Hypoxia and the pulmonary circulation: a brief review.
Hypoxia constricts small pulmonary arteries. Local hypoxia regulates blood flow/ventilation ratios, while general hypoxia elevates pulmonary artery pressure (Ppa). There is a continuum of responses from flow reduction to Ppa elevation dependent on the proportion of lung involved. Stimulus-response curves to hypoxia show the effect is maximal within the physiological range and resemble that for the carotid body. In widespread lung disease so much of the lung becomes hypoxic, that blood flow/ventilation matching fails, hypoxaemia and pulmonary hypertension follow. In chronic hypoxia structural changes take place which maintain a high pressure even when hypoxia is removed; small arterioles become muscularized and there is right ventricular hypertrophy and polycythaemia. Animal models of hypoxic pulmonary hypertension have brought some understanding of the growth processes involved and shown that several drugs will prevent these changes. The reactivity of the restructured pulmonary vessels in chronic hypoxia is altered.