Human Physiology

Human Physiology is the single largest unit in the NEET Biology syllabus, typically contributing the greatest number of questions from Class 11 Zoology. It asks a simple question in many forms: how do the organ systems of one human body work separately, yet cooperate to keep the internal environment steady?

The Organising Idea: Homeostasis and Division of Labour

A single-celled organism can exchange everything it needs directly with its surroundings. A human body of some 10¹³ cells cannot — the cell deep inside your thigh muscle is far from air, food or the outside world. Multicellular animals solved this by developing specialised organ systems, each handling one step of the exchange, and a circulating internal fluid that links them all.

The state that all these systems defend is called homeostasis — the maintenance of a nearly constant internal environment (temperature, pH, glucose, ions, water, O₂ and CO₂ levels) despite fluctuations outside. Homeostasis is almost always achieved through negative feedback: a deviation is detected, and a response is triggered that opposes the deviation. Rising blood glucose triggers insulin, which lowers glucose; a falling body temperature triggers shivering, which raises it. Positive feedback loops (which amplify a change) are rare in physiology — oxytocin-driven labour contractions are the standard example.

It helps to map the seven physiology chapters onto the "task" each performs:

• Digestion and absorption — converting bulk food into absorbable molecules
• Breathing and exchange of gases — bringing in O₂, removing CO₂
• Body fluids and circulation — transporting everything between organs
• Excretory products and elimination — removing nitrogenous waste, balancing water and ions
• Locomotion and movement — mechanical work and displacement
• Neural control and coordination — fast, electrical, point-to-point regulation
• Chemical coordination and integration — slow, hormonal, broadcast regulation

Taking In and Breaking Down: Digestion and Absorption

The human alimentary canal runs mouth → buccal cavity → pharynx → oesophagus → stomach → small intestine (duodenum, jejunum, ileum) → large intestine (caecum, colon, rectum) → anus, assisted by the salivary glands, liver with gall bladder, and pancreas.

Humans have thecodont, dicyodont, heterodont dentition with the dental formula 2123/2123 in each half-jaw (32 teeth in an adult). Chemical digestion proceeds in stages, each with its own pH optimum:

1. Mouth — salivary amylase (ptyalin) hydrolyses about 30% of starch to maltose at pH ~6.8; lysozyme in saliva is antibacterial.
2. Stomach — HCl from parietal (oxyntic) cells makes pH ~1.8, converting pepsinogen (from peptic/chief cells) to pepsin, which cleaves proteins into peptones. Mucus and bicarbonate from mucous cells protect the lining. Infant stomachs also secrete rennin for milk casein. Gastric lipase acts weakly.
3. Small intestine — bile (no enzymes; bile salts emulsify fat), pancreatic amylase, trypsin/chymotrypsin/carboxypeptidase (activated from trypsinogen by enterokinase), pancreatic lipase and nucleases, then brush-border enzymes (maltase, sucrase, lactase, dipeptidases, nucleosidases) finish the job.
4. Large intestine — no digestion; absorbs water, some minerals and drugs, and forms faeces.

Absorption of glucose and amino acids is largely by active transport into intestinal capillaries; fatty acids and glycerol are too hydrophobic and are packaged as micelles, re-formed into triglycerides in the mucosa, coated as chylomicrons, and released into the lymph (lacteals) — a favourite exam point. Fructose and some amino acids move by facilitated transport; water follows osmotically.

Gas Exchange and Transport

Breathing brings air to the alveoli; exchange of gases happens by simple diffusion across the alveolar–capillary membrane (barely 0.2 mm thick in total) driven by partial pressure gradients. Inspiration is active: the diaphragm flattens and external intercostals lift the ribs and sternum, increasing thoracic volume, so intrapulmonary pressure falls below atmospheric and air rushes in. Normal expiration is passive elastic recoil.

Key respiratory volumes worth memorising: tidal volume ≈ 500 mL, inspiratory reserve ≈ 2500–3000 mL, expiratory reserve ≈ 1000–1100 mL, residual volume ≈ 1100–1200 mL. Vital capacity = ERV + TV + IRV; total lung capacity = vital capacity + residual volume (≈ 5–6 L).

Transport differs sharply for the two gases:

• Oxygen — about 97% carried as oxyhaemoglobin (each Hb binds 4 O₂); ~3% dissolved in plasma. The oxygen dissociation curve is sigmoid; it shifts right (unloading favoured) with high pCO₂, high H⁺/low pH, high temperature — the Bohr effect — exactly the conditions in active tissue.
• Carbon dioxide — about 70% as bicarbonate, ~20–25% as carbamino-haemoglobin, ~7% dissolved. Carbonic anhydrase in RBCs makes the bicarbonate conversion fast in both directions.

Respiratory rhythm is set by the respiratory rhythm centre in the medulla, modulated by the pneumotaxic centre in the pons and by chemosensitive areas that respond chiefly to CO₂ and H⁺, not to O₂.

Circulation: The Internal Transport Grid

Blood is a fluid connective tissue: plasma (~55%, mostly water with albumins, globulins, fibrinogen, ions, glucose) plus formed elements — erythrocytes (5–5.5 million/mm³, biconcave, enucleate in mammals, ~120-day lifespan), leucocytes (6000–8000/mm³; granulocytes: neutrophils ~60–65%, eosinophils, basophils; agranulocytes: lymphocytes and monocytes) and platelets (1.5–3.5 lakh/mm³). Blood groups follow the ABO antigen–antibody rule, and Rh incompatibility in pregnancy causes erythroblastosis foetalis.

