Gas exchange across the alveolar wall.

O2 and CO2 diffuse across the alveolar membrane down their partial-pressure gradients; solubility, membrane thickness and surface area set the rate, and emphysema, fibrosis or asthma degrade it.

Keep blood leaving the lung at pO2 ≥ 95 mm Hg after setting surface to 40 % (emphysema), using only the other controls.

CO2 crosses slowly because its gradient is small.

• 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.

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