penicilling t1_itpocmm wrote on October 25, 2022 at 12:10 PM

>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.

scoliendo OP t1_itpu5x5 wrote on October 25, 2022 at 1:01 PM

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?

Emily_Ge t1_itqneku wrote on October 25, 2022 at 4:23 PM

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.