Exchange of gases

Gases are exchanged at two places: in the alveoli (between air and blood), the main site, and in the tissues (between blood and cells). Both happen by simple diffusion: each gas moves from where its pressure (or concentration) is higher to where it is lower. How easily a gas dissolves, and how thick the membrane is, also matter.

In a mixture of gases like air, each gas contributes part of the total pressure. That part is its partial pressure: pO₂ for oxygen, pCO₂ for carbon dioxide. Here are the values (in mm Hg) to remember:

Read the table to see the direction. Oxygen is highest in the alveoli (104), lower in the blood arriving there (40), so it moves alveoli → blood; then from oxygenated blood (95) to the tissues (40), blood → tissues. Carbon dioxide does the opposite: highest in the tissues (45), so it moves tissues → blood → alveoli (40), and out.

CO₂ is 20 to 25 times more soluble than O₂, so far more CO₂ can diffuse across the membrane for the same difference in partial pressure.

The membrane gases cross is extremely thin, much less than a millimetre. It has three main layers: the thin flat (squamous) cells lining the alveolus, the endothelium of the blood capillary, and the basement substance between them.