Digestion, respiration and circulation

In short

Physiology of digestion, respiration and circulation is the study of how an animal takes in food and converts it to absorbable molecules, how it exchanges O₂ and CO₂ with its environment, and how blood transports these along with heat, hormones and wastes. The three systems are studied together because they form one supply chain: digestion loads nutrients into blood, respiration loads O₂, and circulation delivers both to every cell. In B.Sc Zoology under UGC-CBCS this is the heaviest-weighted unit of Animal Physiology, with enzyme tables, the oxygen dissociation curve and the cardiac cycle appearing every year. It is examined in CSIR-UGC NET Life Sciences, GATE-XL (Animal Science section), ICMR/DBT-JRF, AIIMS/JIPMER nursing and paramedical entrances, and in interviews for clinical-laboratory, pharma clinical-research, animal-house and NABL diagnostic-lab roles.

Explained

Alimentary canal and the mechanics of digestion

• Digestion: hydrolytic breakdown of macromolecules into absorbable units — polysaccharides to monosaccharides, proteins to amino acids, fats to fatty acids and monoglycerides, nucleic acids to nucleotides and bases.
• Types by site: *extracellular* (in a gut lumen — all vertebrates), *intracellular* (in food vacuoles — protozoa, sponges), *extra-corporeal* (enzymes secreted outside — spiders, some insects).
• Canal layers (inside out): mucosa, submucosa (Meissner's plexus), muscularis externa — inner circular + outer longitudinal (Auerbach's myenteric plexus between them), serosa.
• Mouth: mastication increases surface area; saliva (1–1.5 L/day, pH 6.8) from parotid, submandibular and sublingual glands carries salivary α-amylase (ptyalin), lingual lipase, lysozyme, mucin and bicarbonate. Human dentition is thecodont, diphyodont, heterodont, formula 2123/2123 × 2 = 32.
• Deglutition: buccal phase voluntary; pharyngeal and oesophageal phases reflex, controlled by the deglutition centre in the medulla. The epiglottis closes the glottis.
• Peristalsis: a travelling wave of contraction behind and relaxation ahead of the bolus; in the small intestine *segmentation* (mixing) and *migrating motor complexes* (housekeeping, interdigestive) also occur.
• Stomach: cardia, fundus, body, pyloric antrum. Gastric glands hold mucous (neck) cells, parietal (oxyntic) cells → HCl + intrinsic factor, chief (peptic/zymogen) cells → pepsinogen + gastric lipase, G-cells (pyloric) → gastrin, ECL cells → histamine, D-cells → somatostatin.
• HCl secretion: CO₂ + H₂O → H₂CO₃ (carbonic anhydrase) → H⁺ + HCO₃⁻; H⁺ pumped into the lumen by the H⁺/K⁺-ATPase (proton pump), Cl⁻ follows through a channel, HCO₃⁻ leaves into blood (the "alkaline tide"). Lumen pH 1.5–2.5.
• Functions of HCl: activates pepsinogen → pepsin (autocatalytic below pH 5), denatures protein, kills microbes, converts Fe³⁺→Fe²⁺ for absorption.
• Mucosal barrier: mucus + HCO₃⁻ gel, tight junctions, rapid cell turnover, prostaglandin-driven blood flow. NSAIDs and *Helicobacter pylori* break it → peptic ulcer; treated with proton-pump inhibitors.
• Small intestine (≈6 m): duodenum, jejunum, ileum. Surface amplification ≈600× by *plicae circulares* (valves of Kerckring) → *villi* → *microvilli* (brush border). Crypts of Lieberkühn hold enterocytes, goblet cells, Paneth cells (lysozyme, defensins), enteroendocrine cells and stem cells. Brunner's glands (duodenal submucosa) secrete alkaline mucus.
• Large intestine: no villi; absorbs water and electrolytes, hosts microbiota that synthesise vitamin K, B₁₂, biotin and folate and ferment fibre to short-chain fatty acids. Caecum and vermiform appendix are vestigial in humans but large and functional in herbivores (hindgut fermenters such as horse and rabbit); ruminants ferment in a four-chambered foregut — rumen, reticulum, omasum, abomasum (only the abomasum is a true glandular stomach).

