Venous Return - Hemodynamics

Venous return (VR) is the flow of blood back to the heart. Under steady-state conditions, venous return must equal cardiac output (CO) when averaged over time because the cardiovascular system is essentially a closed loop (see figure at right). Otherwise, blood would accumulate in either the systemic or pulmonary circulations. Although cardiac output and venous return are interdependent, each can be independently regulated.

The circulatory system is made up of two circulations (pulmonary and systemic) situated in series between the right ventricle (RV) and left ventricle (LV) as depicted in the figure to the right. Balance is achieved, in large part, by the Frank-Starling mechanism. For example, if systemic venous return is suddenly increased (e.g., changing from upright to supine position), right ventricular preload increases leading to an increase in stroke volume and pulmonary blood flow.  Increased pulmonary venous return to the left atrium leads to increased filling (preload) of the left ventricle, which in turn increases left ventricular stroke volume by the Frank-Starling mechanism.  In this way, an increase in venous return can lead to a matched increase in cardiac output.

Hemodynamically, venous return (VR) to the heart from the venous vascular beds is determined by a pressure gradient (venous pressure, P_V, minus right atrial pressure, P_RA) and venous resistance (R_V) as shown to the right.  Therefore, increases in venous pressure or decreases in right atrial pressure or venous resistance will lead to an increase in venous return, except when changes are brought about altered body posture (see below). Although the above relationship is true for the hemodynamic factors that determine the flow of blood from the veins back to the heart, it is important not to lose sight of the fact that blood flow through the entire systemic circulation can be represented by either the cardiac output or the venous return, because these are equal in the steady-state owing to the circulatory system being closed. Therefore, one could just as well say that venous return is determined by the mean aortic pressure minus the mean right atrial pressure, divided by the resistance of the entire systemic circulation (i.e., the systemic vascular resistance).

Venous return is influenced by several factors.

1. Muscle contraction. Rhythmical contraction of limb muscles as occurs during normal locomotory activity (walking, running, swimming) promotes venous return by the muscle pump mechanism.

2. Decreased venous compliance. Sympathetic activation of veins decreases venous compliance, increases central venous pressure and promotes venous return indirectly by augmenting cardiac output through the Frank-Starling mechanism, which increases the total blood flow through the circulatory system.
3. Respiratory activity. During respiratory inspiration, the venous return increases because of a decrease in right atrial pressure.
4. Vena cava compression. An increase in the resistance of the vena cava, as occurs when the thoracic vena cava becomes compressed during a Valsalva maneuver or during late pregnancy, decreases return.
5. Gravity. The effects of gravity on venous return seem paradoxical because when a person stands up hydrostatic forces cause the right atrial pressure to decrease and the venous pressure in the dependent limbs to increase. This increases the pressure gradient for venous return from the dependent limbs to the right atrium; however, venous return actually decreases. The reason for this is when a person initially stands and before the baroreceptor reflex is activated, cardiac output and arterial pressure decrease because right atrial pressure and ventricular preload falls, which decreases stroke volume by the Frank-Starling mechanism.  The flow through the entire systemic circulation falls because arterial pressure falls more than right atrial pressure, therefore the pressure gradient driving flow throughout the entire circulatory system is decreased.

Revised 04/07/10