Human Physiology I — Digestion, Respiration and Circulation — NEET Biology
Human physiology is the largest and most reliably tested part of NEET Biology, and these three systems alone yield 7–9 questions every year. They reward exact recall: which enzyme acts where, the respiratory volumes and their definitions, how oxygen and carbon dioxide travel in blood, and the events of the cardiac cycle. This chapter lays out all three systems in the ordered, value-rich form NCERT uses — the enzymes, the numbers and the mechanisms the exam asks almost verbatim.
Part A — Digestion and Absorption
1. The alimentary canal and glands
The alimentary canal runs mouth → pharynx → oesophagus → stomach → small intestine (duodenum, jejunum, ileum) → large intestine → rectum → anus. Associated digestive glands: salivary glands, liver (with gall bladder) and pancreas.
- Teeth are thecodont (embedded in sockets), diphyodont (two sets), heterodont (four types: incisors, canines, premolars, molars). Dental formula of an adult human: 2123/2123.
- The liver is the largest gland; it secretes bile (no enzymes) which emulsifies fats.
- The pancreas is a mixed gland (digestive enzymes + hormones insulin/glucagon).
2. Digestion of the three food types
Digestion is enzymatic hydrolysis, region by region:
| Region | Secretion/enzyme | Substrate → product |
|---|---|---|
| Mouth | salivary amylase (ptyalin) | starch → maltose |
| Stomach | pepsin (from pepsinogen, HCl activates) | proteins → peptides |
| (rennin in infants) | milk protein | |
| Small intestine | pancreatic amylase | starch → maltose |
| trypsin, chymotrypsin (from inactive zymogens) | proteins → peptides | |
| pancreatic lipase (bile emulsifies first) | fats → fatty acids + glycerol | |
| intestinal (brush-border) enzymes: maltase, lactase, sucrase, peptidases | to monosaccharides, amino acids |
HCl in the stomach kills microbes and provides the acidic pH for pepsin; bile raises pH and emulsifies fats. The small intestine is the main site of digestion and absorption.
Worked example 2.1. Which enzyme begins protein digestion, and what activates it? Pepsin begins protein digestion in the stomach. It is secreted as inactive pepsinogen and activated by the HCl (acidic pH) of gastric juice.
3. Absorption and disorders
Digested end-products are absorbed mainly in the small intestine across the villi (which greatly increase surface area; each has microvilli and a lacteal). Glucose and amino acids enter blood capillaries; fatty acids and glycerol re-form fats and enter lacteals (lymph) as chylomicrons. The large intestine absorbs water and forms faeces.
Disorders: jaundice (bile pigment in blood, yellow skin — liver), PEM (protein-energy malnutrition: kwashiorkor, marasmus), constipation, diarrhoea.
Worked example 3.1. How are the products of fat digestion absorbed differently from those of carbohydrate digestion? Glucose (carbohydrate) is absorbed directly into blood capillaries of the villi. Fatty acids and glycerol are re-formed into fats, packaged as chylomicrons, and enter the lacteals (lymph) first — not the blood directly.
Part B — Breathing and Respiration
4. The respiratory system and breathing
Air passes: nostrils → pharynx → larynx → trachea → bronchi → bronchioles → alveoli (the thin-walled sacs where gas exchange occurs). The lungs sit in the pleural cavity; the diaphragm and intercostal muscles drive breathing.
Mechanism of breathing (pressure-driven):
- Inspiration — diaphragm contracts (flattens) + external intercostals raise ribs → thoracic volume ↑ → intrapulmonary pressure ↓ below atmospheric → air rushes in.
- Expiration — muscles relax → volume ↓ → pressure ↑ → air pushed out (usually passive).
Respiratory volumes and capacities (learn the definitions and typical values):
| Term | Meaning | Value |
|---|---|---|
| Tidal volume (TV) | air per normal breath | ~500 mL |
| Inspiratory reserve (IRV) | extra inhaled forcibly | ~2500–3000 mL |
| Expiratory reserve (ERV) | extra exhaled forcibly | ~1000–1100 mL |
| Residual volume (RV) | air left after forced expiration | ~1100–1200 mL |
| Vital capacity (VC) | TV + IRV + ERV (max breathe out after max in) | ~3400–4600 mL |
| Total lung capacity (TLC) | VC + RV | ~5000–6000 mL |
Worked example 4.1. Define vital capacity and total lung capacity in terms of the basic volumes. Vital capacity = TV + IRV + ERV (the maximum air a person can expel after a maximum inhalation). Total lung capacity = vital capacity + residual volume (the total air the lungs can hold).
5. Gas exchange and transport
Gases move by diffusion down partial-pressure gradients. At the alveoli, O₂ (high pO₂) diffuses into blood and CO₂ diffuses out; the reverse happens at the tissues.
Oxygen transport — ~97% carried as oxyhaemoglobin (bound to haemoglobin), ~3% dissolved. The oxygen-haemoglobin dissociation curve is sigmoid; a shift to the right (more O₂ release) occurs with high CO₂, low pH (Bohr effect), high temperature — exactly the conditions in active tissue.
Carbon dioxide transport — ~70% as bicarbonate (HCO₃⁻), ~20–25% as carbamino-haemoglobin, ~7% dissolved. Carbonic anhydrase in RBCs converts CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (with a chloride shift).
Regulation — the respiratory centre in the medulla oblongata (and pons) controls breathing rate, mainly in response to CO₂ (H⁺) levels in blood, not oxygen primarily.
