Venous Return - Hemodynamics
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.
Venous return is influenced by several factors.
- Muscle contraction. Rhythmical contraction of limb muscles as occurs during normal locomotory activity (walking, running, swimming) promotes venous return by the muscle pump mechanism.
- 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.
- Respiratory activity. During respiratory inspiration, the venous return increases because of a decrease in right atrial pressure.
- 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.
- 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.