THE ACTION OF CARDIAC EJECTION ON VENOUS RETURN

Abstract

The mechanical relations between cardiac ejection, lung air pressure, chest wall movements and venous return are discussed. The excess of arterial outflow over venous inflow to the chest during systole tends to create a partial vacuum of about 15 mm. Hg in the lungs. Collapse of the chest wall, however, permits only a small fraction of this to be realized, about 0.5 mm. Hg according to derivations from measurements of transient increase of body volume with each heart beat. This partial vacuum will assist venous return but it is not nearly as important as the negative intra-pleural pressure in performing this function. The relative sizes of cardiac stroke volume and lung volume determine the partial vacuum which tends to be created in the lungs by the heart beat and therefore the potential aspirating forces on venous blood and chest wall. The ratio of cardiac stroke volume to lung volume is approx. constant in warm blooded animals. Consequently, the relative mechanical effects of ejection are similar in animals of all sizes. Decrease of lung vol. caused by accumulation of blood in the chest and the accompanying changes of elasticity may be expected to increase the aspirating action of arterial ejection. That this occurs is not proved directly, but it is shown that the normal mechanical relations may be greatly altered in valvular disease of the heart. In advanced mitral stenosis, for example, the chest may expand rather than contract during systole.