Maybe this can help you... It helped me

http://www.usmle-forums.com/usmle-step-1-bits-pieces/295-cardiac-output-venous-return-curves.html

http://www.usmle-forums.com/usmle-step-1-forum/5831-cardiac-output-tpr-relationship.html

 

Yea that didn't help.

 

....bump

 

Got it??? Are you still confused???

 

Perhaps this might help you understand?

 

most probably after a acute fistula ,body got more venous return and more cardiac output..so more blood is accumulating in the body in normal venous pull.that explains the increase in blood volume.to pump out more blood sympathetic system increase contractility and vessle tone.so more blood is pump out of the vein.

 

Under normal conditions, Blood will flow from arteries -->arterioles--->capillaries ---> venules---> veins.

Arterioles are resistance vessels. They provide resistance.

In AV fistula, there is a fistula (direct connection between arteries and veins), that means blood flows directly from arteries into veins bypassing resistance.So blood will flow easily.

Blood is under high pressure in arteries as compared to that in veins. So blood will flow from artery to vein.

Now we have more blood in veins (veins store blood) --> Increase in venous return but cardiac contractility isn't increased yet (Its acute). So we are not pumping out all the blood out of the heart. After a while, this problem will become chronic.

There will be chronic decrease in perfusion to organs, specially kidneys. So, Kidneys will activate Renin angio. aldo. system. This will increase Blood volume. Angio II will increase vascular tone.

Also, decreased blood flow will decrease firing of baroreceptors which will increase sympathetic tone. This will increase contractility.

 

nice explanation.Thanks.
so first there is a systolic failure of the heart.so in response to that kidney and sympathetic sysmtem compensate.it is nothing but a systolic heart failure situation??