Human Physiology

1Breathing volumes and vital capacity

A quiet breath moves about 500 mL of air — the tidal volume (TV). A forcible inspiration can add another 2500–3000 mL on top of that (inspiratory reserve volume, IRV), and a forcible expiration can push out another 1000–1100 mL after a quiet breath (expiratory reserve volume, ERV). Air that stays in the lungs even after the hardest blow is residual volume (RV), about 1100–1200 mL, and it is why the alveoli never fully collapse.

Capacities are just named sums of those volumes. Inspiratory capacity is TV + IRV. Vital capacity is ERV + TV + IRV — the deepest breath you can take after emptying as far as you can, or the deepest blow after filling as far as you can. Total lung capacity adds residual volume on top of vital capacity. A healthy adult at rest moves roughly 6000–8000 mL of air per minute at a quiet rate, which is just TV times breaths per minute.

Figure. The three volumes that add to vital capacity, drawn at the mid-range sizes used in the worked example. Residual volume is deliberately absent — it is not part of VC.

2How blood carries O₂ and CO₂

About 97 per cent of oxygen rides inside red cells bound to haemoglobin as oxyhaemoglobin; only 3 per cent dissolves in plasma. Each haemoglobin molecule can bind up to four O₂ molecules, and the binding is reversible. A plot of percentage saturation against partial pressure of oxygen is a sigmoid — the oxygen dissociation curve — because binding at one site makes the next easier until the molecule is full.

Carbon dioxide travels the other way around: nearly 70 per cent as bicarbonate, 20–25 per cent bound on haemoglobin, and about 7 per cent dissolved in plasma. In the tissues, where pO₂ is low and pCO₂ and H⁺ are high, oxyhaemoglobin gives up oxygen. In the alveoli the gradients reverse, so haemoglobin loads oxygen and bicarbonate is driven back to CO₂ for exhalation. Carbonic anhydrase in the red cell is what makes the bicarbonate route fast enough to matter.

Figure. Shape of the oxygen dissociation curve only: saturation rises steeply through the tissue range and flattens toward the alveolar range. No invented percentage is marked on the axis — the point is the sigmoid shape and where unloading is steep.

3Alveolar exchange and the diffusion membrane

Alveoli are the primary sites of gas exchange. O₂ and CO₂ move by simple diffusion down their partial-pressure gradients: alveolar pO₂ is about 104 mm Hg against 40 mm Hg in deoxygenated blood, so oxygen enters the capillary; tissue and venous pCO₂ sit near 45 mm Hg against 40 mm Hg in alveoli, so carbon dioxide leaves. CO₂ is 20–25 times more soluble than O₂, which is why a smaller pressure difference still moves enough of it.

The diffusion membrane has three layers — the thin squamous epithelium of the alveolus, the endothelium of the alveolar capillary, and the basement substance between them — and its total thickness is much less than a millimetre. That thinness, the huge alveolar surface, and the continuous blood flow are what make the exchange fast enough for life.

Figure. Schematic partial-pressure drive across the three-layer alveolar membrane. Layers are named stacks, not a cupped anatomical alveolus.

Which way each gas moves

1. At the alveoluspO₂ is higher in alveolar air than in arriving blood, so O₂ diffuses into the capillary; pCO₂ is higher in the blood, so CO₂ diffuses out.
2. At the tissuepO₂ is higher in the capillary than in the respiring cell, so O₂ leaves the blood; pCO₂ is higher in the tissue, so CO₂ enters the blood.
3. Why CO₂ keeps upEven though its pressure differences are smaller, CO₂ is 20–25 times more soluble, so enough of it crosses the same thin membrane.

4Double circulation: pulmonary and systemic

Humans have complete double circulation: blood passes through the heart twice for every trip around the body, on two separate pathways. The right ventricle pumps deoxygenated blood into the pulmonary artery; it is oxygenated in the lungs and returns by the pulmonary veins into the left atrium — the pulmonary circulation. The left ventricle then pumps that oxygenated blood into the aorta for the systemic circulation, and deoxygenated blood returns by the venae cavae into the right atrium.

The four chambers keep the two streams from mixing. Pulmonary veins are the only veins that carry oxygenated blood; pulmonary arteries are the only arteries that carry deoxygenated blood. That separation is the significance of double circulation: tissues receive blood at high oxygen tension, and the pressure needed for the long systemic circuit is set by the thick left ventricle without forcing the same pressure through the delicate lung capillaries.

Figure. Pulmonary and systemic circuits as a rectangular loop: body → right heart → lungs → left heart → body. The return leg runs outside the boxes, not as a diagonal across the middle.

Oxygenated blood from lung to aorta

1. Leave the lungOxygenated blood leaves by the pulmonary veins.
2. Left atriumIt enters the left atrium, then crosses the bicuspid (mitral) valve into the left ventricle.
3. Into the aortaThe left ventricle contracts and opens the aortic semilunar valve into the aorta for systemic distribution.

