Interaction between the Heart and the Circulation

Question 0

Define venous return?

Answer 0

is the flow (ml/min) returning to the central venous compartment from the peripheral venous compartment.

Question 1

What happens to the cardiac output from the left ventricle after perfusing the entire body?

Answer 1

It is returned to the heart via the venous system.

Question 2

What is the central venous compartment?

Answer 2

are the great veins in the thorax and in the right atrium

Question 3

The venous return is directly proportional to ________ and inversely proportional to_______

Answer 3

1) Pressure gradient b/n the peripheral venous compartment and central venous compartment
2) Venous resistance

Question 4

What affect the volume of blood in the central venous compartment?

Answer 4

Difference b/n venous return (VR; inflow) and the cardiac output (CO;outflow)

Question 5

Define compliance.

Answer 5

is the amount of volume increase per unit increase in pressure.

Question 6

Compare venous resistance and arterial resistance.

Answer 6

Venous resistance is always ver low compared to arterial resistance.
What does it mean?

Question 7

What determines the filling of the right ventricle?

Answer 7

The pressure in the right atrium which in turn is determined by the CENTRAL VENOUS PRESSURE (pressure in the central venous compartment).

Question 8

Right ventricle-pulmonary circulation-left atrium constitute of a passive conduit conveying blood to the left ventricle. What does this mean?

Answer 8

It is passive means that it does not participate in any of the reflex responses that affect flow in the systemic circulation.

NOTE: We consider that left ventricular filling is determined by CVP and right atrial pressure ALTHOUGH technically its the left atrial pressure that directly determines left ventricular filling.

Question 9

In the steady state what is the relationship b/n CO and Venous Return (VR)?

Answer 9

CO=VR

Question 10

How long does it take a labeled blood ejected from the left ventricle on average to return to the right atrium?

Answer 10

1 min (the so called “mean circulation time”) 5L/min is the cardiac output pumped through the circulation in 1minute.

Question 11

What happens if we change the CO say by suddenly increasing HR?

Answer 11

Circulation is compliant so the changes in CO don’t instantly change the flow via the system. The change will take time to appear downstream (back at the heart).
If CO or VR changes, there will be a brief transient period when CO doesn’t equal VR. This results in change in the distribution of blood volume b/n the venous and arterial compartments.

Question 12

Label Figure 2.3

Answer 12

pp. 168 of week 3 notes.

Question 13

What is the total peripheral resistance? MAP=102 mmHG, CVP= 2mmHG, CO=5L/min

Answer 13

MAP-CVP=CO(TPR)
TPR=100/5 mmHg=20 mmHg
CO=5 L/min
Pressure gradient=MAP-CVP
(MAP-mean arterial pressure)
(CVP-central venous pressure)

Question 14

What happens when the heart is suddenly stopped?

Answer 14

Blood will continue to flow with blood being transferred from the arterial system to the venous system. Increase in the volume of the venous system causes increase in CVP while decrease in volume on the arterial side will cause MAP to fall. Flow continues until the 2 pressures are equal.

Question 15

What happen if the arteries and veins have the same compliance (Ca=Cv) when the heart stops?

Answer 15

The decline in MAP will equal the increase in CVP, because a decrease in the arterial volume equals in an increase in venous volume. Thus, MAP and CVP will attain an average of the 2 initial pressures
(102+2)/2=52
NOTE: BUT veins are much more (19x) compliant than arteries. This means for the same change in volume the pressure will fall 19x.
MAP=CVP 102-19X=2+X X=5mmHg. So the CVP=MAP=7mmHg when blood flow stops.

Question 16

What is the equilibrium pressure which exists in the absence of flow?

Answer 16

Mean circulatory pressure, which is equal to 7mmHg for both MAP and CVP.
NOTE: It reflects the total volume of blood and the overall compliance of the system.

Question 17

What happens to CVP and MAP when CO is 5L/min or 0L/min?

Answer 17

At CO=5L/min
MAP=102mmHg, CVP=2mmHg
At CO=0L/min
MAP=7mmHg, CVP=7mmHg

Question 18

What does the vascular function curve reflect?

Answer 18

-shows how CVP rises and falls when heart is slowed or accelerated.

