Lecture 13: Venous return

Question 1

Describe how venous return is calculated

Answer 1

VR=Q=ΔP/Rvr = (MSFP-Pra)/(Rv+Ra(Ca/Ca+Cv))
CO=Q=ΔP/R= (MAP-Pra)/TPR

Mean systemic filling pressure is defined as the theoretical pressure that the body would equilibrate at if the heart stopped pumping. it is the driving force for filling of the right atrium.

Question 2

Describe the vascular function curve, as well as what the changes are based on resistance and blood volume

Answer 2

Has plateau due to competing effects. As vacuum in RA, sucks more blood from vena cava, but also causes constriction. At very rightmost point, Ra=MSFP therefore no VR.

When there is increased resistance, slope (1/R) changes but MSFP doesn’t.
When there is increased volume, but compliance/resistance doesn’t change, the slope doesn’t change but MSFP does.
Note: VENOconstriction is similar to increase in blood volume due to decrease in venous reserve, therefore increase in effective circulating volume

Question 3

Describe how the body gets back to set-point.

Answer 3

when RAP increases, there is an increase in EDV, so there is an increase in CO. This sucks blood out of the RA, and causes an increase in venous return, and also decreases RAP and EDV, leading to the decrease in CO and increase in VR.

Question 4

Describe how the steady-state point can change

Answer 4

If there is transfusion, Δ inotropy, or SNS stimulation.
transfusion causes shift in vascular function.
Δ inotropy means that there is a shift in the cardiac function curve.
SNS stimulation means there is change in both.

Question 5

Describe compensated heart failure

Answer 5

When ischemia occurs, cardiac output drops significantly
At first, there is vasoconstriction through the baroreflex, causes increase in contractility and MSFP
Later on, there is increased fluid retention, increases MSFP

Question 6

Describe decompensated heart failure

Answer 6

When the cardiac output is so low that fluid balance can’t be maintained, fluid retention keeps increasing to try to raise CO, but eventually stretches the myocardium to the point where there is edema occuring
To treat this, can either use something like digitalis to increase contractility, or furosemide which inhibits the NaCl symporter to reduce fluid retention