There is a substantial safety margin for the uptake of gases in the alveolus because at rest, blood is completely oxygenated after traveling only 1/3 the length of the pulmonary capillary bed.

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This is because the blood flowing through the pulmonary capillaries is exposed to alveolar gas, and the diffusion of oxygen into the blood is very efficient. This efficiency allows the blood to become fully saturated with oxygen early as it passes through the capillaries, providing a safety margin for conditions where oxygen demand increases or diffusion capacity decreases.

There Is a Substantial Safety Margin for the Uptake of Gases