veins...blood pressure

[IntelInside]

i know from the continuity equation that flow rate is constant and is equal to AxV....So i was wondering why is it that blood pressure is lower in the veins than in the capillaries according to a diagram in EK bio review (pg 143). I thought that blood pressure would be lowest in the capillaries, which they also state..

---

Members don't see this ad.

 

[linkin06]

it's lowest in veins because of the overall resistance/friction from blood flowing throughout the body.

bonus:
blood pressure higher at feet than head because of gravity -- i think i've seen that on a couple aamcs, so useful tidbit

 

[engineeredout]

bonus:
blood pressure higher at feet than head because of gravity -- i think i've seen that on a couple aamcs, so useful tidbit

More bonus: This is why you can get dizzy if you stand up too fast. Orthostatic hypotension resulting from decreased venous return because the blood is pooled in your lower extremities.

 

[IntelInside]

it's lowest in veins because of the overall resistance/friction from blood flowing throughout the body.

but isnt the cross-sectional area of veins about four times that of arteries.....so how is there more resistance. the bigger teh area the less resistance you have. EK also says that blood pressure increases near the heart

Oh and ty for that useful tidbit on BP in the feet

 

[engineeredout]

but isnt the cross-sectional area of veins about four times that of arteries.....so how is there more resistance. the bigger teh area the less resistance you have. EK also says that blood pressure increases near the heart

Oh and ty for that useful tidbit on BP in the feet

There isn't. Its lower resistance.

 

[IntelInside]

There isn't. Its lower resistance.

wow, major brain fart, i just looked out my notes and read it wrong.....ty for bearing with me

 

[linkin06]

it is lower resistance, but it's been slowed down by the resistance beforehand

 

[Charles_Carmichael]

A big reason why the pressure is lower is because veins have a lot of compliance. Compliance = change in volume/change in pressure. Veins have a huge compliance; they distend easily when filled with blood. They can fill up with a lot of blood and still have a low pressure. Remember that the biggest determinant of blood pressure is blood volume. In the case of veins, they can handle bigger volumes with lower pressure due to their high compliance. Hope this helps.

 

[engineeredout]

A big reason why the pressure is lower is because veins have a lot of compliance. Compliance = change in volume/change in pressure. Veins have a huge compliance; they distend easily when filled with blood. They can fill up with a lot of blood and still have a low pressure. Remember that the biggest determinant of blood pressure is blood volume. In the case of veins, they can handle bigger volumes with lower pressure due to their high compliance. Hope this helps.

But keep in mind that most of the physiological control over TPR (vasodilation/constriction) is on the arteriolar side

 

[linkin06]

But keep in mind that most of the physiological control over TPR (vasodilation/constriction) is on the arteriolar side

yes, vein pumping is passive and depends on the overall movement of leg to pump it back to the heart w/ valves to prevent backflow. that's why you shouldn't sit still for 5 hrs as you study for mcat! move around.

 

[Charles_Carmichael]

But keep in mind that most of the physiological control over TPR (vasodilation/constriction) is on the arteriolar side

I realize that, but I'm confused as to why you brought it up regarding my post. I didn't mention anything about TPR or its regulation. Veins can also be constricted/dilated via sympathetic activity, etc. This should affect the pressures in the arterial system as well since veins store blood and venoconstriction would shift blood from the venous system to the arterial system, which increases arterial blood volume, which affects arterial pressure. But you're right about the most resistance being in the arterioles.

Regarding the compliance of veins, what you have to realize is that cardiac output, venoconstriction, etc. all affect either compliance or volume and thus, affect the venous pressure. That's what I meant when I posted previously; I guess I wasn't clear about it.

For example, venous constriction via sympathetic activity decreases the compliance of veins. Based on the equation C = dV/dP, you can see that this decrease in compliance causes an increase in pressure.

The muscle pump, venodilation/constriction, cardiac output, standing up, etc. all affect compliance and/or volume in some way and this, in turn, affects the pressure. Hope this helps.

 

[engineeredout]

I wasn't looking to contradict you, just adding more information in general.

 

[Charles_Carmichael]

I wasn't looking to contradict you, just adding more information in general.

Hey, that's what I thought but the "but" threw me off a bit heh. Anyways, hope I didn't come off as attacking you or something; I was just confused about it, is all. Thanks for clarifying it!