Question 19

What if we restart the heart and change the CO from 0L/min to
1L/min?

Answer 19

Heart restarts and CVP falls while MAP rises (increment in MAP 19x greater than the decrement in CVP as the same volume is transferred).
Change in pressure=Q(R)=(1L/min) (20mmHg)=20mmHg
7+19X=7-X HUH?
NOTE: gradient can be achieved by 19mmHg rise in MAP
(to 26mmHg) and a 1mmHg fall in CVP( to 6mmHg).
CO=1L/min then CVP=6mmHg and MAP=26mmHg

Question 20

Draw the vascular function curve.

Answer 20

pp. 11
NOTE: the relationships in this function curve and the ones below (both vascular and cardiac) demonstrate concepts which are only a part of circulatory control (it assumes that there are NO reflexes).

Question 21

So what is the point of learning the vascular function curve in relation to congestive heart failure patient?

Answer 21

CHD-decreased SV and CO. From the curve this leads to an increase in venous pressure-leads to increased mid-capillary hydrostatic pressure-increased filtration rate-lead to edema (The patient can’t fit into her shoes!)

Question 22

What profoundly affects the vascular function curve?

Answer 22

It is mainly affected by blood volume. Mean circulatory pressure is normally 7mmHg when heart is stopped. BUT if you decrease the tot. volume of blood the pressure will decrease too and volume increase will result in increase in pressure.

Question 23

How would the vascular function curve change for volume expansion (transfusion) and hemorrhage?

Answer 23

They are simply parallel lines which are shifted upward or downward respectively. Figure on pp 14.

Question 24

When the heart is stopped, describe how the arteriolar tone affects the vascular function.

Answer 24

Figure 2.9 on pp. 180

Question 25

When the heart stops, what happens to the blood volume in the arterioles?

Answer 25

There is minimal amount of blood (3%) in the arterioles.

Question 26

How would a vasoconstriction or vasodilation (assuming it affects the arterioles) change the vascular function curve?

Answer 26

It will change the slope of the vascular function curve without changing the mean circulatory pressure. Figure 2.9 pp 180

Question 27

How is CO related to CVP?

Answer 27

Higher CVP results in greater ventricular filling and stretch of the ventricular fibers (Frank Starling’s Law) causing increased SV. This increases CO.

Question 28

What is the relationship b/n CO and CVP?

Answer 28

Cardiac function curve

Figure pp.16

Question 29

So how can CVP go down when CO goes up (Fig 2.9) and up or CVP goes up when CO goes up (Fig. 2.10)? pp182

Answer 29

It depends on what we are varying:
1) ventricular filling pressure (CVP or, equivalently, right arterial pressure,RAP), HR=constant and measure CO-Cardiac function curve
2) the cardiac output (CO) by varying the HR and measure CVP- the vascular function curve.
NOTE: physiologically both variables change not just one or the other
BUT how do we explain the weird relationship??????

Question 30

Define the operating point (equilibrium point).

Answer 30

Steady state where the CO and CVP will rest under a given condition and is defined by the intersection b/n the vascular function curve and the cardiac function curve. (Figure 2.12)

Question 31

What happens in heart failure (with no reflexes present)?

Answer 31

Decrease in CO at any given filling pressure (CVP) or a shift downward in the cardiac function curve=negative inotropy (b/c filling pressure determines cardiac stretch)
Acute heart failure-decrease CO-increase CVP-increase in CO-decrease in CVP. This happens until we reach a new equilibrium point. The end result is a shift in the equilibrium point from A to B where CVP is increased. Fig 2.13 p 188

Question 32

What is compensated heart failure?

Answer 32

If the heart failure becomes chronic, body will increase blood volume via fluid retention by the kidneys-result in upward shift in the vascular function curve (the same as transfusion)-equilibrium point moves to yet another equilibrium (D) Figure 2.13 pp.188) at a higher CO largely compensating for the decreased cardiac function raising the CO to nearly normal.

Question 33

What is uncompensated heart failure?

Answer 33

If the heart failure become severe, volume expansion by the kidneys is insufficient to compensate for the decline in CO dictated by the downward shifting cardiac function curve. The new operating point in this severe case is at a very high CVP and low CO (Figure 2.14 pp. 190).