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VENOUS RETURN
(VR) is the flow of blood back to the heart.
Under steady-state conditions, venous return must equal cardiac output
SYNCHRONOUS CO
CVS is essentially a closed loop.
Otherwise, blood would accumulate in either the systemic or
pulmonary circulations
.
Increase in venous return can lead to a matched increase in cardiac output
Two circulations (pulmonary and systemic) situated in series between the right ventricle (RV) and left ventricle (LV).
Balance is achieved, in large part, by the Frank–Starling mechanism
FRANK–STARLING MECHANISM
If 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.
The left ventricle experiences an increase in pulmonary venous return,
Increases left ventricular preload and stroke volume by Frank–Starling mechanism.
DETERMINANT OF VR
(VR) to the heart from the venous vascular beds is determined by a pressure gradient 1. Venous pressure –2 Right atrial pressure 3.Venous resistance
Therefore, increases in venous pressure or decreases in right atrial pressure or venous resistance will lead to an increase in
venous return,
MATHEMATICALLY
VR is determined by the mean aortic pressure minus the mean right atrial pressure, divided by the resistance of the entire systemic circulation .
MAP-RAP/R
FACTORS AFFECTING VENOUS RETURN MECHANISM
Musculovenous pump:
Decreased Veno capacitance
Respiratory pump
Venacaval compression
Gravity
Pumping action of the heart
MUSCLE PUMP
Determinants of Venous Return
Mean systemic filling pressure
Right Atrial Pressure
Resistance to Flow
Pressure change is slight. Thus, small increase in RA Pressure causes dramatic reduction in venous return. (mean systemic filling pressure).
Venous Return & Cardiac Output
Cardiac output increases with atrial pressure.
Normal atrial pressure is about 0 mm Hg. Venous return (with heart and lungs
removed) decreases with atrial pressure. Working cardiac output is where venous
return curve meets cardiac output curve.
Normal Venous Return Curve
5 L/min
VR (CO)
Rt. Atrial Pressure (mm Hg)
0-4
Mean systemic filling pressure ~ 7 mm Hg
Plateau: collapse of large veins ( => increased resistance)
Venous return with heart and lung removed.
Cardiac Output Curve
Working Cardiac Output
Changes in Intrapleural Pressure
Generally shift the cardiac output curve in proportion to pressure change (breathing, Valsalva maneuver).
Cardiac Tamponade (filling of pericardial sac with fluid) lowers rate of change of CO with right atrial pressure
HeartPericardial Sac
Rt. Atrial Pressure
CO
tamponade15 L/min