Venous return refers to the amount of blood and blood flow returned back to the heart from veins. The cardiovascular system, consisting of both systemic and pulmonary circulations, is a closed-loop system, with the right ventricle receiving blood from the systemic circulation and the left ventricle receiving blood from the pulmonary circulation. At steady state conditions, cardiac output (CO) equals venous return. If CO did not equal venous return, then blood volume would collect in one part of the circulation or another. Therefore, since CO is 5 L/min, venous return also equals 5 L/min.
VASCULAR FUNCTION RELATIONSHIP
Arthur Guyton correlated the relationship between mean systemic filling pressure (MSFP) and right atrial pressure (RAP) to control venous return. Baseline values are as follows:
A change in RAP (which is equivalent to central venous pressure [CVP]) is produced by a change in CO. As CO (or venous return) increases, CVP decreases and more blood is pumped from veins to the right atrium. Consequently, increases in MSFP or decreases in RAP lead to increased venous return.
Venous return increases as the pressure difference between RAP and MSFP increases. If RAP (or CVP) is low and MSFP high, then there will be a maximum change in pressure and a maximum venous return of blood to the right heart. On the other hand, if RAP increases, but there is no change in MSFP, then there will be a small difference between the two variables and venous return will decrease.
FACTORS AFFECTING VENOUS RETURN
Increases in blood volume will increase MSFP. Higher blood volumes lead to greater vasculature stretch (preload) and end-diastolic volume, resulting in an increased gradient for flow to the right atrium and venous return. Anything that causes volume retention increases MSFP. Examples include blood transfusion and fluid retention by renal mechanisms (ADH, renin-angiotensin, and aldosterone). Alternatively, situations that decrease blood volume, such as hemorrhage and dehydration, decrease MSFP and venous return.
Skeletal Muscle Contraction
During physical activity, venous pressure is increased due to muscle contraction, causing greater venous blood return to the heart. Peripheral veins have one-way valves that direct flow of blood away from the limbs and toward the heart. Muscle contraction causes venous compression, and muscle relaxation causes venous decompression. The alternative contraction and relaxation patterns cause blood to be pumped back to the heart, and unidirectional valves prevent blood from flowing back toward the limbs, enhancing venous return.
As stated previously, venous return increases as the pressure difference between RAP and MSFP are increased. Respiratory activity influences venous return to the ...