📖 Chapter BN03 · NDA Class 11–12 Level🎯 NDA Level : High Priority
Human Physiology is the largest and most heavily tested Biology chapter in NDA. Questions span all seven organ systems — digestive, respiratory, circulatory, excretory, nervous, skeletal, and endocrine. Factual recall of gland-hormone pairs, blood group properties, heart chambers, kidney nephron steps, and cranial nerve count are direct NDA favourites. This chapter rewards systematic revision of one system at a time.
Major site of digestion and absorption. Duodenum receives bile (liver) + pancreatic juice. Jejunum and Ileum absorb nutrients via villi. SI = 6–7 m long. Succus entericus (intestinal juice) completes digestion.
Liver
Produces bile (stored in gall bladder). Bile emulsifies fats (breaks into droplets for lipase). Also: detoxification, glycogen storage, plasma protein synthesis, heat production.
Pancreas
Exocrine: secretes pancreatic juice (trypsin, lipase, amylase, nuclease). Endocrine: secretes insulin and glucagon. Only gland with dual function (exo + endocrine).
Large Intestine
Absorbs water and electrolytes. Bacteria synthesise Vitamin K and some B vitamins. Compacts waste into faeces. No digestion of nutrients.
📌 Digestive Enzyme Table — NDA Most Tested:
Enzyme
Source
Substrate
Product
Salivary Amylase (Ptyalin)
Salivary glands
Starch
Maltose
Pepsin
Gastric glands (stomach)
Proteins
Peptides
Rennin
Stomach (infants)
Casein (milk protein)
Paracasein (clot)
Trypsin
Pancreas
Proteins/Peptides
Amino acids
Pancreatic Amylase
Pancreas
Starch/Glycogen
Maltose
Pancreatic Lipase
Pancreas
Emulsified fats
Fatty acids + Glycerol
Maltase / Sucrase / Lactase
Small intestine (brush border)
Maltose / Sucrose / Lactose
Glucose / Fructose / Galactose
🧠 Absorption Sites:
Fats → absorbed into lacteals (lymph capillaries of villi) → lymphatic system → blood
Glucose, amino acids, vitamins, minerals → absorbed into blood capillaries → portal vein → liver
Water → mainly large intestine (also small intestine)
Vitamin B12 requires intrinsic factor (from stomach) for absorption in ileum.
PYQTopic-Wise PYQs — Digestive System
Q1. Which enzyme is responsible for the digestion of proteins in the stomach?
A. Amylase
B. Lipase
C. Pepsin
D. Trypsin
Answer: C — Pepsin. Pepsin is secreted as inactive pepsinogen by gastric chief cells. HCl activates it. Pepsin breaks proteins into smaller peptides. Trypsin (pancreas) and further intestinal enzymes complete protein digestion to amino acids. Amylase = starch; Lipase = fats.
Q2. Bile is produced by the liver and stored in the gall bladder. Its primary function in digestion is:
A. Digesting proteins
B. Emulsifying fats
C. Neutralising stomach acid
D. Converting starch to sugar
Answer: B — Emulsifying fats. Bile does NOT contain digestive enzymes — it contains bile salts (emulsifiers) that break large fat globules into tiny droplets, increasing the surface area for pancreatic lipase to act. Bile also neutralises HCl from the stomach (alkaline pH 7.6–8.6), making Option C partially correct, but emulsification is the primary digestive function tested in NDA.
Q3. Fat-soluble vitamins are absorbed from the small intestine through:
A. Blood capillaries directly
B. Lacteals (lymph capillaries)
C. Portal vein
D. Hepatic artery
Answer: B — Lacteals. Fat-soluble vitamins (A, D, E, K) and dietary fats are absorbed into lacteals (lymphatic capillaries inside villi), not blood capillaries directly. They enter the lymph as chylomicrons and eventually reach the bloodstream via the thoracic duct. Water-soluble vitamins (B, C) and glucose/amino acids go into blood capillaries.
TRICKY🧐 Digestive System Traps
⚠️ "Pancreas is only an endocrine gland." True or False?
False. The pancreas is a mixed gland — both exocrine AND endocrine. Exocrine function: acinar cells secrete pancreatic juice (enzymes) into the duodenum via the pancreatic duct. Endocrine function: Islets of Langerhans secrete insulin (β cells) and glucagon (α cells) directly into the blood. NDA has repeatedly asked about this dual function.
