Venous Return

Question 1

For CO to go up, must venous return go up?

Answer 1

YES

Question 2

What happens to CO if right atrial pressure goes up?

Answer 2

it goes up

Question 3

What happens to CO if right atrial pressure goes down?

Answer 3

it goes down

Question 4

What is the independent and dependent variable, respectively on a graph of CO vs. Right atrial pressure?

Answer 4

Right atrial pressure= x axis (independent variable)
CO= y axis (dependent variable)
*So this graph does NOT tell you what happens to right atrial pressure as CO goes up.

A graph where these axes are switched will tell you the opposite, but this graph is completely linear and reversed.
CO=x axis
right atrial pressure= y axis
As CO goes up right atrial pressure (how much blood is entering vs. how much blood is leaving; if more blood enters, right atrial pressure goes up) goes down because more blood is leaving than is entering.

Question 5

What does the vascular function curve show us?

Answer 5

plots venous return (y axis; dependent variable) vs. right atrial pressure (x axis). Aka: we are looking at the amount of blood returning to the right atrium as a function of right atrial pressure.

Question 6

If central venous pressure (SVC and IVC) doesn’t change, but right atrial pressure (RAP) goes up, what must happen to venous return?

Answer 6

it DECREASES, because venous return is proportional to a difference in pressure between CVP and RAP (CVP-RAP).
Think about it this way too: if you fill up a balloon (right atrium) with air, is it easier or harder to blow it up more as the pressure increases? Harder. so that’s why venous return (the air in this analogy) decreases.

Question 7

What is the mean circulatory pressure (mean systemic filling pressure)?

Answer 7

pressure of right atrium (about 7 mm Hg) strictly due to the presence of blood volume. aka pressure if you were dead.

Question 8

So what happens to venous return when RAP decreases?

Answer 8

it INCREASES, because the driving pressure to fill the right atrium is going up.

Question 9

What happens when right atrial pressures drop to about 0?

Answer 9

veinous return reaches a maximum (plateaus on the graph) as veins collapse. Think about sucking so much blood out of the veins that they collapse because they are flaccid.
So if you try to suck the blood out of a vein with a syringe, you can only pull back on the syringe with a certain speed, or else the vein will collapse!

Question 10

What happens to the vascular function curve (venous return vs. RAP) if blood volume increases (hypervolemia) or venoconstriction increases, for a given RAP?

Answer 10

It also increases (shifts up and to the right).

Question 11

What happens to the vascular function curve (venous return vs. RAP) if blood volume decreases or venoconstriction decreases, for a given RAP?

Answer 11

It decreases (shifts down and to the left).

Question 12

What happens to venous return with increases arterial tone (aka increasing arterial resistance)?

Answer 12

it DECREASES (rotates curve COUNTERCLOCKWISE), because you are decreasing the amount of blood flowing from the arterial side to the venous side. 
This is different than the effects of changes in blood volume or venoconstriction, because when arterial tone (resistance) changes, the curve pivots around the same mean systemic filling pressure, compared to the curve shifting in parallel.

Question 13

What happens to venous return with decreases in arterial tone (aka decreasing arterial resistance)?

Answer 13

in INCREASES (rotates curve CLOCKWISE).

Question 14

What does the equilibrium point indicate on a graph that superimposes CO and venous return as functions of RAP?

Answer 14

that CO and venous return are identical in terms of RAP :)

Question 15

What happens if we increase RAP from 2 to 4 for example in terms of CO and venous return?

Answer 15

you will increase CO, and therefore a drop in venous return due to the higher pressures in the right atrium preventing more blood from entering it.
This means that there is more blood leaving the heart than entering it, and this cannot persist, so overtime you will see a gradual decrease in right atrial pressure (because less blood is being pumped around overall, meaning less blood and therefore pressure in the right atrium) and therefore a gradual decrease in CO. This also means that you will see an equilibrating increase in venous return.

Question 16

What happens to RAP when CO is greater than venous return? aka what happens for the heart to equilibrate?

Answer 16

CO will go down and venous return will increase

Question 17

How does CO relate to venous return when we increase blood volume (hypervolemia; shifting the curve to the right)?

Answer 17

venous return will be greater initially than CO for at least a certain amount of time, meaning the RAP will have to increase. However, as this equilibrates, CO will increase and venous return will decrease.

Question 18

How does CO relate to venous return when we increase CONTRACTILITY (via sympathetic stimulation; shift cardiac function curve up and to the left)?

Answer 18

the CO will initially be greater than venous return. So RAP must go down (bc more blood is entering the right ventricle) causing venous return to increase, thus equilibrating the new curve up and to the left.

Question 19

How does CO relate to venous return when we increase ARTERIAL RESISTANCE (via sympathetic stimulation; shift cardiac function curve down and to the right or clockwise)?

Answer 19

This in effect will increase afterload (increase in aortic pressure) thus DECREASING CO below venous return, initially. This causes right atrial pressure to increase, and overtime CO will increase and venous return will decrease to reach a new equilibrium point at a lower CO.

Question 20

How does an increase in arterial resistance (lowering CO on the cardiac function curve) influence the vascular function curve? (see diagrams)

Answer 20

This is just another way of wording what happens to venous return when we increase arterial resistance (afterload). It will rotate the curve COUNTERCLOCKWISE, decreasing venous return.
This will give us a new equilibrium point at a lower CO and lower venous return.

Question 21

** So what collectively happens with sympathetic stimulation?

Answer 21

HR increases and contractility increases (increasing dP/dT) shifting the cardiac function curve (CO) up and to the left (rotating it counterclockwise).
Venous (veins) tone will also increase with sympathetic stimulation (meaning you’ll reduce the size of the venous compartment, shifting more of the blood (remember veins have 2/3 of all blood) from the venous side to the arterial side, and resistance won’t change much bc veins are larger overall), giving the same response as if you would increase blood volume. Aka you shift the vascular function curve up and to the right; increasing venous return and RAP :)
* So the new equilibrium point of both the cardiac function curve (CO) and vascular function curve (venous return) due to sympathetic stimulation will INCREASE BOTH!

Question 22

In regards to changes in these curves, must CO and venous return always match with equilibration?

Answer 22

YES!!! When changes occur, they happen briefly and then equilibrate. That’s all these curves are showing us.