Non-respiratory functions of the lung

The lung has an extremely distensible vasculature, which enables it to cope with large variations in venous return, especially during postural changes, exercise, and increased intravascular volume. The two mechanisms involved are recruitment and distension of the pulmonary vasculature. At resting cardiac output, the pulmonary vascular bed is not fully perfused. In the face of an increased cardiac output, underperfused areas of the pulmonary vasculature are ‘recruited’ to accommodate an increase in blood flow and to prevent an increase in pulmonary arterial pressures. The walls of the pulmonary vasculature are thin and contain relatively little smooth muscle, which make it compliant to increased blood volume. The pressure in the pulmonary circulation is approximately six times less than that of the systemic circulation and both arteries and veins increase in calibre with lung expansion. This ‘distension’ along with ‘recruitment’ helps in altering the lung blood volume by 500–1000 ml.