⚠️ "Food moves through the oesophagus because of gravity." True or False?
False. Food moves by peristalsis — rhythmic wave-like muscular contractions of the oesophageal wall. This is why you can swallow food even upside down. If it were gravity, astronauts in space couldn't swallow food, which they can. Peristalsis operates throughout the alimentary canal.
2. Respiratory System
2.1
Organs, Breathing Mechanism & Gas Transport
Know the pathway of air and how O₂/CO₂ are transported in blood — both are NDA tested
📌 Gas Exchange at Alveoli: Alveoli walls are one cell thick (simple squamous epithelium). O₂ diffuses from alveolar air into blood (high → low pressure). CO₂ diffuses from blood into alveolar air. This is simple diffusion — no active transport. Alveoli are surrounded by dense capillary networks (pulmonary capillaries).
PYQTopic-Wise PYQs — Respiratory System
Q4. Most of the CO₂ produced during cellular respiration is transported in blood as:
A. Dissolved CO₂ in plasma
B. Carbaminohaemoglobin
C. Bicarbonate ions (HCO₃⁻)
D. Oxyhaemoglobin
Answer: C — Bicarbonate ions (HCO₃⁻). About 70% of CO₂ is converted to bicarbonate ions in RBCs by the enzyme carbonic anhydrase, and transported in plasma. ~23% is carried as carbaminohaemoglobin, and ~7% is dissolved. Oxyhaemoglobin transports O₂, not CO₂.
Q5. The site of gas exchange in the human respiratory system is:
A. Trachea
B. Bronchi
C. Bronchioles
D. Alveoli
Answer: D — Alveoli. The ~300 million alveoli provide a massive surface area (~70 m² — size of a tennis court) for gas exchange. Their extremely thin walls (one cell thick) and rich capillary supply enable rapid diffusion of O₂ into blood and CO₂ out.
3. Circulatory System
3.1
Human Heart — Structure & Double Circulation
4 chambers, 4 valves, pacemaker, and the two circuits — all tested in NDA
Fig. 1 — Human Heart: RA/LA = atria (receive blood); RV/LV = ventricles (pump blood). Left ventricle wall is thickest — pumps blood to the entire body via the aorta.
Blood group, RBC lifespan, clotting, and vessel comparison — all direct NDA MCQ topics
RBC (Erythrocytes)
4–5 million/mm³
No nucleus (mature mammalian RBC)
Lifespan: 120 days
Destroyed in spleen ("graveyard")
Formed in red bone marrow
Contain haemoglobin (O₂ transport)
Biconcave disc shape
WBC (Leukocytes)
5,000–10,000/mm³
Have nucleus; colourless
Lifespan: days to years (variable)
Immunity and defence
Neutrophils (most common ~60–70%): phagocytosis
Lymphocytes: antibody production
Monocytes: phagocytosis
Platelets (Thrombocytes)
1.5–4 lakh/mm³
Smallest; no nucleus
Lifespan: 5–10 days
Blood clotting (coagulation)
Release thrombokinase → clotting cascade
Low platelets → dengue/thrombocytopenia
Plasma
~55% of blood volume
~90% water; pale yellow fluid
Proteins: albumin, globulin, fibrinogen
Transports: nutrients, hormones, waste
Fibrinogen → fibrin (clotting)
Serum = plasma minus fibrinogen
📌 Blood Group System (ABO + Rh) — NDA Direct Questions: O⁻ (O Rh-negative) = Universal donor | AB⁺ (AB Rh-positive) = Universal recipient
Blood group O: no A/B antigens on RBC; both anti-A and anti-B antibodies in plasma
Blood group AB: both A and B antigens on RBC; no antibodies in plasma
Rh factor: Rh+ has Rh antigen; Rh– does not. Rh incompatibility → erythroblastosis foetalis in baby
Vessel
Direction
Wall
Valves
Blood type
Artery
Away from heart
Thick, muscular, elastic
Absent
Mostly oxygenated (exception: pulmonary artery)
Vein
Toward heart
Thin, less elastic
Present (prevent backflow)
Mostly deoxygenated (exception: pulmonary vein)
Capillary
Connects artery to vein
One cell thick (endothelium)
Absent
Mixed; site of gas/nutrient exchange
PYQTopic-Wise PYQs — Circulatory System
Q6. Which blood group is called the "Universal Donor"?
