How does vasoconstriction reduce blood pressure in haemostasis?

Submitted by scoliendo t3_ycx8eo in askscience

I'm a nursing student, and I'm currently studying my second physiology unit. My materials all say that when a blood vessel is damaged, vasoconstriction occurs to reduce the pressure and flow of blood, so that the platelet plug is not dislodged. This mostly makes sense to me, however it's my understanding that vasoconstriction means the same amount of blood flows through a smaller opening, thereby INCREASING blood pressure. Is there a different mechanism in place during haemostasis? Is the constriction occurring proximally to the clot, reducing the volume of blood? Is the heart output of blood changed? I can see the heart rate increasing in major injuries, thus lowering blood pressure, but what about for minor injuries?

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CardiOMG t1_itpgls6 wrote

The blood can go through more than just that blood vessel. If you have 2 vessels in parallel and one gets injured and constricts, the resistance in that vessel will increase. In that way, more blood will flow through the other vessel and less blood will flow through the injured vessel.

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scoliendo OP t1_itpgzsr wrote

So are all capillary beds fed by more than one artery/arteriole network? Or are some capillary beds only fed by one? And if the latter is the case, what happens to that capillary bed when the blood flow is reduced?

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CardiOMG t1_itpsy4p wrote

The blood flow doesn’t have to go to 0, just reduced. But also, depending on the part of the body, you don’t need constant circulation. Your legs can go 6-8 hours with no circulation and still survive.

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penicilling t1_itpocmm wrote

>my understanding that vasoconstriction means the same amount of blood flows through a smaller opening, thereby INCREASING blood pressure.

No. Let's look at a simple model of circulation:

There is a certain amount of pressure in the arterial system. Overall, the pressure is determined by the cardiac output, the amount of blood, and the vascular tone. If you change any of these variables, then you change the pressure in the system. If you constrict ALL of the blood vessels, then you could increase the overall pressure.

But in an injury, there is localized vasoconstriction in the arterioles proximal to the injury. Let's hold the other variables the same in this simplified model.

The cross section of the arteriole proximal to the injury is reduced. The pressure of the system as a whole is unchanged. Thus the VOLUME OF BLOOD passing through the constricted arteriole is smaller, and the pressure AFTER THE CONSTRICTION is reduced.

One of the things that is important to remember is that, while we talk about the arterial system as having a specific pressure, the "blood pressure", it does not have one pressure! In fact, for blood to flow (or any liquid to move in a system of pipes), there must be a pressure differential! The pressure of blood in the thoracic aorta is higher than the pressure of the blood in the peripheral arteries, or the blood does not move. The system is constantly adjusting the resistance of the arterioles to manage how much blood is sent here or there depending on need.

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scoliendo OP t1_itpu5x5 wrote

So as I theorised, the vasoconstriction is occurring proximal to the injury, rather than at the site of the injury? This makes the most sense to me, which is why I theorised it, I just wish my course materials had explained this!

From what I understand based on this... blood is flowing at standard pressure until it reaches an area of vasoconstriction. Beyond this area is a damaged blood vessel. The constricted vessels allow less blood through per second, thus reducing the blood pressure in areas distal to this vasoconstriction, along the same artery/arteriole network, reducing the pressure on the damaged area. Is this accurate?

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Emily_Ge t1_itqneku wrote

It‘s like pinching a hose. Despite the pressure at the pinch point being higher, distal to the pinchinung the pressure is massively reduced, and only a tricky come out the end of the hose.

Just like you said at the end.

The part you missed is that the injury site itself, keeps the same diameter. To any reduction in diameter proximal to the injury reduces the blood flow. Which reduces the pressure at the site of injury.

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ExtraVacation t1_itq4ejc wrote

Consider the inflamatory process. Lets say a nail punctured your skin- there will be a brief vasoconstriction, then mast cells degranulate releasing histammine, prostaglandins, etc. Histamine is a major player in vasodiolation with the purpose of increasing blood flow to that LOCALIZED area-- with this increased flow includes clotting factors too. So widespread vasocinstriction would indeed raise blood presure, but when localized to capilary bed wouldn't have any meaningful impact on BP.

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nmb-ntz t1_itqhuvu wrote

It's only local. Think of a garden hose. If you squeeze the tip, water will squirt much further. If you squeeze it a meter up the hose, water will drizzle out. In the latter situation, pressure at the opening drops, but it increases before the pinching point. Vasoconstriction works the same way. There is less pressure on the opening as not to dislodge the platelet plug. But opposed to a garden hose the pressure before the constricted point dissipates through the rest of the system.

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mironoprea t1_itr43ge wrote

Some fluid dynamics in addition to some of the other answers: Locally, the law of Laplace states that parietal tension is proportional to the pressure and to the radius, meaning as the radius decreases, the parietal tension also decreases.

Also, as per Bernoullis principle, this decrease in radius doesn't increase blood pressure. Velocity is increased, but pressure decreases, as the total energy of the blood in the vessel leading up to the narrowing has to be equal to its energy as it passes through the narrowing.

This also explains the increased risk of aneurysms rupturing. Their increased diameter leads to higher, not lower, pressure.

Moreover, this works locally, regardless of parallel blood vessels: a dilated ventricle leads to increased parietal tension not by increased blood flow from parallel blood vessels - there are none - but from the same quantity of blood passing through.

Edit: typo

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Ape-Ostate t1_itrff2x wrote

True, plus its a closed system so both principles work without adjustments for pressure differentials across input/output flow rates needed..

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auraseer t1_itualil wrote

Vasoconstriction shrinks the size of blood vessels.

It does not change the amount of blood inside your body.

If you make a container smaller, but keep the same amount of fluid inside, the walls of the container press harder against that fluid. That is the same as saying the pressure goes up.

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scoliendo OP t1_ituatr8 wrote

That's what I thought, but apparently when a blood vessel is injured, the vessel constricts in order to reduce blood pressure, which is the opposite of my understanding. Hence the question.

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auraseer t1_itubsvr wrote

It reduces blood pressure very locally at the site of the injury, increasing it elsewhere.

Imagine you have a cut on your hand. In response, the arteries in your wrist constrict. That constriction means they become narrower pipes, which means blood cannot flow through them as easily, which in turn means there is less blood inside your hand. Therefore, the blood pressure specifically in your hand decreases.

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scoliendo OP t1_ituc714 wrote

So as I hypothesised, the vasoconstriction is occurring proximally to the injury in order to reduce blood flow and hence blood pressure at the site of the injury. Thank you! I just wish my texts had made this more clear - they all state it as though the vasoconstriction is happening AT THE INJURY SITE.

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auraseer t1_itud5ph wrote

Now that you understand that part, I'll admit I was oversimplifying slightly.

Vasoconstriction is happening all around the region of injury. It's not really affecting just that big artery in the wrist. It's affecting all the little arteries and arterioles near the wound.

Constriction proximal to the injury will reduce blood flow to it, no matter how far proximal. It will work even if it's just happening in the arterioles a few centimeters or millimeters away from the injury.

That's what they mean by "at the injury site."

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