VASCULAR: VENOUS RETURN Flashcards
What determines venous return?
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)
Define: mean systemic filling pressure (Pmsf)
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
Define: mean systemic filling pressure (Pmsf)
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
What is the total peripheral resistance? Show your calculations
Resistance = TPR = P/Q = 20 mmHg/l/min
Because Parterial is equal to 102 mmHg and Pvenous is 2 mmHg
Q = 5 L/min
Describe the vascular function curve
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
What happens when right atrial pressure is 7 mmHg?
When right atrial pressure is 7 mmHg it matches mean systemic filling pressure and thus there is 0 driving force for venous return
What happens when right atrial pressure becomes less than 0?
When right atrial pressure becomes less than 0, the veins tend to collapse
Describe what the vascular function curve slope tells us
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
How does vasodilation and vasoconstriction influence the vascular function curves
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)
Describe how changes in blood volume effect the vascular function curves
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
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?
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
How does the vascular function curve change with a blood transfusion?
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.
How does the cardiac function curve shift with a positive inotrope?
When a positive inotrope → contractility increases → new steady state with higher cardiac output, higher venous return, lower right atrial pressure.
How does the cardiac function curve change with sympathetic nerve stimulation?
With sympathetic nervous system → new steady state with much higher cardiac output, much higher venous return, and similar right atrial pressure
What is heart failure?
What is it often due to ?
What is the focus of recovery from heart failure?
heart failure: compromised cardiac pumping actviity
often due to myocardial infarction
focus of recovery means improving function of the remaining cells
define: myocardial infarction
Myocardial infarction - complete cessation of blood flow & tissue death
cell death can either be due to nercosis or apoptosis
define: myocardial ischemia
reduced blood flow and o2 supply; blood flow and o2 supply is less than demand
define: necrosis
necrosis (unregulated cell death)
define: apoptosis
apoptosis (highly ordered cell death)
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
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
Discuss how the cardiac function curve changes during decompensated heart failure
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
What are the treatments for decompensated heart failure?
Cardiac glycosides: example - digitalis; results in increased contractility
Diuretic drugs: example - furosemide; results in increase in renal excretion and decrease in blood volume.
