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Old 01-09-2011
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Heart Cardiac Output and TPR relationship?

I've never been very good at physiology probably because I was never good at physics in school.
Ok so generally if you have some problem and cardiac output falls >> effective arterial blood volume will fall. This will trigger the baroreceptor system and lead to vasoconstriction of peripheral arterioles (increasing DBP), venoconstriction (increasing venous return to heart), increasing contractility etc. So all these mechanisms are intended to increase blood flowing back to the heart to make it work better.

However my question is if you're constricting peripheral arterioles, this increases TPR and surely increasing TPR increases afterload and this means the heart has to push harder. Surely this is counterproductive???

Thanks in advance.
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Old 01-09-2011
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Cardiac and Vascular Function Curves:

These are simultaneous plots of cardiac output and venous return as a function of end diastolic volume (or right atrial pressure.)



Cardiac Function Curve : This is simply the Frank-Starling curve for the ventricle showing the relationship of cardiac output as a function of end diastolic volume.

Venous Return Curve : This is the relationship between blood flow in the vascular system (venous return) and right atrial pressure.

Mean Systemic Pressure : This is the point where the venous return curve intersects the X axis. The mean systemic pressure reflects the right atrial pressure when there is ‘no flow’ in the system. At this point the pressure is equal throughout the circulatory system.

Equilibrium: This is the steady-state where the two curves intersect; it reflects the point where cardiac output is equal to venous return.

Total Peripheral Resistance Changes :

TPR is determined by the resistance of the arterioles. Changes in TPR will change the slope of both the cardiac function curve and the venous return curve.



An increase in TPR (shown above) will cause blood to be retained on the arterial side of circulation and will increase the aortic pressure against which the heart must pump. This will act to shift both slopes downward. As a result of this simultaneous change, both the cardiac output and the venous return are decreased, however the right atrial pressure remains the same .


A decrease in TPR (not shown) will allow more blood to flow to the venous side of circulation and will lower the aortic pressure against which the heart must pump. This will shift both slopes upward. Both cardiac output and venous return will be simultaneously increased; again, right atrial pressure will remain the same .

Hope this help you...
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Arrow The heart pushing harder is a good thing

Quote:
Originally Posted by prim View Post
However my question is if you're constricting peripheral arterioles, this increases TPR and surely increasing TPR increases afterload and this means the heart has to push harder. Surely this is counterproductive???
Making the heart pushing harder is not counterproductive. It's useful!
The heart pushing harder means increased pressure and that's what we need in acute flight and fight (sympathetic response).
In the example you mentioned, you lost cardiac output for a reason or another. You are not going to bring back blood (unless you hook up the patient on IV line!) so the body has to deal with the new reduced amount of blood that it has right now. So to keep the flow (Cardiac Output) it has to increase the pressure and in order to do that it has to increase the TPR.

Cardiac Output = Pressure / TPR, you increased the TPR, which then forced the pressure to increase to maintain the same cardiac output.

Hopefully, I got to your point
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Old 01-10-2011
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if it helps, reduced eabv+less pressure= less perfusion

but, reduced eabv + more pressure = better perfusion in emergency state.

this is just a key to remember the "thing" real quick.

Dulbee said the basic.
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Old 08-24-2012
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Quote:
Originally Posted by aktorque View Post
Cardiac and Vascular Function Curves:

These are simultaneous plots of cardiac output and venous return as a function of end diastolic volume (or right atrial pressure.)



Cardiac Function Curve : This is simply the Frank-Starling curve for the ventricle showing the relationship of cardiac output as a function of end diastolic volume.

Venous Return Curve : This is the relationship between blood flow in the vascular system (venous return) and right atrial pressure.

Mean Systemic Pressure : This is the point where the venous return curve intersects the X axis. The mean systemic pressure reflects the right atrial pressure when there is ‘no flow’ in the system. At this point the pressure is equal throughout the circulatory system.

Equilibrium: This is the steady-state where the two curves intersect; it reflects the point where cardiac output is equal to venous return.

Total Peripheral Resistance Changes :

TPR is determined by the resistance of the arterioles. Changes in TPR will change the slope of both the cardiac function curve and the venous return curve.



An increase in TPR (shown above) will cause blood to be retained on the arterial side of circulation and will increase the aortic pressure against which the heart must pump. This will act to shift both slopes downward. As a result of this simultaneous change, both the cardiac output and the venous return are decreased, however the right atrial pressure remains the same .


A decrease in TPR (not shown) will allow more blood to flow to the venous side of circulation and will lower the aortic pressure against which the heart must pump. This will shift both slopes upward. Both cardiac output and venous return will be simultaneously increased; again, right atrial pressure will remain the same .

Hope this help you...


Thanks a lot.
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