Secretions, enzymes and absorption

• Pancreatic juice (≈1.2–1.5 L/day, pH 7.1–8.2, HCO₃⁻-rich): trypsinogen, chymotrypsinogen, procarboxypeptidase, proelastase, pancreatic α-amylase, pancreatic lipase + colipase, phospholipase A₂, cholesterol esterase, nucleases. Enterokinase (enteropeptidase) from the duodenal brush border converts trypsinogen → trypsin, which then activates all the other zymogens — the key cascade question in exams.
• Bile (0.8–1 L/day from hepatocytes, concentrated 5–10× in the gall bladder): bile salts (glycocholate, taurocholate), lecithin, cholesterol, bilirubin, HCO₃⁻. No enzymes. Bile salts are amphipathic: they emulsify fat into micelles, raising lipase access, and carry fat-soluble vitamins A, D, E, K. 95% are recycled by the enterohepatic circulation through the ileum and portal vein.
• Brush-border enzymes: maltase, sucrase-isomaltase, lactase, trehalase, aminopeptidases, dipeptidases, nucleotidases, alkaline phosphatase. Lactase declines after weaning in most Indian adults → lactose intolerance (very high prevalence in India, especially south and east).
• Absorption mechanisms: glucose and galactose by SGLT1 (Na⁺-coupled secondary active transport) and out through GLUT2; fructose by GLUT5 (facilitated). Amino acids by Na⁺-coupled carriers; di-/tripeptides by PepT1 (H⁺-coupled). Fatty acids diffuse from micelles, are re-esterified in the enterocyte, packed with apoB-48 into chylomicrons, and leave by lacteals → thoracic duct → left subclavian vein (lymph, not portal blood). Short-chain fatty acids and glycerol go into portal blood. Ca²⁺ needs calcitriol-induced calbindin; iron as Fe²⁺ via DMT1, exported by ferroportin; vitamin B₁₂ needs intrinsic factor and is absorbed in the terminal ileum.
• GI hormones: gastrin (G-cells; acid and pepsinogen secretion), secretin (S-cells; pancreatic and biliary HCO₃⁻, inhibits gastrin — the first hormone ever discovered, Bayliss and Starling 1902), cholecystokinin (I-cells; gall-bladder contraction, pancreatic enzymes, satiety), GIP and GLP-1 (incretins; insulin release), motilin, ghrelin (hunger), VIP, somatostatin (universal inhibitor).
• Nervous control: enteric nervous system ("gut brain", ~10⁸ neurons) with vagal parasympathetic stimulation and sympathetic inhibition. Gastric secretion has cephalic (vagal, ~30%), gastric (distension + peptides, ~60%) and intestinal (~10%) phases.
• Liver's digestive role: bile production, glycogenesis/glycogenolysis/gluconeogenesis, deamination and the urea cycle, plasma-protein and clotting-factor synthesis, detoxification, storage of A, D, B₁₂, iron and copper.
• Disorders examined: marasmus and kwashiorkor, jaundice (pre-hepatic, hepatic, post-hepatic), steatorrhoea, coeliac disease, lactose intolerance, gallstones, dehydration from diarrhoea (ORS works because glucose drives SGLT1-linked Na⁺ and water uptake).

