Influence of hydrostatic pressure on the pulmonary circulation.

Abstract

The uneven topographical distribution of blood flow in the lung is an inevitable consequence of the intimate apposition of blood and air which results in large pressure differences across the pulmonary vessels at different levels in the lung. The resulting imbalance of blood flow and ventilation causes large regional differences in gas exchange but interference with overall gas transfer is negligible unless the normal pulmonary arterial pressure is reduced or alveolar pressure is increased when an alveolar dead space may develop. The uneven distribution of blood flow results in peculiar overall pressure-blood flow relations in that the pulmonary vascular resistance falls considerably when either the pulmonary arterial or venous pressures are raised. These characteristics are advantageous to the lung, the one organ of the body which receives the whole cardiac output during a variety of physiological states.In general, the patterns of distribution of blood flow observed in health and disease can apparently be explained by the interactions between the pulmonary arterial, venous and alveolar pressures. However patients with an increased pulmonary venous pressure as in mitral stenosis or left ventricular failure may show a reduction of blood flow in the lower zones and a similar pattern has been found in the isolated lung where it is caused by interstitial oedema. The mechanism is postulated to be an interference with the normal expanding action of the inflated lung parenchyma on the extra-alveolar blood vessels.