Pulmonary gas exchange

Perfusion-limited exchange

• gas equilibrates early along the length of the pulmonary capillary, therefore the partial pressure of the gas in the arterial blood equals that in the alveolar air
• the only way to increase diffusion of a gas is therefore to increase blood flow

Diffusion-limited exchange

• gas does not equilibrate by the time the blood has reached the end of the pulmonary capillary
• a partial pressure gradient is therefore maintained between the arterial blood and the alveolar air