![]() Ventilation is also higher at the bases compared to the apices but the differential is to a far lesser degree. Whilst standing, perfusion is better to the bases when compared with the apices due to gravitational and hydrostatic forces. In normal health there is a mismatch in V/Q that differs depending on the part of the lung. In the average adult the alveoli are ventilated by 4L of air and perfused by 5L of blood each minute, giving a V/Q ratio of 0.8 (4:5). This is the ratio of alveolar ventilation to alveolar perfusion. V/Q refers to the ventilation-to-perfusion ratio. Hypercapnia (+/- hypoxaemia) normally results from reduced alveolar ventilation or increased alveolar dead space. Whilst V/Q mismatch, shunts and diffusion limitation cause hypoxaemia they may also feature hypercapnia if the disease process is severe enough. ![]() Though below we discuss them separately they are related and often occur together. There are a number of processes that lead to hypoxaemia and hypercapnia. Hypoxaemia is most commonly caused by a V/Q mismatch, hypercapnia is often caused by alveolar hypoventilation. ![]()
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