Tuesday, February 21, 2012

TOPIC DISCUSSION: "effective blood volume"

Estimates of blood volume distribution indicates that 85% of blood circulates on the low pressures( venous side) and 15% on the high pressure ( arterial side).  The "effective" arterial volume is the arterial side and the high -pressure circulation.  The major determinant of that is cardiac output and vasodilatation of the vessels. For example, in cardiac heart failure, there is decreased cardiac output leading to decreased forward flow and hence low effective arterial volume. The concept is referring to the over and under filling of the heart and how that leads to volume of the high pressure circulation.
This concept is easy to understand when you have  volume depletion and the blood pressure is low and there is orthostatics leading to effective low blood pressure.  What is decreased effective blood volume in edematous disorders like cirrhosis and CHF? 

What is "effective blood volume"? Cardiac output was initially thought to be the main mediator of this effective blood volume and this makes sense. But if the normal kidney is uniformly responding to decrease in cardiac output in edematous states, then there is a problem as there would be worsening water and Na retention. 
So given the above thoughts, increase in total blood volume could occur, even if there is underfilling of the arterial circulation given most of the blood is in the venous circulation( edematous state).  Underfilling is secondary to decrease cardiac output or increase vasodilatation or diminished splanchnic vascular resistance.

So really, one should think of this as more of “decreased effective arterial blood volume” that drives the flow to the kidney and or “arterial underilling” rather than “effective total blood volume” especially in edematous states.
Dr Schrier describes this concept elegantly in a manuscript in JASN from 2007.

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