An alveolus where V>Q (well ventilated but poorly perfused alveolus).There was not blood in this case to take part in gas exchange. Lack of blood flow around an alveolus with normal ventilation (dead space ventilation with V/Q = infinity, eg pulmonary embolism).The blood passing back into the circulation from this alveolus has not taken part in gas exchange and so will have the same PO 2 (partial pressure or tension of oxygen) and PCO 2 (partial pressure or tension of carbon dioxide) as the blood in the pulmonary artery. Atelectic (closed) alveolus with normal perfusion (physiological shunt with V/Q = zero).Extremes of ventilation-perfusion mismatching include:.In conscious horses the vertical gradient of ventilation closely matches the vertical gradient of perfusion, but local mismatches of V or Q can still occur, eg due to local hypoxic pulmonary vasoconstriction.Some parts of the lungs receive more ventilation, and some parts receive more blood.Ventilation-perfusion mismatching describes conditions in which changes in ventilation and/or perfusion → inadequacies in gas exchange.This matching of ventilation and perfusion is the most important determinant of gas exchange in the lungs. Normally, the ratio of pulmonary ventilation (V) to perfusion (Q) when considering the lungs as a whole is close to 1 (V/Q = 0.8 in humans). Ventilation of alveoli is, ideally, matched to their perfusion.
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