; fight infection; phagocytosis of pathogens
Key Fact
Have nucleus; 5 types; increased in infection
Platelets (Thrombocytes)
Normal Count
1.5–4 lakh per mm³
Function
Blood clotting (coagulation); prevents excessive bleeding
Key Fact
No nucleus; smallest blood cell; life span 8–10 days
Plasma (55% of blood)
Composition
Straw-coloured liquid; 90% water
Carries
Digested food, hormones, antibodies, CO₂, urea (waste), fibrinogen (clotting protein)
Part 3 — Excretory & Nervous Systems
3. Excretory System — The Nephron
The kidney's job is to filter waste products from blood and eliminate them as urine. The functional unit doing this work is the nephron — there are about 1 million nephrons in each kidney.
Stage 1 — Ultrafiltration
Where
Glomerulus (in Bowman's capsule)
What
Blood filtered under pressure. Water, glucose, urea, salts pass into Bowman's capsule.
Key Fact
Proteins stay in blood (too large to filter)
Stage 2 — Selective Reabsorption
Where
Proximal Convoluted Tubule, Loop of Henle
What
Useful substances reabsorbed back into blood: glucose, amino acids, water, salts.
Key Fact
Glucose completely reabsorbed (normally)
Stage 3 — Secretion
Where
Distal Convoluted Tubule, Collecting duct
What
Extra waste actively secreted into filtrate: H⁺, K⁺, ammonia, drugs.
Key Fact
Final urine: urea, water, salts, creatinine
4. Nervous System & Endocrine System
Nervous System Key Points
CNS
Brain (cerebrum — thinking; cerebellum — balance; medulla — autonomic) + Spinal cord
PNS
Somatic (voluntary, controls skeletal muscles) + Autonomic (involuntary: sympathetic/parasympathetic)
Reflex arc
Receptor → Sensory neuron → Spinal cord → Motor neuron → Effector. Brain NOT involved
Neuron
Basic unit; impulse travels via dendrite → cell body → axon → synapse
Speed
Nerve impulse: ~120 m/s in myelinated fibres
Endocrine Glands & Hormones
Pituitary
"Master gland"; controls other glands; GH, TSH, ADH
Thyroid
Thyroxine (T4); controls metabolic rate; iodine needed
Adrenal (cortex+medulla)
Adrenaline: fight-or-flight; cortisol: stress
Pancreas
Insulin (β-cells): lowers blood sugar; Glucagon (α-cells): raises blood sugar
Testes/Ovaries
Testosterone/Oestrogen: secondary sex characters
Part 4 — Respiratory & Skeletal Systems
5. Respiratory System
Every cell needs oxygen to produce ATP and generates carbon dioxide as waste. The respiratory system is the exchange gateway — bringing O₂ in and removing CO₂ with each breath.
CDS Direct Point — Haemoglobin Affinity: Carbon monoxide (CO) binds haemoglobin with 240× more affinity than oxygen — forming stable carboxyhaemoglobin. This blocks O₂ transport → CO poisoning causes suffocation even with normal breathing. This is why CO from vehicle exhaust or burning charcoal in an enclosed room is lethal.
6. Skeletal System
Skeletal System — Key Facts
Total bones
206 in adults (infant: ~270; fuse during development)
Longest bone
Femur (thigh bone)
Smallest bone
Stapes (in middle ear)
Hardest substance
Enamel (tooth enamel; made of calcium phosphate hydroxyapatite)
Bone composition
Calcium phosphate (Ca₃(PO₄)₂) gives hardness; collagen fibres give flexibility
Joint types (CDS tested)
Ball-and-socket: shoulder, hip (full rotation) | Hinge: elbow, knee (one plane) | Pivot: atlas-axis (head rotation) | Gliding: wrist, ankle | Fixed (suture): skull bones
Functions of skeleton
Support, protection of organs (skull→brain; ribcage→heart+lungs), movement (with muscles), blood cell production (red marrow), mineral storage (Ca, P)
CDS PYQs — Human Body
Q1. The functional unit of the kidney is the:CDS PYQ
(a) Nephron
(b) Neuron
(c) Alveolus
(d) Glomerulus
✓ Answer: (a) Nephron
The nephron is the structural and functional unit of the kidney. Each human kidney contains approximately 1 million nephrons. Each nephron consists of a Bowman's capsule (containing the glomerulus for filtration), a proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct. The glomerulus is just one component of the nephron — it is the filtration site, not the functional unit itself.
Q2. The natural pacemaker of the human heart is:CDS PYQ
(a) AV node
(b) Bundle of His
(c) SA node
(d) Purkinje fibres
✓ Answer: (c) SA node (Sinoatrial node)
The SA node (sinoatrial node), located in the right atrium wall, generates the electrical impulse that initiates every heartbeat — making it the natural pacemaker. The impulse travels to the AV node → Bundle of His → Purkinje fibres, causing coordinated heart muscle contraction. When the SA node fails, an artificial pacemaker is surgically implanted. Normal resting heart rate: 72 beats/minute.
