Blood flow in pulmonary veins: III. Simultaneous measurements of their dimensions, intravascular pressure and flow.

Vein flow in the large extraparenchymal pulmonary veins is pulsatile and its wave form has an inverse relationship to left atrial pressure. Extraparenchymal pulmonary veins are thin walled and collapsible. This enables them to behave as highly compliant structures. Dimensional measurements of their cross sectional area in living open chested dogs showed them to be non circular at low left atrial pressures. They rapidly assumed a circular cross section as left atrial pressure rose. Only at pressures above 1.5 kPa (11 mmHg) were the pulmonary veins circular in cross section. The aggregate volume of the large extraparenchymal pulmonary veins, when fully distended, was found to be equal to or greater than one stroke volume of the heart. The extraparenchymal pulmonary veins act as a reservoir to the left atrium so that left ventricular stroke volume can be maintained relatively unaffected by beat by beat changes in right ventricular stroke output. Their behaviour at normal mean left atrial pressures also enables them to isolate the lung capillaries from retrograde transmission of positive pressure transients from the left atrium, which could otherwise impede venous outflow of blood from the lung capillary bed.