2.1—Gas exchange surfaces and Fick's Law

A gas-exchange surface is effective when it has a large area, a short diffusion distance and a steep concentration gradient. Fick’s law summarises this as rate ∝ surface area × concentration difference ÷ barrier thickness.

Alveoli provide a huge folded surface, a thin moist epithelium and a dense capillary network. Ventilation refreshes alveolar air and blood flow removes oxygen, maintaining the gradient.

If surface area doubles while thickness and gradient stay constant, the predicted diffusion rate doubles. If mucus thickens the barrier, diffusion slows even if oxygen concentration outside remains unchanged.

Small organisms can rely on their body surface because their surface-area-to-volume ratio is high. Larger organisms need specialised surfaces and transport systems to keep internal cells within diffusion distance.

Fick’s law is a proportional model, not a promise that every biological change is linear. Permeability, binding and active regulation can also limit transfer.