Role of the Veins in Venous Return

"It is axiomatic that the heart can pump only as much blood as it receives. Indeed, the volume of blood returned to the heart is the basic determinant of cardiac output. Since the latter varies enormously under ordinary con-ditions of daily activity, the mechanisms that facilitate venous return have been the subject of discussion for centuries."—C. J. Wiggers1 THE veins are no longer considered pas-sive channels between the capillaries and the heart. However, the direct contribution of the venules and veins to the dynamics of venous return has not been fully explored. Folkow2"4 has noted the disparity which exists in the literature concerning the amount of information on the arterial and on the venous sides of the circulation. He contends that "this is to a great extent due to the diffi-culties involved in studying the reactions of the veins, as there are few good methods avail-able for a continuous and selective recording of active changes in venous tone."2 Franklin,5 Landis and Hortenstine,0 and McDowall7 ' 8 extensively reviewed the litera-ture pertaining to the reactivity of the veins of animals and man to mechanical, thermal, chemical, and nervous stimuli. Numerous in-vestigators,8"31 using widely varying experi-mental methods, have provided evidence that individual veins, or selected venous beds, are under tonic nervous control and react to vasomotor center activity in the same manner as do arterial vessels