Q3. Insulin is produced by which cells of the pancreas?CDS PYQ
(a) Acinar cells
(b) Alpha cells of islets
(c) Beta cells of islets
(d) Delta cells
✓ Answer: (c) Beta cells of islets of Langerhans
The pancreas has two roles: exocrine (digestive enzymes via pancreatic duct) and endocrine. The Islets of Langerhans are clusters of endocrine cells. Beta (β) cells secrete insulin when blood glucose rises — insulin allows cells to absorb glucose, lowering blood sugar. Alpha (α) cells secrete glucagon (raises blood sugar). Deficiency of insulin causes Diabetes mellitus (Type 1). This is one of the most frequently repeated CDS questions.
Q4. A reflex action is controlled by:CDS PYQ
(a) Cerebrum
(b) Cerebellum
(c) Medulla oblongata
(d) Spinal cord
✓ Answer: (d) Spinal cord
A reflex arc bypasses the brain entirely. The pathway: Stimulus → Receptor → Sensory neuron → Spinal cord → Motor neuron → Effector (muscle/gland). The spinal cord integrates the reflex response. Examples: knee-jerk reflex, withdrawal reflex (hand from fire). The brain perceives the sensation afterwards — this is why you pull your hand away BEFORE feeling pain. CDS tests this "where is reflex controlled?" concept directly.
Q5. Which of the following carries oxygenated blood from the lungs to the heart?⚡ Tricky
(a) Pulmonary artery
(b) Pulmonary vein
(c) Aorta
(d) Vena cava
✓ Answer: (b) Pulmonary vein
This is the classic CDS trap: arteries carry blood away from the heart; veins carry blood toward the heart. The pulmonary vein is the exception — it carries oxygenated blood FROM the lungs TO the left atrium. The pulmonary artery carries deoxygenated blood FROM the right ventricle TO the lungs for oxygenation. Remember: pulmonary vein = oxygenated (despite being a vein!).
Q6. The smallest bone in the human body is:CDS PYQ
(a) Patella
(b) Stapes
(c) Malleus
(d) Incus
✓ Answer: (b) Stapes
The stapes (stirrup bone) in the middle ear is the smallest and lightest bone in the human body — approximately 3 mm long. All three middle ear ossicles (malleus, incus, stapes) are tiny, but the stapes is the smallest. They transmit sound vibrations from the eardrum to the inner ear. The largest bone is the femur (thigh bone). This contrast (largest/smallest bone) is a classic direct CDS question.
Q7. Gas exchange in the human lungs occurs in the:CDS PYQ
(a) Trachea
(b) Bronchi
(c) Alveoli
(d) Bronchioles
✓ Answer: (c) Alveoli
The alveoli are tiny, thin-walled air sacs at the end of the bronchioles — around 700 million in both lungs, providing a surface area of ~70 sq metres (size of a badminton court). Their thin walls and rich capillary network enable rapid diffusion of O₂ (into blood) and CO₂ (into alveoli). Breathing moves air to alveoli; actual gas exchange happens here by simple diffusion driven by concentration gradients.
Quick Memory Chart — BC02
Digestion
- Starch: salivary amylase (mouth)
- Protein: pepsin (stomach, acidic)
- All: small intestine (main site)
- Liver: largest gland; bile (emulsifies fat)
- Pancreas: insulin + digestive enzymes
Heart & Blood
- SA node = pacemaker (right atrium)
- RBC: no nucleus; 120-day life; O₂ transport
- WBC: immunity; have nucleus
- Platelets: clotting; no nucleus
- Pulmonary vein: carries oxygenated blood!
Hormones
- Pituitary: master gland; GH, ADH
- Insulin (β-cells): lowers blood glucose
- Glucagon (α-cells): raises blood glucose
- Adrenaline: fight-or-flight
- Reflex: spinal cord (NOT brain)
Practice Exercise
E1. Which part of the brain controls balance and coordination?
(a) Cerebrum
(b) Medulla oblongata
(c) Cerebellum
(d) Hypothalamus
E2. The largest gland in the human body is the:
(a) Pancreas
(b) Thyroid
(c) Liver
(d) Spleen
E3. In diabetic patients, glucose appears in urine because:
(a) Kidneys stop filtering
(b) Blood glucose exceeds renal threshold; tubules cannot reabsorb all of it
(c) Insulin is over-produced
(d) Glomerular filtration rate is zero
Answers:
E1 → (c) Cerebellum [co-ordinates fine motor movement and maintains posture/balance] |
E2 → (c) Liver [~1.5 kg; functions: bile, glycogen storage, detox, plasma protein synthesis] |
E3 → (b) Renal threshold exceeded [normal renal threshold for glucose ~180 mg/dL; above this, tubules cannot reabsorb all filtered glucose → glycosuria]
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