Part of Each Breath Ventilates Dead-Space and Does Not Participate in Gas Exchange

Dead-space ventilation consists of the gas that does not par­ticipate in gas exchange. This includes both anatomic dead­space (see Chapter 45) and alveolar dead-space. The latter con­sists of alveoli that receive ventilation but no blood flow; that is, they have a V/Q ratio of infinity (see Figure 47-5, top). Alveolar dead-space can form when the pulmonary arterial pressure is so low that many capillaries are unperfused, or when the vessels are obstructed by thrombi.