alveolar gas, so the ideal alveolar-arterial PO2 difference equals zero. However, Fig. 1 shows that, in reality, the alveolar-arterial PO2 difference exceeds zero, even in healthy young adults. Several factors can increase the alveolar-arterial PO2 difference, relative to the ideal value of zero, and these are called gas exchange limitations. The alveolar-arterial PO2 difference quantifies the net effect of these limitations on pulmonary gas exchange. Gas exchange limitations do not affect O2 consumption at rest, but they will lower maximal O2 consumption and PO2 values along the O2 cascade in a steady state.

The alveolar-arterial PO2 difference is useful for diagnosing O2 exchange limitations. The physiologic mechanisms responsible for an alveolar-arterial PO2 difference increase gradually with age or dramatically with lung disease. However, different mechanisms respond differently to simple tests such as O2 breathing. This means that changes in the alveolar-arterial PO2 difference with such simple tests can be used to diagnose pulmonary disease.

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