Dead spaces can severely impact breathing, because they reduce the surface area available for gas diffusion. Both produce dead space or shunts, regions of ineffective lung tissue. This difference between the amounts of air and blood reaching the lungs is referred to as ventilation/perfusion (V/Q) mismatch. At times, however, there is a mismatch between the amount of air (ventilation, V) and the amount of blood (perfusion, Q) in the lungs. These capillaries and arteries are not always in use but are ready if needed. As cardiac output increases, the number of capillaries and arteries that are perfused (filled with blood) increases. This is because of a phenomenon called recruitment, which is the process of opening airways that normally remain closed when cardiac output increases. It is also independent of cardiac output. Pulmonary circulation pressure is very low compared to that of the systemic circulation.
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