blood pressure confusion

[ankit1ag]

Hello,

I just got confused about a mcat question. I always thought that blood pressure would increase with vasoconstriction however when I relate this with fluid flow in a pipe, the smaller the diameter, the greater the velocity. Greater velocity corresponds to lower pressure.



Please clarify. Thanks,

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[mc4435]

When they say blood pressure increases with vasoconstriction, they mean total blood pressure in the body, not the blood pressure in those specific vasoconstricted vessels.

 

[Idiopathic]

Blood pressure is composed of a systolic (pump) force and a diastolic (rest) force. If you vasoconstrict, you increase the diastolic component and flow would indeed slow, however, the heart works harder, generating a higher systolic pressure to overcome this. This is why the heart can become enlarged with chronic long term hypertension. Also, the flow/velocity for a pressure of 120/60 is likely higher than 150/100, even though the mean pressure and all components are higher in the latter. The pressure gradient, which is the flow determinant, is less.

Another corollary is in early sepsis, where you have massive vessel dilation, but preserved systolic flow (initially, at least), and you can have pressures something like 150/40, with massive cardiac outputs, due to the lowering of the diastolic pressure. Pressure is indeed less, and until the system corrects (typically by lowering systolic pressure) your velocity will be increased.

 

[trustforward]

Hello,

I just got confused about a mcat question. I always thought that blood pressure would increase with vasoconstriction however when I relate this with fluid flow in a pipe, the smaller the diameter, the greater the velocity. Greater velocity corresponds to lower pressure.



Please clarify. Thanks,

hey, I attempted to answer this in my own words, but think i ended up confusing myself, so here is a response I found to the same Q from a fellow SDNer:

"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 SNS 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."

 

[gdawg102]

Hello,

I just got confused about a mcat question. I always thought that blood pressure would increase with vasoconstriction however when I relate this with fluid flow in a pipe, the smaller the diameter, the greater the velocity. Greater velocity corresponds to lower pressure.



Please clarify. Thanks,

I'm assuming you are referring to the effects of velocity through cappillaries from arteries to the vein side on BP....

Velocity = rate of fluid flow/Area of pipe

Although capillaries seem to have a smaller radius and thus smaller area than the aorta and arteries---> you would think the velocity of cappilaries would be higher.

However, the TOTAL CROSS SECTIONAL AREA of a capillaries is larger than the aorta. Thus the velocity of blood through cappilaries is actually slower (which makes sense since there needs to be time for nutrient exchange with tissues). Hence, vasoconstriction does result in raised BP because velocity through the cappilaries are slower, and thus decreasing rate of blood flowing to the venule side in comparison to blood flowing from the venule side to the aorta.

 

[ParvatiP]

I think it depends what system you are talking about...vasoconstriction is a sympathetic response to low blood pressure because it increases peripheral resistance, thus increasing OVERALL blood pressure.

However if you're looking at the local area, I think pressure decreases. For example, in blood clotting, one of the first responses of the body is to produce paracrines that cause vasoconstriction, decreasing the blood pressure through the vessel and thus decreasing blood flow through the vessel. This will minimize blood loss and allow the clot to form while blood is still flowing.

It's kind of confusing, but I think those 2 examples illustrate the difference between overall and local bp.

 

[lifeistough]

Hello,

I just got confused about a mcat question. I always thought that blood pressure would increase with vasoconstriction however when I relate this with fluid flow in a pipe, the smaller the diameter, the greater the velocity. Greater velocity corresponds to lower pressure.



Please clarify. Thanks,

What you wrote above is correct, to the best of my knowledge. Are you asking for someone to explain why that is so? I am not clear on what is being asked. If it is just clarification to why it is, last part can be explained with Bernoulli's eqn, the middle part with AV=constant, and first part when you constrict flow to an artery the pressure increases, under Poseuille's law constricting an artery by 1/2, in order for eqn to be constant, the pressure must increase by 16 times! The equation has P=something/r^4. Pressure in this case also referred to resistance.