Respiration: structure, mechanics and gas transport

• Levels: *external respiration* (ventilation + exchange at the surface), *transport* in blood, *internal/tissue respiration* (exchange at cells), *cellular respiration* (mitochondrial ATP synthesis — biochemistry, not this topic).
• Organs across animals: body surface (earthworm, cutaneous diffusion), tracheae with spiracles (insects — air delivered directly to tissues, so haemolymph carries little O₂), gills (counter-current exchange in fish gives up to 80–90% O₂ extraction), book lungs (scorpion), lungs (tetrapods). Birds have parabronchi with air sacs and unidirectional flow; amphibians use buccal, cutaneous and pulmonary routes.
• Human pathway: external nares → nasal chambers (filter, warm, humidify) → pharynx → larynx (voice box) → trachea (C-shaped cartilage rings) → bronchi → bronchioles → terminal and respiratory bronchioles → alveolar ducts → alveoli (~300 million, ~70 m² surface area).
• Respiratory membrane: alveolar epithelium + fused basement membranes + capillary endothelium, ~0.5–1 µm thick. Type I pneumocytes for exchange, Type II secrete surfactant (dipalmitoylphosphatidylcholine) which lowers surface tension, prevents alveolar collapse and raises compliance; its absence in premature infants causes respiratory distress syndrome. Alveolar macrophages (dust cells) defend.
• Mechanics: inspiration is active — diaphragm (dome flattens, ~75% of tidal volume) and external intercostals raise the ribs and sternum, thoracic volume rises, intrapulmonary pressure falls ~1–3 mm Hg below atmospheric, air flows in. Expiration at rest is passive elastic recoil; forced expiration uses internal intercostals and abdominal muscles. Intrapleural pressure stays negative (≈ −4 mm Hg at rest, ≈ −6 to −8 on inspiration); puncture of the pleura causes pneumothorax and lung collapse.
• Transport of O₂: ~97% bound to haemoglobin (4 haem groups, Fe²⁺, one O₂ each; fully saturated Hb carries 1.34 mL O₂/g), ~3% dissolved in plasma. The oxygen dissociation curve is sigmoid because of cooperative binding. Right shift (Bohr effect) — unloads O₂ — with ↑CO₂, ↑H⁺ (↓pH), ↑temperature, ↑2,3-BPG, i.e. exactly the conditions in active tissue. Left shift with the opposite, and in foetal Hb (HbF, γ chains) which binds 2,3-BPG weakly and so takes O₂ from maternal blood. Myoglobin has a hyperbolic, left-shifted curve — a store, not a carrier.
• Transport of CO₂: ~70% as bicarbonate (carbonic anhydrase in the RBC; HCO₃⁻ leaves in exchange for Cl⁻ — the chloride (Hamburger) shift), ~20–25% as carbamino-haemoglobin, ~7% dissolved. Haldane effect: deoxygenated Hb carries more CO₂; so O₂ unloading in tissue promotes CO₂ pick-up and O₂ loading in the lung drives CO₂ off.
• Control: the medullary dorsal and ventral respiratory groups set the basic rhythm; the pontine pneumotaxic centre moderates it. Central chemoreceptors (medulla) respond to CSF H⁺ from CO₂ — the dominant drive; peripheral chemoreceptors (carotid and aortic bodies) respond mainly to a large fall in PO₂ (below ~60 mm Hg) and to H⁺ and CO₂. Hering–Breuer stretch reflex limits over-inflation.
• Disorders: asthma (bronchiolar constriction and inflammation), emphysema (alveolar wall destruction, usually smoking), chronic bronchitis, occupational pneumoconioses — silicosis, asbestosis, byssinosis, coal-worker's lung (notifiable occupational diseases; workplace dust and health-surveillance duties now sit under the Occupational Safety, Health and Working Conditions Code, 2020, in force from 21 November 2025, which replaced the Factories Act, 1948), hypoxia (hypoxic, anaemic, stagnant, histotoxic), asphyxia, CO poisoning (CO binds Hb ~240× more strongly than O₂ and left-shifts the curve), acclimatisation at altitude (more 2,3-BPG, erythropoietin-driven polycythaemia, hyperventilation).

