How can vasoconstriction increase blood pressure yet decrease blood flow?

[September24]

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In my mind, if there is a lot of blood pressure, the blood is propelled towards a certain area with force.

When there is vasoconstricton, blood pressure increases due to reduced cross sectional area (thus reduced volume). However, according to the pouiselli's law, the the reduced radius should decrease flow. However, how does flow and pressure differ? What does this mean for the body? When do we need more flow and when do we need more pressure?

Quick bonus question: If area goes down, velocity should go up right? According to bernoullis, shouldn't pressure go down? I assume this is because this is for ideal fluid with laminar flow and blood is not an ideal fluid.

 

[bcurtis]

The radius is not exactly decreased though, my EMT-B instructor explained it like this: imagine you have a water circuit with pipes and a pump. When everything is normal there is a set pressure in the pipes. If you expand the pipes (vasodilation) you lower the volume in the circuit which could lead to hypovolemic shock. On the other hand if you constrict the pipes (vasoconstriction) you increase the volume at a given point and this pressure in that point goes up, usually as anaphylaxis. I'm not to sure about the second one though.

 

[Synapsis]

You could actually think of blood pressure in a similar way as in physics. P = F/A. Now I know there's specifics for that, like the force has to be perpendicular, but this is just a functional way to think about it that's pretty useful. Area goes down (like during a vasoconstriction), pressure goes up. Area goes up, pressure goes down, like in a vasodilation. When smooth muscles contract, area goes down.

Blood flow usually refers to how much blood is entering a region of the body. One area where this is important is in a process called active hyperemia. In active hyperemia, there has been a lot of body cell activity in a certain region of the body. So blood vessels will tend to vasodilate so that the blood flow to the region increases. You want this to happen so that you can get supplies (glucose, for example) to those body cells so that they can make more ATP.