Vasoconstriction, venoconstriction , and venous return

[lemontree52]

Hi, I'm a little confused on the (mathematical) relationships between vascular compliance & resistance and venous return.

I remember learning that venoconstriction causes an increase in VR, and it also makes sense to me conceptually - constricting the veins causes their compliance/capacitance to drop, and more blood is "forced" into the heart (there're valves and the blood can't go backwards).

And then there's this equation which is driving me crazy:
VR = (MSFP - CVP)/RVR
where MSFP = mean systemic filling pressure; CVP = central venous pressure or right atrial pressure; RVR = resistance to venous return.

It's pretty obvious that a decrease in compliance causes an increase in MSFP. But when you constrict the veins, venous resistance should also increase, wouldn't this oppose the increase in VR caused by the increase in MSFP?

I tried assuming that the increase in MSFP overrides the increase in RVR. But what about vasoconstriction of arterioles? I read somewhere that vasoconstriction of arterioles causes VR to decrease (because you're preventing the blood from flowing to the venous compartment from the arterioles), but according to this equation, both MSFP and RVR should also increase in the case of vasoconstriction? There's also this equation that I found in some papers: RVR = Rv + Ra/19, where Rv = venous resistance and Ra = arterial resistance. According to this logic, an increase in venous resistance should be 19 times more apparent than an equivalent increase in arterial resistance? How do I make sense of arteriolar vasoconstriction mathematically?

Also, in the case of sympathetic stimulation, both vaso and venoconstriction should occur, but why do we say that sympathetic stimulation increases venous return?

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[TheNsg300]

First, the mean systemic filling pressure is an "ideal" measurement of how "tightly filled" the venous circulatory system is and it's often approximated by the pressure required to stop the right atrium from filling. It is elevated in conditions that either increase circulatory blood volume (ex: blood transfusions) or increase venoconstriction (^^symp tone) . The formula you mentioned is a modification of Ohm's law where flow(VR) is directly proportional to pressure gradient (in this case MSFP only, since CVP is normally accepted to be Ommhg unless you have Right heart failure) and inversely to resistance to VR.

Second, during times of increased SNS and venoconstriction, there will be marked increment in the MSFP, forcing more blood back to the heart, but with only trace/minimal increment of venous resistance. This is because of the thin smooth muscle of venous circuit and it's high compliance contributing to its property of "stress relaxation" i.e as you stretch it more by constriction and subsequent increased flow, rather than having a myogenic reflex like the arteries to constrict, it relaxes and hence the venous return increment won't be compromised by an increased resistance.

Lastly, yes an increased SNS--^^VR, though it constricts both vascular beds. This serves the purpose of elevating MAP by trapping blood in the arterial tree and enhancing more flow to it. For the reason VR^^ despite arterial constriction, you got to remember at any time (t) normally 64% of the total blood volume is found in the venous system(hence the name capacitance vessels) with only 13% in the arterial system. So even if u prevent the arterial tree from emptying, you still remain with a huge reserve to ^VR by venoconstriction. Hope that settles your query!!

 

[Jabbed]

Pointing out the obvious, but looking at venous return curves only gives you half the story. You need to overlap the Starling curve and adjust it accordingly.