Exchange of Gases

Once inspired air has reached the alveoli deep within the lungs, the actual exchange of gases between this alveolar air and the blood in the surrounding pulmonary capillaries -- and, correspondingly, the exchange of gases between the blood and the body's own respiring tissues -- takes place by a single, entirely passive physical process: simple diffusion, with each gas moving independently down its own concentration, or more precisely its own partial-pressure, gradient, requiring no expenditure of cellular energy at either site.

When a mixture of gases, such as air, is in contact with a liquid, such as blood, each individual gas within that mixture behaves, for the purposes of diffusion, as though it alone were present, moving from the region where its own partial pressure is higher to the region where its own partial pressure is lower, entirely independently of what the other gases in the mixture are doing. In alveolar air, the partial pressure of oxygen (pO2) is considerably higher, at approximately 104 mm Hg, than the partial pressure of oxygen in the deoxygenated blood arriving at the lungs in the pulmonary capillaries, at approximately 40 mm Hg; oxygen therefore diffuses from the alveolar air into the blood, down this favourable gradient, until the blood leaving the lungs is very nearly as oxygenated as the alveolar air itself. Carbon dioxide, at the very same site, moves in exactly the opposite direction for exactly the same reason: its partial pressure in the arriving deoxygenated blood, at approximately 45 mm Hg, is higher than its partial pressure in the alveolar air, at approximately 40 mm Hg, so carbon dioxide diffuses out of the blood and into the alveolar air, to be expelled at the next expiration.

Exactly the same principle, simply with the gradient direction reversed, governs gas exchange a second time, at the level of the body's tissues. Oxygenated blood arriving at a respiring tissue carries oxygen at a substantially higher partial pressure than the tissue cells themselves, which have been steadily consuming oxygen in cellular respiration and so maintain a distinctly lower local pO2; oxygen therefore diffuses out of the blood and into the tissue cells. Carbon dioxide, continuously generated as a waste product of the very same cellular respiration, accumulates within the tissue cells to a higher partial pressure than that of the arriving blood, and so diffuses from the tissue into the blood, to be carried back to the lungs for elimination. …