Circulation: blood, heart and vessels

• Blood (≈5 L, 8% of body weight): plasma 55% (water 90–92%, proteins 7–8% — albumin for oncotic pressure, globulins for immunity, fibrinogen for clotting) and formed elements 45%. RBCs 5–5.5 million/µL (men) and 4.5–5 million/µL (women), biconcave, enucleate in mammals, lifespan 120 days, destroyed in the spleen ("graveyard of RBCs"). WBCs 6,000–8,000/µL — neutrophils 60–65%, lymphocytes 20–25%, monocytes 6%, eosinophils 2–3%, basophils 0.5–1% (mnemonic: Never Let Monkeys Eat Bananas). Platelets 1.5–4.5 lakh/µL. Haematopoiesis in red bone marrow.
• Haemostasis: vascular spasm → platelet plug → coagulation cascade (intrinsic and extrinsic pathways converge on factor X → prothrombin activator → thrombin → fibrinogen to fibrin), needing Ca²⁺ and vitamin K. Heparin and citrate are anticoagulants.
• Blood groups: ABO (antigen on RBC, antibody in plasma; O is the universal donor, AB the universal recipient — true only for packed cells) and Rh. Rh-negative mother with Rh-positive foetus → erythroblastosis foetalis in later pregnancies, prevented by anti-D immunoglobulin. India's transfusion practice is regulated by the Drugs and Cosmetics Act, 1940 and Rules (blood centres were redefined from "blood banks" by the 2020 amendment), with NBTC/SBTC standards and NACO oversight.
• Heart: four chambers, myogenic, in the mediastinum, enclosed in a double pericardium. Right atrium → tricuspid valve → right ventricle → pulmonary (semilunar) valve; left atrium → bicuspid (mitral) → left ventricle → aortic valve. Chordae tendineae and papillary muscles stop AV-valve eversion. Ventricular wall is thicker on the left; cardiac muscle is striated, uninucleate, branched, joined by intercalated discs with gap junctions (a functional syncytium) and has a long absolute refractory period, so it cannot be tetanised.
• Conducting system: SA node (pacemaker, 70–75/min) → internodal pathways → AV node (the only electrical bridge, with a ~0.1 s delay) → bundle of His → right and left bundle branches → Purkinje fibres. Intrinsic rates fall along the chain (AV node 40–60, Purkinje 20–40/min), which is why blocks cause bradycardia.
• Cardiac cycle (0.8 s at 75/min): atrial systole 0.1 s (tops up ventricles by ~20–30%), ventricular systole 0.3 s (isovolumetric contraction → ejection), joint diastole 0.4 s. Sounds: S1 "lub" = AV valves closing at the start of ventricular systole; S2 "dub" = semilunar valves closing. Murmurs indicate valve stenosis or incompetence.
• ECG: P wave = atrial depolarisation; QRS complex = ventricular depolarisation (atrial repolarisation is buried in it); T wave = ventricular repolarisation. PR interval = AV conduction time; a raised ST segment suggests myocardial infarction; absent P waves with an irregular rhythm suggest atrial fibrillation.
• Double circulation: *pulmonary* (right heart → lungs → left atrium, deoxygenated out, oxygenated back) and *systemic* (left heart → body → right atrium). Mammals and birds have complete double circulation; amphibians and most reptiles incomplete (crocodiles have a four-chambered heart but a foramen of Panizza); fish have single circulation. Portal systems: hepatic portal (gut → liver) and hypophyseal portal. Coronary circulation feeds the myocardium itself — blockage causes a heart attack.
• Haemodynamics and pressure: normal adult BP ≈ 120/80 mm Hg; hypertension is now generally defined in Indian practice as ≥140/90 mm Hg on repeated office measurement (ICMR/NPCDCS guidance; some international bodies use lower thresholds, so quote the source). Resistance is greatest in arterioles (the resistance vessels); veins hold ~60–70% of blood volume (capacitance vessels) and return blood by the muscle pump, respiratory pump and valves. Capillary exchange follows Starling forces — hydrostatic pressure out, oncotic pressure in; excess filtrate becomes lymph, and failure of this balance (or hypoproteinaemia) causes oedema.
• Regulation: cardiac output = heart rate × stroke volume. Frank–Starling law — greater venous return stretches the myocardium and raises stroke volume. Sympathetic noradrenaline (β₁ receptors) raises rate, conduction and contractility; vagal acetylcholine (M₂) lowers rate. Baroreceptors in the carotid sinus and aortic arch buffer short-term BP; the renin–angiotensin–aldosterone system, ADH and ANP handle longer-term volume control.
• Lymphatic system: blind-ended capillaries → vessels with valves → nodes → thoracic duct and right lymphatic duct → subclavian veins. Returns ~2–4 L of interstitial fluid and protein daily, absorbs dietary fat, and houses immune cells. Filarial blockage causes elephantiasis.
• Comparative respiratory pigments: haemoglobin (iron, inside RBCs in vertebrates, dissolved in plasma of earthworm), haemocyanin (copper, blue — molluscs, crustaceans), haemerythrin, chlorocruorin.

