venous return

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