Clinical term referring to the volume of air in the lung that does not exchange with the blood during respiration.

When air is inhaled, a lot of it goes into tubes whose only function is to carry the air to alveoli - cells which exchange oxygen and carbon dioxide between the blood and the air. The air that sits in these tubes upon inhalation is known as the anatomical dead space. In addition, if certain alveoli are nonfunctional or if the air is exhaled to quickly, a portion of the air never exchanges with the blood. All of this together is the physiological dead apace. The physiological dead space includes the anatomical dead space and additonal volume that does not exchange gas.