Integration, measurement and clinical/laboratory practice

• Why the three link: O₂ consumed and CO₂ produced depend on digested substrate; the respiratory quotient (RQ) reveals the fuel — carbohydrate 1.0, fat ~0.7, protein ~0.8–0.82, mixed Indian diet ~0.85.
• Lung volumes (typical adult male): TV 500 mL, IRV 2,500–3,000 mL, ERV 1,000–1,100 mL, RV 1,100–1,200 mL; IC = TV+IRV, FRC = ERV+RV, VC = TV+IRV+ERV (≈3,500–4,800 mL), TLC = VC+RV (≈5,000–6,000 mL). Only RV, FRC and TLC cannot be measured by simple spirometry. FEV₁/FVC separates obstructive (reduced ratio — asthma, COPD) from restrictive (normal or high ratio, low volumes — fibrosis) disease. Indian reference values run lower than Caucasian ones, so ICMR/Indian spirometry equations should be used.
• Partial pressures (mm Hg): atmospheric air PO₂ 159 / PCO₂ 0.3; alveolar air 104 / 40; deoxygenated blood entering the lung 40 / 45; oxygenated blood leaving 95 / 40; tissue 40 / 45. Exchange is simple diffusion driven by the partial-pressure gradient; CO₂ diffuses ~20–25× faster than O₂ because of its far higher solubility.
• Standard laboratory/practical work: haemoglobinometry (Sahli's acid-haematin method), RBC and WBC counts on a Neubauer chamber, differential leucocyte count, blood grouping, ESR (Westergren), clotting and bleeding time, BP by sphygmomanometer (auscultatory Korotkoff sounds), spirometry, measurement of RQ, qualitative tests for salivary amylase and pepsin activity against pH and temperature, bile-salt emulsification, Benedict's and Biuret tests on digests, and an isolated frog-heart or earthworm/cockroach gut preparation.
• Indian regulation of animal work: experiments on animals are governed by the Prevention of Cruelty to Animals Act, 1960 and CPCSEA rules; every institution needs an IAEC and the "3Rs" apply — replacement, reduction, refinement. Frog dissection and use of frogs in college practicals have been barred by UGC/MoEFCC circulars, so computer simulations and virtual labs (Amrita Virtual Labs, Olabs) are used. Human-subject work follows the ICMR National Ethical Guidelines (2017) with an Institutional Ethics Committee, and clinical trials follow the New Drugs and Clinical Trials Rules, 2019 and the NDCT/CDSCO framework.
• National programmes using this physiology: NPCDCS (hypertension, diabetes and stroke screening), the National Tuberculosis Elimination Programme, Anaemia Mukt Bharat (iron-folic-acid supplementation — note anaemia definitions and cut-offs are under active revision in India), the National Programme for Control of Blindness and Visual Impairment's nutrition link (vitamin A), and the universal ORS/zinc protocol for childhood diarrhoea.

Key points

• Digestion – enzymatic hydrolysis of macromolecules to absorbable units; extracellular in all vertebrates.
• Salivary amylase – starts starch digestion at pH ~6.8; inactivated by gastric acid.
• Parietal cells – secrete HCl via the H⁺/K⁺-ATPase and intrinsic factor (needed for vitamin B₁₂).
• Trypsin cascade – enterokinase activates trypsinogen; trypsin then activates all other pancreatic zymogens.
• Bile – emulsifies fat and carries fat-soluble vitamins but contains no enzyme; 95% recycled enterohepatically.
• Villi and microvilli – amplify intestinal surface ~600×; brush-border enzymes finish digestion.
• Chylomicrons – carry absorbed fat into lymph (lacteals), not the portal vein.
• GI hormones – gastrin stimulates acid; secretin triggers bicarbonate; CCK empties the gall bladder and releases pancreatic enzymes.
• Respiratory membrane – 0.5–1 µm across ~70 m²; surfactant from Type II cells prevents alveolar collapse.
• Inspiration is active, quiet expiration passive; intrapleural pressure is always sub-atmospheric.
• Sigmoid O₂ curve – cooperative Hb binding; right shift (Bohr) by ↑CO₂, ↑H⁺, ↑temperature, ↑2,3-BPG unloads O₂ in tissue.
• Haldane effect and chloride shift – deoxygenated Hb carries more CO₂; ~70% of CO₂ travels as plasma bicarbonate.
• Respiratory drive – central chemoreceptors sensing CO₂/H⁺ dominate; PO₂ matters only below ~60 mm Hg.
• Cardiac cycle 0.8 s – atrial systole 0.1, ventricular systole 0.3, joint diastole 0.4 s; S1 = AV valves, S2 = semilunar valves.
• SA node – pacemaker at 70–75/min; the AV node is the sole electrical bridge and imposes the conduction delay.
• ECG – P atrial depolarisation, QRS ventricular depolarisation, T ventricular repolarisation.
• Frank–Starling law – stroke volume rises with venous return (end-diastolic stretch) within limits.
• Starling forces and lymph – capillary hydrostatic vs oncotic pressure sets filtration; imbalance gives oedema.

Formulas and examples

Example 1 — Cardiac output, ejection fraction and MAP. A healthy adult has HR 72/min, EDV 140 mL, ESV 65 mL, BP 124/82 mm Hg.

