Compliance and preload

[jds112]

BRS (6th ed, pg 69) gives the formula C = V/P. Compliance is directly proportional to Volume and inversely proportional to Pressure.

Then, on page 80, when talking about ventricular pressure/volume loops, it says an increase in preload "refers to an increase in end-diastolic volume and is the result of increased venous return (e.g., increased blood volume or decreased venous capacitance)". Capacitance=compliance.

I understand the increase in EDV/VR causing an increase in preload. However, If compliance is proportional to volume, why would a decrease in compliance cause an increase in preload?

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[dorster]

well i think they are referring to a decrease in venous compliance.. meaning the veins are more constricted, since less blood is pooling in the veins more blood is returning to the right atrium which increases preload. If they were talking about a decrease in compliance of the right atrium the statement wouldn't make sense. I think you have to focus on the location that they are referring to. C=V/P but Compliance in the "veins" does not equal Volume in the right atrium/ pressure in the right atrium.

 

[zhopv10]

Ya so compliance=change in volume/ change in pressure so if you decrease the compliance of the veins you could see that as being a decrease in the volume (assuming static pressure) this would then send more blood to the heart=increase preload since less blood is pooling in the veins. Like dorster said the equation holds in the location where you are assassin it from


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