Why does vasoconstriction lead to better blood flow?



I keep reading, as a science student, that when blood flow or pressure goes down, arterioles constrict in order to raise blood pressure. But vasoconstriction actually increases resistance, thereby depriving the tissue further of blood flow.

So what is it that I’m not understanding?

In: Biology

Vasoconstriction does not increase blood *flow*. It increases blood *pressure*. If all things were equal, that would normally increase blood flow. But as you note, making the blood vessels smaller increases the resistance to flow.

If you grab a hose or piece of tubing with liquid flowing through it and squeeze, the upstream pressure rises due to the resistance you’ve added. But the rate of flow goes down.

You ever put your thumb in a hose while it’s on? I think that may have something to do with it.

Yes it raises resistance. But why? When the vessels constrict their internal volume decreases. It takes less blood to fill up all the vessels. Someone might be hypovolemic and their body vasoconstricting helps compensate for the lower blood volume.

The circulatory system comprises of various types of vessels with varying sized and functions. Arterioles are a type of vessel that are at the end of the arterial system but before capillaries that’s perfume into tissues. The arterioles contain the valves that do much of the constricting to create a backup in pressure from the heart pumping thus increasing pressure. While increasing resistance may up front sound like it would impede blood from reaching the tissue reframe the perspective as it’s constricting to maintain a balanced blood pressure that is neither too low nor too high. It’s goal is to not completely block flow by constricting. In a healthy individual with properly functioning systems this resistance doesn’t increase to a point that blocks off tissues or capillaries. The diameter of the vessels simply adjust to create a radius that produces adequate flow. Poiseuille’s law can be applied to this situation where radius and pressure are related. If the pressure is low the radius can be decreased to increase pressure and visa versa.

Constricting does not improve flow, it does improve the pressure gradient though. The alternative is to stay dilated and further deprive the tissues of needed oxygen. Constricting the vessels allows the hydrostatic pressure to stay close to the same as before the drop in BP occurred. Further constriction also diverts blood away from less-important tissues like fingers and toes and allows it to be sent to the core organs and brain easier. Also, check out laminar flow and turbulent flow in relation to blood flow.

Okay, so let’s think about this like we would a pipe. Normally, the pump is able to feed enough water into the pipe for the pressure and output on the other end to be reasonable for the pipe size. But let’s say there’s suddenly not a lot of water coming into the pump, and it’s going to be decades before the original flow rate is returned. Now all of a sudden, the pipe acts like a big resevoir rather than a pipe, and it screws with the output’s pressure, making it vary wildly. That’s gonna result in a lot of pissed citizens. So what’s the sane thing to do? Quickly switch out for smaller pipes. Now water flows roughly as it did before, with roughly the same pressure, just with a lower output. Yes, people are still going to be a bit pissed, but there’s not really anything you can do about the output volume. Because of the way water works, the output rate of a pipe system is the same as the input rate (unless something fucks up like the above). You just have to wait for the old input rate to come back.

Now let’s compare to the cardiovascular system. You’ve lost a good deal of blood. You’re not dead, or close to being dead, but you do feel a bit woozy. The key here is the CONSISTENCY of the input/output rate, just like above. So the arterioles constrict, shrinking the pipes of your system. It has restored normal pressure to the system, and now it can do its job of pushing blood along, even if it’s a smaller amount of blood than before. It may not be the IDEAL blood flow of earlier, but at least it’s a CONSISTENT blood flow, and that counts for a lot.

Does that make sense?

It has to do with the areas that it’s happening. Vasoconstriction in one area – say, a lacerated hand – would increase blood pressure and blood flow in the rest of the body. Just like anything else in the body, vasodilation and Vasoconstriction are not uniform throughout the body.

From what little I know, Vasoconstriction would usually occur in an injured area and also tends to occur in extremities in the cold. While this would reduce the blood flow in that area, it would increase blood pressure and blood flow in other areas of the body so that it doesn’t suffer from a sudden pressure drop that the heart cannot keep up with.

So, in a way, it does both. It decreases blood flow in the area that is constricting; however, it raises pressure and flow elsewhere in the body.