• SV = 140 − 65 = 75 mL
• CO = 72 × 75 = 5,400 mL/min = 5.4 L/min
• EF = 75/140 × 100 = 53.6% (normal 55–70%, so borderline-low)
• Pulse pressure = 124 − 82 = 42 mm Hg; MAP = 82 + 42/3 = 82 + 14 = 96 mm Hg
• At HR 150/min during exercise with SV 110 mL, CO = 150 × 110 = 16,500 mL/min = 16.5 L/min — a cardiac reserve of ~3.1×.

Example 2 — Dead space and the value of deep breathing. TV 500 mL, anatomical dead space 150 mL, RR 12/min.

• Minute volume = 500 × 12 = 6,000 mL/min
• Alveolar ventilation = (500 − 150) × 12 = 350 × 12 = 4,200 mL/min
• Shallow panting at TV 250 mL, RR 24/min: minute volume = 250 × 24 = 6,000 mL/min (identical), but alveolar ventilation = (250 − 150) × 24 = 100 × 24 = 2,400 mL/min — a 43% fall despite the same minute volume. Deep, slow breathing ventilates alveoli far better.

Example 3 — Oxygen content, delivery and anaemia. Hb 15 g/dL, arterial saturation 97%, venous saturation 75%, CO 5 L/min.

• Arterial O₂ = 1.34 × 15 × 0.97 = 19.5 mL/dL (plus ~0.3 mL dissolved ≈ 19.8)
• Venous O₂ = 1.34 × 15 × 0.75 = 15.1 mL/dL; a–v difference = 4.4 mL/dL
• Delivery = 50 dL/min × 19.5 = 975 mL O₂/min; consumption (Fick) = 50 × 4.4 = 220 mL O₂/min — close to the standard resting 250 mL/min.
• An anaemic patient with Hb 8 g/dL at the same saturation carries 1.34 × 8 × 0.97 = 10.4 mL/dL — only 53% of normal. To keep VO₂ at 220 mL/min the a–v extraction and CO must both rise; this is why severe anaemia (common in Indian women and adolescents) causes tachycardia and breathlessness on effort.

Example 4 — RQ and fuel use. A student in a closed-circuit respirometer consumes 15.0 L O₂ and produces 12.8 L CO₂ per hour.

• RQ = 12.8/15.0 = 0.853 → a mixed carbohydrate-and-fat diet (pure carbohydrate would give 1.0, pure fat ~0.70).
• Energy ≈ 20.3 kJ per litre of O₂ at this RQ → 15.0 × 20.3 ≈ 305 kJ/h ≈ 73 kcal/h, i.e. ~1,750 kcal/day at rest — a plausible BMR.

Example 5 — Starch digestion arithmetic. 100 g of starch is completely hydrolysed.

• Starch (C₆H₁₀O₅)ₙ, residue mass 162; glucose 180. Each bond broken adds one H₂O.
• Glucose yielded = 100 × 180/162 = 111.1 g, i.e. 11.1 g of water consumed.
• At 17 kJ/g, this gives ≈ 1,889 kJ ≈ 451 kcal of metabolisable energy.

Where it is used

• Diagnostic laboratories (NABL-accredited; Dr Lal PathLabs, Metropolis, SRL, Thyrocare, government district labs): CBC and haemogram, haemoglobin estimation, blood grouping and cross-matching, liver and pancreatic enzyme panels, arterial blood gases — all read through this physiology.
• Hospitals and diagnostics equipment (Philips, GE Healthcare, Mindray, Transasia, Agappe): pulse oximeters, spirometers, ECG machines, ventilators, blood-gas and haematology analysers — sales, applications and service roles need the underlying physiology.
• Pharma and clinical research (Sun, Cipla, Dr Reddy's, Biocon, Syngene, CROs): proton-pump inhibitors, antacids, bronchodilators, antihypertensives, anticoagulants and statins act on exactly these pathways; CRA/clinical-data roles test the end points.
• Animal husbandry, poultry, aquaculture and veterinary nutrition (NDDB, ICAR institutes, Godrej Agrovet, Venky's): ruminant fermentation, feed conversion efficiency, gill function and dissolved-oxygen management in fish ponds.
• Food, nutrition and public health (FSSAI labelling and fortification standards, NIN Hyderabad RDA tables, ICDS, Anaemia Mukt Bharat, ORS programmes): digestibility, glycaemic response, iron and B₁₂ bioavailability.
• Occupational health, sports science and higher study: lung-function surveillance for dust-exposed workers under the OSH Code, 2020 (in force 21 November 2025); VO₂-max and lactate testing at SAI and NSNIS Patiala; MSc/PhD and NET/GATE-XL entry into physiology and biomedical research.

