What defines physiologic dead space?

Other alveolar units have ventilation, but no perfusion. These units have a ventilation-perfusion ratio of infinity and are what is referred to as physiologic dead space.

Conceptually, alveolar units with ventilation-perfusion ratios of greater than one, but not infinity, can be described as if the units were functionally equivalent to areas of physiologic dead space.

In diseases characterized by areas of high ventilation-perfusion ratios, hypercapnia usually does not occur because individuals can compensate for the increase in dead space by increasing the overall level of ventilation.

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