Venous Return

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  • 1. (VR) is the flow of blood back to the heart. Under steady-state conditions, venous return must equal cardiac output
  • 2. CVS is essentially a closed loop. Otherwise, blood would accumulate in either the systemic or pulmonary circulations
  • 3. 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
  • 4. 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.
  • 5. (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,
  • 6. 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
  • 7. Musculovenous pump: Decreased Veno capacitance Respiratory pump Venacaval compression Gravity Pumping action of the heart
  • 8. Mean systemic filling pressure Resistance to Flow Pressure change is slight. Thus, small increase in RA Pressure causes dramatic reduction in venous return. (mean systemic filling pressure). Right Atrial Pressure
  • 9.     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.
  • 10. Plateau: collapse of large veins ( => increased resistance) Working Cardiac Output 5 L/min VR (CO) Venous return with heart and lung removed. Cardiac Output Curve -4 0 Rt. Atrial Pressure (mm Hg) Mean systemic filling pressure ~ 7 mm Hg
  • 11. Heart Pericardial Sac   tamponade 15 L/min CO 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 Rt. Atrial Pressure