Gaseous Exchange

From surface to alveolus

Gaseous exchange is the bulk and diffusive movement of O₂ and CO₂ between an organism and its environment. Punjab Textbook Board Biology XII Chapter 14 traces the progression from cutaneous exchange in flatworms, to tracheal systems in insects (which bypass blood entirely and deliver O₂ directly to tissues via tracheoles ~1 µm in diameter), to gills in fish (counter-current flow extracts up to 80 % of dissolved O₂), to lungs in tetrapods. Campbell 12e Chapter 42 emphasises that all designs satisfy Fick's law of diffusion: rate ∝ A·ΔP/d, where A is surface area, ΔP the partial-pressure gradient, and d the diffusion distance. The human alveolar surface area is about 70 m² with a blood–gas barrier of only 0.5 µm.

Mechanics of human breathing

Quiet inspiration is active: the diaphragm flattens (contributing ~75 % of tidal volume) and external intercostals lift the rib cage, lowering intrapleural pressure to about −8 cm H₂O and intra-alveolar pressure to −1 cm H₂O. Quiet expiration is passive, driven by elastic recoil of lung parenchyma and surface tension. Surfactant, a mixture of dipalmitoylphosphatidylcholine and proteins SP-A through SP-D secreted by Type II pneumocytes from week 24 of gestation, reduces alveolar surface tension and prevents collapse — its deficit causes neonatal respiratory distress syndrome. Tidal volume in a resting adult is ≈500 mL, vital capacity ≈4.5 L, and total lung capacity ≈6 L.

Gas transport in blood

Each haemoglobin tetramer (MW ≈ 64 500 Da) binds four O₂ molecules with positive cooperativity, producing the sigmoid oxyhaemoglobin dissociation curve. At alveolar P_O₂ = 100 mm Hg saturation is ≈98 %; at tissue P_O₂ = 40 mm Hg it falls to ≈75 %, releasing about a quarter of the bound O₂. The Bohr effect — discovered by Christian Bohr in 1904 — shifts the curve right when pH falls or P_CO₂ rises, unloading more O₂ in metabolically active tissue. The Haldane effect describes the reciprocal: deoxygenated blood carries more CO₂. Roughly 70 % of CO₂ travels as bicarbonate (formed by red-cell carbonic anhydrase, CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻), 23 % as carbamino compounds on haemoglobin, and 7 % dissolved.

Control of ventilation

The dorsal and ventral respiratory groups in the medulla, modulated by the pontine pneumotaxic centre, set respiratory rhythm. Central chemoreceptors near the ventral medulla respond to CSF pH (a proxy for arterial P_CO₂); peripheral chemoreceptors in the carotid and aortic bodies detect P_O₂, P_CO₂, and pH. A rise in arterial P_CO₂ from 40 to 45 mm Hg roughly doubles minute ventilation. Hypoxic drive becomes important only below P_O₂ ≈ 60 mm Hg — a fact relevant to high-altitude physiology and chronic obstructive pulmonary disease.

Common disorders

Asthma involves bronchoconstriction and mucosal oedema mediated by IgE-driven mast-cell degranulation; treatment uses β2-agonists like salbutamol and inhaled corticosteroids. Emphysema (a form of COPD) destroys alveolar walls, often from α1-antitrypsin deficiency or smoking-induced elastase imbalance, reducing surface area and elastic recoil. Tuberculosis, caused by Mycobacterium tuberculosis, forms granulomas with central caseous necrosis. Pulmonary fibrosis thickens the diffusion barrier, raising d in Fick's law and reducing transfer factor for CO (DLCO) — a clinical marker.