Breathing & Exchange of Gases

Respiration in humans involves a finely regulated system that draws atmospheric oxygen into the body and expels carbon dioxide, a metabolic waste product. The respiratory tract begins at the external nares and continues through the nasal cavity (which warms, moistens, and filters incoming air), pharynx, larynx (housing the vocal cords), trachea (C-shaped cartilaginous rings prevent collapse), primary bronchi, secondary and tertiary bronchi, bronchioles, and finally the alveoli — the functional units of gas exchange. The lungs are paired organs enclosed in double-layered pleural membranes with intrapleural fluid reducing friction; the right lung has three lobes while the left lung has two lobes (accommodating the cardiac notch).

Breathing involves two phases controlled by pressure differences. During inspiration, the diaphragm contracts and flattens while external intercostal muscles lift the ribs outward and upward, increasing thoracic volume. This decreases intra-pulmonary pressure below atmospheric pressure, drawing air in. Expiration at rest is largely passive — the diaphragm and intercostals relax, elastic recoil of lungs compresses alveolar air, and air exits. During forced expiration, internal intercostal muscles and abdominal muscles actively contract to push air out more forcefully.

Respiratory volumes and capacities are clinically important. Tidal Volume (TV) is approximately 500 mL of air moved in a normal breath. Inspiratory Reserve Volume (IRV, 2500-3000 mL) is the extra air that can be forcibly inhaled. Expiratory Reserve Volume (ERV, 1000-1100 mL) is the additional air that can be forcibly exhaled. Residual Volume (RV, 1100-1200 mL) is air that remains in the lungs even after the most forceful expiration and can never be voluntarily expelled. Key capacities include: Vital Capacity (VC = TV + IRV + ERV, approximately 3500-4600 mL) and Total Lung Capacity (TLC = VC + RV, approximately 5000-6000 mL).

Gas exchange occurs at the alveoli by simple diffusion along partial pressure gradients across a thin diffusion membrane composed of three layers: the alveolar squamous epithelium, the shared basement membrane, and the capillary endothelium. Oxygen diffuses from alveoli (pO₂ = 104 mmHg) into blood (pO₂ = 40 mmHg), and CO₂ moves from blood (pCO₂ = 45 mmHg) to alveoli (pCO₂ = 40 mmHg).

Oxygen is transported primarily (97%) as oxyhaemoglobin within RBCs. The oxygen-haemoglobin dissociation curve is sigmoid-shaped, reflecting cooperative binding. The Bohr effect describes how increased pCO₂, decreased pH, elevated temperature, and higher 2,3-BPG levels shift this curve rightward, promoting oxygen release to active tissues. Carbon dioxide is transported in three forms: 70% as bicarbonate ions (HCO₃-) formed via carbonic anhydrase in RBCs (the chloride shift or Hamburger's phenomenon maintains electrical balance as HCO₃- moves out and Cl- moves into RBCs), 23% bound to haemoglobin as carbaminohaemoglobin, and 7% dissolved in plasma.

Breathing rhythm is regulated by the respiratory rhythmicity centre in the medulla oblongata, modulated by the pneumotaxic centre in the pons (which limits inspiratory duration). Peripheral chemoreceptors in aortic and carotid bodies detect changes in pO₂, pCO₂, and H+ concentration to fine-tune ventilation rate.

The key testable concept is the quantitative breakdown of O₂ and CO₂ transport mechanisms and the factors affecting the oxygen dissociation curve.