A. AB positive
B. O negative
C. A positive
D. B negative
Answer: B — O negative (O⁻). O⁻ RBCs have no A, B, or Rh antigens, so they can be given to patients of any blood group without triggering an immune reaction. AB⁺ is the universal recipient (can accept all blood types). This is one of NDA's most repeated Biology facts.
Q7. The pacemaker of the human heart is the:
A. AV node
B. Bundle of His
C. SA node
D. Purkinje fibres
Answer: C — SA node (Sinoatrial node). The SA node is located in the wall of the Right Atrium and generates electrical impulses that set the heart's rhythm (~72 beats/min). It is called the natural pacemaker. The AV node relays the impulse to ventricles. Purkinje fibres distribute it through the ventricular wall.
Q8. Which of the following carries deoxygenated blood?
A. Aorta
B. Pulmonary vein
C. Pulmonary artery
D. Carotid artery
Answer: C — Pulmonary artery. The pulmonary artery is the only artery that carries deoxygenated blood (from RV to lungs). The pulmonary vein is the only vein that carries oxygenated blood (from lungs to LA). This reversal of the "arteries = oxygenated" rule is one of NDA's classic Biology traps.
TRICKY🧐 Circulatory System Traps
⚠️ "The lifespan of RBCs is 120 days — where are they destroyed?"
Spleen — called the "graveyard of RBCs." After 120 days, old RBCs are phagocytosed by macrophages in the spleen and liver. Haemoglobin is broken down: haem → bilirubin (pigment; excreted in bile, giving stool its colour); globin → recycled amino acids; iron → stored and reused for new RBC production in bone marrow. NDA may ask: "Where is haemoglobin broken down?" → Spleen and liver.
4. Excretory System
4.1
Kidney Structure & Urine Formation
Three-step urine formation (filtration → reabsorption → secretion) is the key NDA sequence
🟡 Kidney — Key Facts
Bean-shaped; one pair; retroperitoneal (behind peritoneum)
Right kidney slightly lower (due to liver)
Each kidney: ~1 million nephrons (functional units)
Outer cortex + inner medulla + renal pelvis
Receives blood via renal artery (from abdominal aorta)
Urine leaves via ureter → urinary bladder → urethra
PCT (Proximal Convoluted Tubule): glucose, amino acids, 70% water reabsorbed
Loop of Henle: concentration of urine (ascending = salt, descending = water)
DCT (Distal Convoluted Tubule): selective reabsorption and secretion
Collecting duct: final water reabsorption (ADH-regulated); opens into renal pelvis
Step 1: Glomerular Filtration
Blood filtered under pressure in glomerulus into Bowman's capsule. Filtrate contains: water, glucose, urea, salts, amino acids. Proteins and RBCs do NOT pass (too large). Rate: ~180 L/day (Glomerular Filtration Rate = GFR).
Step 2: Selective Reabsorption
Useful substances reabsorbed back into blood: all glucose, amino acids, 99% water, Na⁺, K⁺. Glucose in urine (glycosuria) indicates diabetes — glucose not fully reabsorbed when blood level exceeds renal threshold.
Step 3: Tubular Secretion
Additional waste actively secreted into tubule from peritubular capillaries: H⁺, K⁺, NH₃, certain drugs. This further cleans the blood. Final urine: ~1.5 L/day (of the 180 L filtered).
🧠 Urine Composition & Clinical Facts (NDA):
Normal urine: ~95% water, urea (2%), uric acid, creatinine, salts; no glucose, no proteins Glycosuria (glucose in urine) = Diabetes mellitus | Proteinuria = kidney damage ADH (Anti-Diuretic Hormone) from posterior pituitary → controls water reabsorption in collecting duct Dialysis: artificial filtration of blood when kidneys fail (haemodialysis)
PYQTopic-Wise PYQs — Excretory System
Q9. The functional unit of the kidney is the:
A. Nephron
B. Glomerulus
C. Renal tubule
D. Loop of Henle
Answer: A — Nephron. Each kidney contains about 1 million nephrons. The nephron is the complete structural and functional unit responsible for filtration, reabsorption, and secretion. The glomerulus is just one part of the nephron (filtration component), not the complete unit.
