alveolar-capillary membrane

D is a test that measures the ability of gases to diffuse across the alveolar-capillary membrane.

It is possible to become hypoxemic during exercise with certain lung diseases due to impaired diffusion of oxygen across the alveolar-capillary membrane.

ABG testing is mainly used in pulmonology and critical care medicine to determine gas exchange which reflect gas exchange across the alveolar-capillary membrane.

The first is that the test measures not just diffusion across the alveolar-capillary membrane, but also takes into account factors affecting the chemical combination of a given gas with hemoglobin.

For example, in the alveoli of mammalian lungs, due to differences in partial pressures across the alveolar-capillary membrane, oxygen diffuses into the blood and carbon dioxide diffuses out.

This test gas mixture contains a small amount of carbon monoxide (usually 0.3%) and a tracer gas that is freely distributed throughout the alveolar space but which doesn't cross the alveolar-capillary membrane.

The concentration of carbon dioxide (CO) in healthy alveoli is known - it is equal to its concentration in blood since CO rapidly equilibrates across the alveolar-capillary membrane.

When alveolar ventilation (in liters of air per minute) and alveolar capillary blood flow (in liters of blood per minute) are approximately equal, oxygen equilibrates across the alveolar-capillary membrane well before the blood has traversed the alveolus.