Worked example 5.1. What is the main form in which CO₂ is transported in blood, and what mainly regulates breathing? CO₂ is transported mainly (~70%) as bicarbonate ions (HCO₃⁻) in plasma. Breathing is regulated by the respiratory centre in the medulla oblongata, responding chiefly to rising CO₂/H⁺ in the blood.
Disorders: asthma (bronchiole inflammation), emphysema (alveolar wall damage — smoking), occupational lung diseases (silicosis).
Part C — Circulation
6. Blood, blood groups and lymph
Blood is a fluid connective tissue: plasma (~55%, mostly water with proteins albumin, globulin, fibrinogen) + formed elements (~45%):
- RBCs (erythrocytes): biconcave, no nucleus (in mammals), carry O₂ via haemoglobin; made in bone marrow; ~120-day lifespan.
- WBCs (leucocytes): defence; granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes). Neutrophils most abundant.
- Platelets (thrombocytes): clotting.
Blood groups: ABO (antigens A/B on RBC, antibodies in plasma) — O is universal donor, AB universal recipient. Rh factor — Rh⁺/Rh⁻; erythroblastosis foetalis can occur if an Rh⁻ mother carries an Rh⁺ foetus in a later pregnancy.
Lymph is the tissue fluid returned to blood; it carries WBCs, absorbs fats (via lacteals) and has no RBCs.
Worked example 6.1. Why is blood group O called the universal donor? Group O RBCs carry neither A nor B antigen, so they are not attacked by anti-A or anti-B antibodies in any recipient's plasma — allowing O blood to be given to all ABO groups (universal donor). AB, having no plasma antibodies, is the universal recipient.
7. The heart and the cardiac cycle
The human heart is four-chambered (two atria, two ventricles) with valves ensuring one-way flow: tricuspid (right AV), bicuspid/mitral (left AV), and semilunar valves at the aorta and pulmonary artery.
Conduction: the SA node (pacemaker) in the right atrium initiates each beat → AV node → bundle of His → Purkinje fibres. The heart is myogenic (beat originates in the muscle itself).
Cardiac cycle (~0.8 s at 72 beats/min): atrial systole → ventricular systole (blood ejected, AV valves close — "lub") → ventricular diastole (semilunar valves close — "dub"). The ECG records the electrical cycle: P (atrial depolarisation), QRS (ventricular depolarisation), T (ventricular repolarisation).
- Cardiac output = stroke volume × heart rate ≈ 70 mL × 72 ≈ 5 L/min.
Double circulation — blood passes through the heart twice per cycle: pulmonary (heart → lungs → heart, oxygenation) and systemic (heart → body → heart). This keeps oxygenated and deoxygenated blood fully separate.
Worked example 7.1. Calculate cardiac output for a stroke volume of 70 mL and a heart rate of 72/min. Cardiac output = stroke volume × heart rate = 70 × 72 = 5040 mL/min ≈ 5 L/min — the volume of blood pumped by each ventricle per minute.
Blood pressure — normal ~120/80 mmHg (systolic/diastolic); persistent high BP is hypertension (a risk for heart and kidney disease). Other disorders: coronary artery disease (CAD), angina, heart failure.
8. Common traps NEET sets here
- Pepsin (stomach, acidic) vs trypsin (intestine, alkaline); both digest protein but at opposite pH.
- Bile has no enzymes — it only emulsifies fats.
- Fats absorbed via lacteals (lymph); glucose/amino acids via blood.
- VC = TV + IRV + ERV; TLC = VC + RV — residual volume is not in vital capacity.
- O₂ mostly as oxyhaemoglobin (~97%); CO₂ mostly as bicarbonate (~70%).
- Breathing is regulated by CO₂/H⁺ in the medulla, not primarily O₂.
- O universal donor, AB universal recipient; Rh incompatibility → erythroblastosis foetalis.
- SA node = pacemaker; heart is myogenic; "lub" = AV valves close, "dub" = semilunar valves close.
- Cardiac output = stroke volume × heart rate ≈ 5 L/min.
- Tricuspid right, bicuspid left — don't reverse.
9. Memory aids
- "Amylase-mouth, Pepsin-stomach, Trypsin-intestine" — where each enzyme acts.
- "Bile breaks fat into bits (emulsifies), no enzyme" — the role of bile.
- "VC has no residual" — vital capacity excludes residual volume.
- "97 oxy, 70 bicarb" — O₂ and CO₂ transport percentages.
- "O gives to all, AB takes from all" — universal donor/recipient.
- "Lub AV, Dub semilunar" — the two heart sounds.
- "Tri-right, Bi-left" — atrioventricular valves.
10. Exam protocol
- Digestion: match enzyme → region → substrate/product; bile emulsifies (no enzyme); small intestine main site.
- Absorption: glucose/amino acids to blood; fats to lacteals (lymph).
- Breathing: pressure-driven inspiration/expiration (diaphragm); define TV, IRV, ERV, RV, VC (=TV+IRV+ERV), TLC (=VC+RV).
- Transport: O₂ ~97% oxyhaemoglobin; CO₂ ~70% bicarbonate; medulla regulates via CO₂/H⁺.
- Blood: plasma + cells; ABO/Rh (O donor, AB recipient); lymph carries fats/WBCs, no RBCs.
- Heart: four chambers, SA-node pacemaker, myogenic; cardiac cycle sounds; CO = SV × HR ≈ 5 L/min; double circulation; BP 120/80.