Q10. Presence of glucose in urine (glycosuria) is associated with:
A. Kidney stones
B. Diabetes insipidus
C. Diabetes mellitus
D. Proteinuria
Answer: C — Diabetes mellitus. In diabetes mellitus, blood glucose exceeds the renal threshold (~180 mg/dL). The kidney cannot reabsorb all the glucose, so it spills into urine. Diabetes insipidus is caused by ADH deficiency and results in large volumes of dilute urine (but no glucose). Proteinuria indicates kidney membrane damage.
5. Nervous System
5.1
CNS, PNS & ANS
Cranial nerve count, brain parts and functions, autonomic divisions — NDA direct recall facts
🧠 CNS — Brain
Cerebrum (Forebrain): voluntary actions, intelligence, memory, speech, sensory interpretation. Largest part (~85% brain weight)
Cerebellum (Hindbrain): balance, posture, coordination of voluntary movements. "Little brain."
Medulla oblongata (Hindbrain): controls involuntary functions — breathing rate, heartbeat, vomiting, swallowing. Most vital centre.
📌 Reflex Arc (NDA Favourite):
Stimulus → Receptor → Afferent nerve (sensory) → Spinal cord (integration) → Efferent nerve (motor) → Effector (muscle/gland) → Response Reflex actions bypass the brain — they are mediated by the spinal cord. Examples: knee-jerk reflex (patellar reflex), withdrawal reflex (touching hot object), pupillary reflex. The time from stimulus to response is called reflex time.
🧠 Cranial Nerves — 12 Pairs Mnemonic: "On Old Olympus Towering Top A Finn And German Viewed Some Hops"
I-Olfactory · II-Optic · III-Oculomotor · IV-Trochlear · V-Trigeminal · VI-Abducens · VII-Facial · VIII-Auditory(Vestibulocochlear) · IX-Glossopharyngeal · X-Vagus · XI-Accessory · XII-Hypoglossal
PYQTopic-Wise PYQs — Nervous System
Q11. The number of pairs of cranial nerves in humans is:
A. 10 pairs
B. 31 pairs
C. 12 pairs
D. 24 pairs
Answer: C — 12 pairs. Humans have 12 pairs of cranial nerves originating from the brain. There are 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal). Confusing 12 and 31 is a classic NDA trap — 12 = cranial, 31 = spinal.
Q12. Which part of the brain is responsible for maintaining body balance and coordination?
A. Cerebrum
B. Medulla oblongata
C. Hypothalamus
D. Cerebellum
Answer: D — Cerebellum. The cerebellum coordinates voluntary movements, maintains posture, and regulates balance. Damage to the cerebellum causes ataxia (loss of coordination). Cerebrum handles voluntary decisions; Medulla controls involuntary functions (breathing, heartbeat); Hypothalamus controls autonomic functions and hormone release.
TRICKY🧐 Nervous System Traps
⚠️ "Reflex actions involve the brain." True or False?
False — for spinal reflexes. Spinal reflex actions (knee-jerk, withdrawal from pain) are mediated entirely through the spinal cord without brain involvement. The impulse goes: receptor → sensory neuron → spinal cord → motor neuron → effector. The brain is informed after the reflex has occurred (you feel the pain after you've withdrawn your hand). However, some cranial reflexes (pupillary reflex, blink reflex) DO involve the brain stem, not the spinal cord.
6. Skeletal System
6.1
Skeleton — Functions, Key Bones & Joints
Total bone count, joint types with examples, and axial vs appendicular — NDA recall facts
🦵 Functions of the Skeleton
Support: provides structural framework; maintains body shape
📌 Bone vs Cartilage:
Bone: hard; matrix of calcium phosphate (hydroxyapatite) + collagen fibres; vascularised (has blood supply)
Cartilage: flexible; matrix of chondroitin sulphate + collagen; avascular (no blood supply) — heals slowly after injury
Cartilage found at: nose tip, ear pinna, trachea rings, intervertebral discs, joint surfaces (articular cartilage)
PYQTopic-Wise PYQs — Skeletal System
Q13. The smallest bone in the human body is:
A. Stapes
B. Femur
C. Patella
D. Malleus
Answer: A — Stapes. The stapes (stirrup) is the smallest bone in the human body, located in the middle ear. It is one of the three ossicles (malleus, incus, stapes) that transmit sound vibrations from the eardrum to the inner ear. Femur is the longest and strongest bone.