Interview questions

• Q: Why does bile help fat digestion though it contains no enzyme? A: Bile salts are amphipathic and emulsify large fat droplets into micelles, hugely increasing the surface area on which pancreatic lipase can act; they also solubilise the products for absorption.
• Q: Why is pepsin secreted as pepsinogen? A: A zymogen is inactive, so the enzyme cannot digest the cells that make it; HCl in the lumen converts pepsinogen to pepsin, which then autocatalyses more conversion.
• Q: Why is the oxygen dissociation curve sigmoid and not hyperbolic? A: Haemoglobin has four cooperative subunits — binding of the first O₂ raises the affinity of the rest, giving an S-shape; monomeric myoglobin shows the hyperbolic curve.
• Q: What is the Bohr effect and where does it matter? A: A rightward shift of the O₂ curve caused by high CO₂, low pH and high temperature, so haemoglobin releases more O₂ exactly in actively metabolising tissue.
• Q: Why does carbon monoxide poisoning not raise the respiratory rate much at first? A: CO lowers the O₂ *content* but arterial PO₂ stays near normal, and peripheral chemoreceptors respond to PO₂, not content — so the main drive is not triggered until acidosis develops.
• Q: How does fat absorption differ from glucose absorption in its route? A: Glucose enters portal blood via SGLT1 and goes straight to the liver; fat is re-esterified, packaged into chylomicrons and enters lymph through lacteals, reaching blood at the left subclavian vein.
• Q: What happens if the AV node is blocked? A: The atria continue at the SA rate but ventricles fall back to their own slow pacemaker (bundle/Purkinje, 20–40/min), giving bradycardia, low cardiac output and possibly syncope — treated with a pacemaker.
• Q: Why can cardiac muscle not be tetanised? A: Its long absolute refractory period (from the plateau phase of the action potential) means a second stimulus cannot excite it until the contraction is nearly over, so summation cannot occur.
• Q: Why does the first heart sound occur at the start of systole? A: Rising ventricular pressure shuts the tricuspid and mitral valves; the resulting vibration is S1, while S2 is the closure of the semilunar valves at the end of ejection.
• Q: Shallow and deep breathing can give the same minute volume — why is deep breathing better? A: Dead-space volume is fixed, so a larger tidal volume delivers a greater *fraction* of each breath to the alveoli, raising alveolar ventilation at the same minute volume.
• Q: Why does ORS contain glucose and not just salt and water? A: Glucose uptake through SGLT1 is Na⁺-coupled, so adding glucose drives sodium absorption and water follows osmotically — the reason low-osmolarity ORS works even during secretory diarrhoea.
• Q: Why do fish extract oxygen far more efficiently than a human lung? A: Gills use counter-current flow, so a diffusion gradient exists along the whole lamella; mammalian alveoli use a tidal, dead-ended system where alveolar PO₂ can only approach, never exceed, equilibrium.

Common mistakes

• Saying bile contains enzymes, or that it digests fat chemically — it only emulsifies.
• Writing that digestion of protein begins in the mouth, or that salivary amylase keeps working in the stomach (it is destroyed below pH ~4).
• Confusing vital capacity with total lung capacity (VC excludes residual volume) and claiming spirometry measures residual volume.
• Reversing the Bohr shift — placing high CO₂ and low pH on the *left* — or mixing up Bohr (O₂ release) with Haldane (CO₂ carriage).
• Saying the AV node generates the heartbeat; it only delays and relays — the SA node is the pacemaker.
• Attributing the T wave to atrial repolarisation (it is ventricular; atrial repolarisation hides inside QRS).
• Describing amphibian and fish circulation loosely — fish have single circulation, amphibians incomplete double circulation; only birds and mammals are fully double.
• Dropping units in calculations — mixing mL/dL with L/min in Fick or O₂-content problems, or quoting cardiac output in mL when the answer is wanted in L/min.

Related topics

Nervous and endocrine systems · Reproductive biology · Excretion and osmoregulation · Cell biology and membrane transport · Biochemistry: enzymes and metabolism · Comparative animal physiology and adaptation