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.
Ref:
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