VASCULAR: VENOUS RETURN

Question 1

What determines venous return?

Answer 1

VR = Q = Change in pressure/ resistance

Change in pressure: P mean systemic filling - P right atrial

R: resistance expressed for the entire vasculature; Total peripheral resistance (TPR)

Question 2

Define: mean systemic filling pressure (Pmsf)

Answer 2

theoretical pressure if the circulation is experimentally stopped and arterial and venous pressures equilibrate (= ~7 mmHg); represents the driving force for filling of the right atrium

Question 3

Define: mean systemic filling pressure (Pmsf)

Answer 3

theoretical pressure if the circulation is experimentally stopped and arterial and venous pressures equilibrate (= ~7 mmHg); represents the driving force for filling of the right atrium

Question 4

What is the total peripheral resistance? Show your calculations

Answer 4

Resistance = TPR = P/Q = 20 mmHg/l/min

Because Parterial is equal to 102 mmHg and Pvenous is 2 mmHg

Q = 5 L/min

Question 5

Describe the vascular function curve

Answer 5

Photo discussion: As you increase right atrial pressure, venous return falls because the driving force for venous return gets smaller. Remember that teh driving force for venous return is delta P and delta P = Pmean systemic filling - P right atrial

Question 6

What happens when right atrial pressure is 7 mmHg?

Answer 6

When right atrial pressure is 7 mmHg it matches mean systemic filling pressure and thus there is 0 driving force for venous return

Question 7

What happens when right atrial pressure becomes less than 0?

Answer 7

When right atrial pressure becomes less than 0, the veins tend to collapse

Question 8

Describe what the vascular function curve slope tells us

Answer 8

The slope of the vascular function curve is Q/P; Thus the slope is equal to 1/R. The steeper the slope of the vascular function curve, the lower the resistance. A shallow slope means we have higher resistance

Question 9

How does vasodilation and vasoconstriction influence the vascular function curves

Answer 9

with vasoconstriction, there is a decreased vascular function curve as there is a lower venous return for a given right atrial pressure (remember also higher resistance) .

With vasodilation there is an increased slope meaning there is greater venous return for a given right atrial pressure (remember also lower resistance)

Question 10

Describe how changes in blood volume effect the vascular function curves

Answer 10

Increase in blood volume → increase in mean systemic filling pressure → right shift in the vascular function curve → increase in venous return

Decrease in blood volume (hemorrhage) → decrease in mean systemic filling pressure → left shift in vascular function curve → decrease in venous return

Question 11

When right atria pressure increases, this results in an increase in cardiac output. Now, there is a mismatch between venous return and cardiac output. How is equilibrium restored?

Answer 11

Point B → Point C: Increase in right atrial pressure will result in an increase in cardiac output but also a decrease in venous return. This mismatch must be corrected ASAP. The increase in cardiac output leads to increase in venous pressure. Higher cardiac output also sucks blood from the right atria which decreases right atrial pressure. This results in an increase in venous return

Question 12

How does the vascular function curve change with a blood transfusion?

Answer 12

With transfusion: there is slightly vasodilation; the slope is slightly steeper. There is now a new steady-state with higher cardiac output, higher venous return, and higher right atrial pressure.

Question 13

How does the cardiac function curve shift with a positive inotrope?

Answer 13

When a positive inotrope → contractility increases → new steady state with higher cardiac output, higher venous return, lower right atrial pressure.

Question 14

How does the cardiac function curve change with sympathetic nerve stimulation?

Answer 14

With sympathetic nervous system → new steady state with much higher cardiac output, much higher venous return, and similar right atrial pressure

Question 15

What is heart failure?

What is it often due to ?

What is the focus of recovery from heart failure?

Answer 15

heart failure: compromised cardiac pumping actviity

often due to myocardial infarction

focus of recovery means improving function of the remaining cells

Question 16

define: myocardial infarction

Answer 16

Myocardial infarction - complete cessation of blood flow & tissue death

cell death can either be due to nercosis or apoptosis

Question 17

define: myocardial ischemia

Answer 17

reduced blood flow and o2 supply; blood flow and o2 supply is less than demand

Question 18

define: necrosis

Answer 18

necrosis (unregulated cell death)

Question 19

define: apoptosis

Answer 19

apoptosis (highly ordered cell death)

Question 20

Discuss how the cardiac function curve changes immediately after heart failure and within a few hours to days after heart failure if the heart can compensate

Answer 20

When heart failure occurs, there is acute compensation within minutes or hours of the event. The decrease in cardiac output activates the sympathetic nervous system (baroreflex). This increases contractility. Vasoconstriction also increases mean systemic filling pressure (MSFP) and reduces the slope of the vascular function curve. Hours or days after heart failure, there is potentially partial recovery of the heart tissue. Renal fluid retention increases blood volume and mean systemic filling pressure. The stunned myocardium recovers

Question 21

Discuss how the cardiac function curve changes during decompensated heart failure

Answer 21

Photo discussion: renal fluid retention is not enough to increase cardiac output but results in overstretch of the myocardium and pulmonary edema. Cardiac output descends due to the overstretch

Question 22

What are the treatments for decompensated heart failure?

Answer 22

Cardiac glycosides: example - digitalis; results in increased contractility

Diuretic drugs: example - furosemide; results in increase in renal excretion and decrease in blood volume.