5Cardiac output and the conduction chain

During each cardiac cycle every ventricle ejects about 70 mL of blood — the stroke volume. Multiply that by the heart rate and you have cardiac output: the volume each ventricle pumps per minute. At the resting average of 72 beats per minute that product is about 5000 mL, or 5 litres. The body can raise either factor, which is why an athlete's cardiac output in exercise far exceeds the resting figure.

The beat is started by the sino-atrial node in the right atrium, which can fire 70–75 action potentials per minute on its own and is therefore the pacemaker. The impulse spreads across the atria, pauses at the atrio-ventricular node, then races down the bundle of His and out through the Purkinje fibres so both ventricles contract almost together. The SAN sets the pace because it fires fastest; anything downstream only follows.

Figure. The impulse path as a four-box chain. The SA node is marked as pacemaker because it fires at the highest intrinsic rate; everything downstream only conducts.

The conduction chain

1. SA nodeFires first (70–75 min⁻¹) and spreads across both atria — atrial systole.
2. AV nodeDelays the impulse briefly so the atria finish emptying into the ventricles.
3. Bundle of HisCarries the impulse into the interventricular septum.
4. Purkinje fibresDistribute it through the ventricular muscle so both ventricles contract together.

6Four chambers, valves and the great vessels

The human heart has two atria and two ventricles. The right atrium receives deoxygenated blood from the superior and inferior venae cavae; the left atrium receives oxygenated blood from the pulmonary veins. The right ventricle sends blood into the pulmonary artery; the left ventricle, with the thickest wall, sends blood into the aorta. Atrio-ventricular valves — tricuspid on the right, bicuspid (mitral) on the left — stop ventricular blood returning to the atria, and semilunar valves at the roots of the pulmonary artery and aorta stop arterial blood falling back into the ventricles.

Normal arterial blood pressure is quoted as about 120/80 mm Hg — systolic over diastolic. The first heart sound (lub) is the AV valves closing at the start of ventricular systole; the second (dub) is the semilunar valves closing at the start of ventricular diastole. Those two facts are clinical, but they are also the shortest way to remember which valves do what.

Figure. Functional four-chamber circuit: right side deoxygenated to lungs, left side oxygenated to body. Valves named on the atrioventricular drops; great-vessel anatomy is not drawn.

7Nephron, filtration and urine formation

The nephron is the functional unit of the kidney. About 1200 mL of blood is filtered by the glomeruli each minute to form roughly 125 mL of filtrate in Bowman's capsule — the glomerular filtration rate (GFR). Over a day that is about 180 litres of filtrate, of which nearly 99 per cent is reabsorbed, so the daily urine volume is a tiny fraction of what was filtered.

Urine formation has three processes. Glomerular filtration is ultrafiltration under capillary pressure through three layers into Bowman's capsule. Tubular reabsorption takes useful solutes and water back into the blood, mostly in the PCT. Tubular secretion adds substances from the blood into the tubule. The loop of Henle and the vasa recta then run a counter-current arrangement that maintains the medullary osmotic gradient needed to concentrate urine — a mechanism you must name even when the anatomy cannot be drawn here.

Figure. Schematic nephron path: filtration at Bowman, bulk reabsorb in PCT, hairpin loop for counter-current, DCT, then collecting duct. Geometry is a labelled flow, not a histological outline.

Three processes that make urine

1. Glomerular filtrationBlood pressure forces water and small solutes from the glomerular capillaries into Bowman's capsule — about 125 mL/min in a healthy adult.
2. Tubular reabsorptionThe tubule reclaims glucose, amino acids, salts and water into the peritubular capillaries; PCT does most of this work.
3. Tubular secretionThe tubule adds ions and wastes from the blood into the filtrate, fine-tuning composition before the collecting duct.

8Nerve impulse, synapse, insulin and glucagon

A resting axon is polarised: the outside is positive relative to the inside, and that electrical difference is the resting potential. A stimulus makes the membrane freely permeable to Na⁺; sodium rushes in, the polarity reverses, and that reversed difference is the action potential — the nerve impulse. The spike is brief: K⁺ then leaves, the resting potential is restored, and the fibre can fire again. At a chemical synapse the impulse opens vesicles of neurotransmitter into the synaptic cleft; the transmitter binds receptors on the post-synaptic membrane and starts a new potential there.

Hormones carry the slower half of coordination. In the pancreatic islets, β-cells secrete insulin and α-cells secrete glucagon. Insulin promotes cellular uptake of glucose and glycogenesis, lowering blood glucose; glucagon promotes glycogenolysis and gluconeogenesis, raising it. The two together keep glucose homeostasis, and insulin deficiency or resistance is diabetes mellitus.

Figure. Shape of one action-potential spike only — resting, depolarised peak, brief undershoot, restored resting level. No millivolt numbers: NCERT describes the polarity change without quoting them here.

One action potential at a membrane site

1. RestingMembrane polarised; outside positive relative to inside — resting potential.
2. DepolarisationStimulus opens Na⁺ permeability; Na⁺ influx reverses polarity — action potential.
3. RepolarisationNa⁺ permeability collapses; K⁺ exits and restores the resting potential.
4. At the synapseThe arriving impulse releases neurotransmitter into the cleft; receptors on the next neuron open ion channels.