***Contradiction between SNS and Bernoulli's????

[Dr. Gigglez]

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this has been bugging me for the past 3 months...

check this out:

the bernoulli effect states that pressure is lower where flow speed is higher right?

so, how do you reconcile this with the sympathetic nervous system response which raises blood pressure AND vasoconstricts in the same time??

if vasoconstriction occurs, then according to Av=Av, the cross sectional area would increase, and the flow speed would increase => lower pressure?!?!

help?!?! 😕

 

[estairella]

Bernoulli's equation applies only to ideal fluid flow.

Blood flow is definitely NOT ideal. Intuitively, you know that the blood coming out of the aorta is going to be the fastest blood. However, according to Av = Av, the flow in the capillaries (which have the smallest cross-sectional area) should be the fastest.. yet, also intuitively, you know that capillary flow is the slowest.

So.. just don't use ideal fluid concepts for blood flow.

 

[luckycharms08]

you can use it, but know that when they say area, it's total cross sectional area, not just of one blood vessel. capillaries have the largest cross sectional area (total) so it flows the slowest there.

 

[stiffany]

Hmm, so now I'm confused. If we use the continuity equation, a smaller area will experience a faster flow. However, we define pressure as force/area. So, if we have a smaller area, wouldn't we have a larger pressure as a result??? Thus, increased pressure would coorespond to a faster flow?

 

[redsoxfan]

try to think of it this way:

in terms of DIAMETER, arteries are larger than veins, and thus undergo higher pressures. in terms of AREA, capillaries have the largest surface area and thus have the lowest pressure.

as far as symp. induced vasoconstriction leading to higher BP, there is a pressure buildup BEHIND the area being constricted. also, try and think of an artery about to rupture--it's alot bigger in diameter than a normal artery, and thus is under greater pressure.

i've also been very confused about these concepts and appropr. use of formulas. i've concluded that it is best to use P=F/A when you are actually talking about a force, ie a piston on an enclosed container of gas or something.

use bernouli's when relating pressure to speed and diameter. this seems to apply more to the human body.

hope this helps...