Exchange of gases

Alveoli are the primary sites of gas exchange; exchange also takes place between blood and tissues. In both places O₂ and CO₂ move by simple diffusion driven by pressure or concentration gradients.

Diffusion rate also depends on the solubility of the gases and the thickness of the membrane involved.

Partial pressure, written pO₂ or pCO₂, is the pressure that an individual gas in a mixture contributes.

Partial pressures of O₂ (mm Hg): atmosphere 159, alveoli 104, deoxygenated blood 40, oxygenated blood 95, tissues 40.

Partial pressures of CO₂ (mm Hg): atmosphere 0.3, alveoli 40, deoxygenated blood 45, oxygenated blood 40, tissues 45.

So O₂ moves from alveoli into blood and from blood into tissues, while CO₂ moves the opposite way: tissues to blood, and blood to alveoli.

CO₂ dissolves 20–25 times more readily than O₂, so for the same partial-pressure difference far more CO₂ than O₂ crosses the diffusion membrane.

The diffusion membrane is made of three major layers: the thin squamous epithelium of the alveoli, the endothelium of the alveolar capillaries, and the basement substance lying between them. Together they are far thinner than a millimetre, which helps diffusion.