The human heart is four-chambered with double circulation: right side pumps deoxygenated blood to the lungs, left side pumps oxygenated blood to the body. Cardiac features to fix in memory:

• SAN (pacemaker, right atrial wall) → AVN → bundle of His → Purkinje fibres; the heart is myogenic.
• Cardiac cycle ≈ 0.8 s at 72 beats/min; stroke volume ~70 mL; cardiac output ≈ 5 L/min.
• Valves: tricuspid (right AV), bicuspid/mitral (left AV), semilunar valves at the aorta and pulmonary artery. First sound (lub) = AV valves closing; second (dup) = semilunar valves closing.
• In an ECG, the P wave is atrial depolarisation, QRS ventricular depolarisation (ventricular contraction begins just after Q), and T ventricular repolarisation.

Lymph, the tissue-fluid drainage returned via lymphatic vessels, carries fats, lymphocytes and interstitial fluid back to the blood.

Excretion, Movement, and the Two Coordinating Systems

Excretion. Humans are ureotelic — the liver converts toxic ammonia to urea in the ornithine cycle. Each kidney holds about a million nephrons. Urine formation has three steps: (1) glomerular filtration (GFR ≈ 125 mL/min, i.e. ~180 L/day) at the podocyte-lined filtration membrane; (2) selective reabsorption, with the PCT reclaiming ~70–80% of electrolytes and water and all glucose; (3) tubular secretion of H⁺, K⁺ and NH₃ to maintain ionic and acid–base balance. The counter-current mechanism between the loop of Henle and the vasa recta builds a medullary osmotic gradient allowing urine to be concentrated up to about four times plasma. ADH promotes water reabsorption (its absence causes diabetes insipidus); the JGA releases renin when renal blood flow drops, activating the renin–angiotensin–aldosterone pathway; ANF from the atria opposes it.

Locomotion. Skeletal muscle fibres contain myofibrils with sarcomeres — thick filaments (myosin, with ATPase heads) and thin filaments (actin plus tropomyosin and troponin). In the sliding filament theory, a nerve impulse releases Ca²⁺ from the sarcoplasmic reticulum; Ca²⁺ binds troponin, exposing actin's binding sites; myosin heads form cross-bridges, pivot to pull actin inward, then detach on binding fresh ATP. The A band stays constant while the I band and H zone shorten. The human skeleton has 206 bones (axial 80, appendicular 126); joint types include ball-and-socket (shoulder), hinge (knee), pivot (atlas–axis) and gliding.

Neural control. The neuron's resting membrane potential (about −70 mV) is maintained by the Na⁺/K⁺ pump (3 Na⁺ out : 2 K⁺ in) and selective K⁺ permeability. A stimulus opens Na⁺ channels → depolarisation → action potential, propagated fast and saltatorily along myelinated axons and transmitted chemically across most synapses (acetylcholine and others). The CNS = brain + spinal cord; forebrain (cerebrum, thalamus, hypothalamus, limbic system), midbrain, hindbrain (pons, cerebellum, medulla). The autonomic division splits into sympathetic ("fight or flight") and parasympathetic ("rest and digest").

Chemical coordination. Hormones are non-nutrient chemical messengers acting on specific receptors — membrane receptors with second messengers (cAMP, Ca²⁺) for protein/peptide hormones, intracellular receptors for steroids and thyroid hormones. Essentials:

• Hypothalamus → releasing/inhibiting hormones controlling the anterior pituitary; posterior pituitary stores hypothalamic oxytocin (uterine contraction, milk ejection) and ADH/vasopressin (water reabsorption in the kidney, vasoconstriction at high doses). The anterior pituitary itself secretes six major hormones: GH (growth hormone — excess causes gigantism/acromegaly, deficiency causes dwarfism), TSH (stimulates the thyroid), ACTH (stimulates the adrenal cortex), FSH and LH (gonadotropins acting on the gonads), and prolactin (stimulates milk production, distinct from oxytocin's milk-ejection role). The pituitary's intermediate part secretes MSH, regulating melanocyte pigmentation.

The thyroid, the largest endocrine gland, secretes iodine-dependent T3 and T4, which set the basal metabolic rate, and calcitonin, which lowers blood calcium. Iodine deficiency causes goitre and, in early development, cretinism with impaired growth. The parathyroid glands secrete PTH, which raises blood calcium via bone, kidney and (indirectly) gut action — PTH and calcitonin thus oppose each other, a favourite contrast question.

The adrenal glands have two parts: the outer cortex secretes glucocorticoids (cortisol — raises blood glucose, suppresses inflammation) and mineralocorticoids (aldosterone — promotes Na⁺ reabsorption and K⁺ excretion); the inner medulla secretes adrenaline and noradrenaline, the "fight or flight" hormones that rapidly raise heart rate, blood pressure and blood glucose under stress.

The pancreas is a mixed gland: its endocrine islets of Langerhans have α-cells secreting glucagon (raises blood glucose) and β-cells secreting insulin (lowers blood glucose) acting antagonistically; their imbalance causes diabetes mellitus. The gonads (testis: testosterone; ovary: oestrogen and progesterone) are endocrine glands as well as gamete producers, linking this chapter directly to reproduction.

Common Mistakes and Exam Traps

1. Reversing insulin and glucagon. Insulin lowers blood glucose; glucagon raises it — commonly swapped under time pressure, especially in "after a meal" (insulin) versus "during fasting" (glucagon) framing.
2. Confusing PTH and calcitonin. PTH raises blood calcium; calcitonin lowers it — the reverse of what the name "para-" might suggest to a rushed reader.
3. Attributing gas exchange at the alveolus to active transport. Both O₂ and CO₂ cross the alveolar membrane by simple diffusion along partial pressure gradients — no ATP or carrier is involved at that step, even though their onward transport in blood does use carriers (haemoglobin, bicarbonate).
4. Assuming falling oxygen drives breathing rate. Respiratory centres respond chiefly to rising CO₂ and falling pH, not to falling O₂ — a common but incorrect intuition.