The partial pressure of oxygen (oxygen tension) of mixed venous blood is 40 mmHg. It rises to the alveolar oxygen tension of 100 mmHg in about 0.25 seconds after entering the pulmonary capillaries.
With mild or moderate diffusion impairment—for example, in diseases producing thickening of the alveolar-capillary membrane—equilibration of capillary blood oxygen tension and alveolar gas is essentially complete by the time the aliquot of blood transits through the capillary. However, with significant diffusion impairment, end-capillary blood oxygen tension may be significantly lower than alveolar oxygen tension, creating a widened alveolar-arterial oxygen gradient.
The effect of alveolar hypoxia on arterial oxygen tension is accentuated during exercise, when the transit time of red blood cells in the pulmonary capillaries is shortened.
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