Mechanism of pulmonary arterial hypertension

Pulmonary circulation in the adult is a low pressure, low resistance system. Among the factors able to influence pulmonary arterial pressure the most important are hydrostatic pressure, intraalveolar pressure, left atrial pressure and alveolar gases, whilst the role of vasomotor tone is relatively limited. Granted that various hypertensive mechanisms can work jointly, the main forms of pulmonary arterial hypertension are the "passive" or "postcapillary" variety characterized by an elevated wedge pressure and the "precapillary" variety associated with a normal wedge pressure. The "precapillary" variety may be subdivided into hypertension due to increased blood flow, "obstructive" hypertension (e.g. thromboembolism), reactive hypertension (due to alveolar hypoxia) and hypertension transmitted from the bronchial circulation (bronchiectases?). In chronic lung disease pulmonary arterial hypertension is partly fixed and partly reversible.