Gas exchange surfaces
Efficient exchange surfaces follow Fick's law: rate ∝ (surface area × concentration difference) ÷ thickness. Alveoli: huge surface area, one-cell-thick squamous epithelium next to capillary endothelium, steep gradient kept by ventilation and blood flow, elastic fibres for recoil. Airways: cartilage (trachea, bronchi) keeps them open; goblet cells make mucus; cilia sweep it away; smooth muscle controls diameter. Ventilation: inspiration (diaphragm and external intercostals contract, volume ↑, pressure ↓); expiration largely passive. Pulmonary ventilation = tidal volume × breathing rate. Fish gills use countercurrent flow to keep a gradient along the whole lamella; insects use tracheae and spiracles; leaves use stomata and air spaces.
≈ 45 min