Q14. The type of joint found at the shoulder is:
A. Hinge joint
B. Pivot joint
C. Ball and socket joint
D. Fixed joint
Answer: C — Ball and socket joint. Ball and socket joints (shoulder and hip) allow maximum range of motion in all directions (360°). Hinge joints (elbow, knee) allow movement in one plane only. Pivot joints (atlas-axis) allow rotation. Fixed joints (skull sutures) allow no movement.
7. Endocrine System
7.1
Important Glands, Hormones & Disorders
Gland → hormone → function → deficiency/excess — the complete NDA format
🔵 Pituitary Gland
Base of brain; "Master gland" — controls other glands
📌 Hormone Deficiency/Excess Summary — NDA Most Repeated Disorders:
Hormone
Deficiency Disorder
Excess Disorder
GH (child)
Dwarfism
Gigantism
GH (adult)
–
Acromegaly
ADH
Diabetes Insipidus (excess dilute urine)
–
Thyroxine (child)
Cretinism
–
Thyroxine (adult)
Myxoedema (hypothyroidism)
Graves' disease (goitre + protruding eyes)
Insulin
Diabetes Mellitus
Hypoglycaemia (low blood sugar)
PTH
Tetany (muscle spasms)
Bone resorption
Cortisol
Addison's disease
Cushing's syndrome
PYQTopic-Wise PYQs — Endocrine System
Q15. Deficiency of insulin causes:
A. Diabetes insipidus
B. Diabetes mellitus
C. Myxoedema
D. Cretinism
Answer: B — Diabetes mellitus. Insulin (from β cells of islets of Langerhans in pancreas) lowers blood glucose. Deficiency → high blood glucose (hyperglycaemia) → Diabetes mellitus. Diabetes insipidus is caused by ADH deficiency — these two diseases share "diabetes" in the name but are completely unrelated. Myxoedema = hypothyroidism; Cretinism = thyroid deficiency in childhood.
Q16. Which gland is called the "Master gland" of the endocrine system?
A. Thyroid gland
B. Adrenal gland
C. Pituitary gland
D. Pancreas
Answer: C — Pituitary gland. The pituitary gland (hypophysis) secretes tropic hormones (TSH, ACTH, FSH, LH) that control the activity of other endocrine glands. This is why it's called the master gland. However, the pituitary itself is controlled by the hypothalamus via releasing and inhibiting hormones — so the hypothalamus is sometimes called the "master of the master gland."
Q17. Cretinism is caused by deficiency of which hormone during childhood?
A. Growth hormone
B. Insulin
C. Thyroxine
D. Cortisol
Answer: C — Thyroxine. Thyroxine deficiency during foetal development or early childhood causes cretinism — characterised by stunted physical growth (dwarfism-like) and severe mental retardation. This differs from GH deficiency (dwarfism) in that cretinism also involves mental impairment. Iodine deficiency in diet is the root cause.
TRICKY🧐 Endocrine & Multi-System Traps
⚠️ "Diabetes insipidus and Diabetes mellitus are caused by the same hormone deficiency." True or False?
False — completely different glands and hormones. Diabetes mellitus → insulin deficiency (Pancreas → Islets of Langerhans). Diabetes insipidus → ADH (vasopressin) deficiency (Posterior Pituitary). Common feature: "diabetes" means excessive urination. DM: urine contains glucose (sweet); DI: urine is very dilute (insipid = tasteless). NDA has tested this as a direct match-the-column question in 2023 and 2024.
⚠️ "Adrenaline is a steroid hormone." True or False?
False. Adrenaline (epinephrine) is an amino acid-derived hormone (catecholamine), not a steroid. Steroid hormones are derived from cholesterol — examples: cortisol, aldosterone (adrenal cortex), testosterone, oestrogen, progesterone. The adrenal medulla (adrenaline) and adrenal cortex (cortisol, aldosterone) are in the same gland but produce chemically different hormones. This distinction is tested in NDA statement-type questions.
📄 Quick-Reference Fact Sheet — BN03
🨘 Digestive Enzyme Quick-Ref
Salivary amylase → starch → maltose (mouth)
Pepsin → protein → peptides (stomach, HCl activates)
Rennin → casein in milk (infants only)
Trypsin → protein → amino acids (pancreas)
Pancreatic lipase → fats → fatty acids + glycerol
Bile: emulsifies fats (no enzymes); from liver, stored in gall bladder
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