Step I what am I missing here??

[artsydoc]

I just read in BRS cardio phys section that because of autoregulation of circulation if perfusion pressure of the heart suddenly decreased compensatory vasodilation would occur in the arterioles to maintain constant flow. I thought that would have caused sympathetic activation of alpha adrenergic receptors resulting in vasoconstriction to increase resistance and maintain pressure. Can someone explain please?

 

[imnestea]

I think you are confusing 2 different scenarios. The main point is that you want to maintain blood flow to critical organs (brain and heart).

1) Drop in SYSTEMIC BP (i.e. some sort of acute blood loss) will activate your baroreceptors ultimately leading to increase CO and increase SVR via the mechanism you mentioned. BP = CO x SVR. These baroreceptors are found in the carotid artery (near the bifurcation if I remember correctly) and aortic arch. Maintenance of BP will ensure adequate perfusion to vital organs such as brain and heart.

However, lets say we have a drop in blood flow to one of these vital organs. More specifically, lets say one of the coronary arteries is being blocked by an atherosclerotic plaque. We do not refer to this as a drop in "blood pressure" to the area that is no longer getting blood. Instead, we use the term "perfusion" (in this case, cardiac perfusion).

We are faced with 2 issues. First, we do not have baroreceptors to sense this drop in perfusion located in the myocardium so we must rely on another mechanism of regulation. Second, our goal is no longer to shunt blood from periphery to core but rather to reperfuse the potentially ischemic area (called "penumbra"). Both of these issues can be addressed by the blood vessel's ability of myogenic autoregulation. If a decrease in perfusion is detected, the blood vessel will automatically dilate to ensure there is adequate blood flow. If you want to look at it from an equation standpoint, blood flow = perfusion pressure / vascular resistance (we want to decrease vascular resistance here to increase blood flow and subsequently perfusion pressure)

 

[artsydoc]

Thank you for that beautiful explanation! It's perfectly clear now and yes, I did in fact confuse system pressure and organ perfusion. What a relief.