Physics behind blood vessel constriction and blood pressure?

[scilearner]

First of all I have to say I have really confused myself on this one by overthinking and I'll ask my questions step by step. Any help would be greatly appreciated.

 

General rule is when blood vessels constrict blood pressure increases.

 

1. Ok there is a container and gas molecules in it. If I decrease the volume of the container the pressure increases. This analogy can not be used for blood vessels right? I mean it is a liquid and has a blood flow.

2. It is said that when blood vessels constrict blood pressure increases. Blood pressure means the pressure at which the heart pumps the blood right? So when they say an increase do they mean an increase in blood pressure overall(increase in heartbeat) or just at the place where blood vessel was constricted?

3.Why does blood pressure increase when blood vessels constrict? When a blood vessel constrict, obviously more blood hits the walls and there is higher pressure, but I'm thinking this higher pressure is countered by the increased resistance, meaning at the end increased resistance decreases pressure. However sources say otherwise. Is the reason behind this is that sensory neurones tell heart to increase blood pressure? If heart did not have sensory neurones and was unable to change its pumping rate willl the constriction of blood vessel decrease pressure?

4. Now how does this all apply when you consider heart as a network of vessels. Does it work like a circuit? If you have two branches and if one branch was dilated, more blood would follow the dilated branch right (path of least resistance), in this case how is there a more blood flow to that branch if heart decreases pressure. I mean isn't more blood countered by low pressure so no change.

 

Thank you so much for anyone who is going to attempt to answer these for me.

Edited January 4, 2010 by scilearner

[Mokele]

Think of it like this:

 

You have a pump, and it moves fluid through a long tube which circles back to the pump. It does this by generating pressure to push the fluid through. But the resistance of the pipes saps that pressure. If the pipe is too long, or too narrow, there won't be enough pressure to move the fluid all the way through the system, and it'll fail. You can fix this by increasing the pressure of the pump. So when your blood vessels constrict, there's more resistance, and the heart needs to generate more pressure to overcome this resistance.

 

When considering forking, you're right about the path of least resistance, but you're forgetting that blood vessels are all normally relaxed. Imagine our pump system has a fork in it, both paths of which lead back to the pump. Each path has a diameter of X. If one path contracts to 1/2 X, then blood will preferentially flow through the unconstricted pipe. However, the total area available for blood flow has decreased - you used to have two pipes of diameter X, now you have one pipe of that size and one pipe of 1/2 X, so there's less total pipe space for the blood.

[Rickdog]

One little detail to consider, permeability of the blood vessels. Their behaviour, also should consider this aspect, since if it is bigger due to a pathological (disease or traumatic) condition, if they constrict, blood pressure inside of it will remain steady and all you`ll have is the loose of different elements of the blood towards the external tissue, without the effect on the blood pressure inside the vessel.

[Mokele]

Actually, that's not the case - permeability is pretty much nil outside of the capilaries, which includes the arteries with muscular walls. Basically, if there's smooth muscle inside that can contract, it's walls are too thick for it to act as anything other than a pipe.

[Rickdog]

permeability is pretty much nil outside of the capilaries,

 

And Capillaries aren´t blood vessels ?

 

Also there is internal permeability in other blood vessels, such as venous valves who regulate the return of blood towards the heart in peripheral veins. In case of the "venous syndrome", there is is a disfunction of these valves, which causes an increase in total blood pressure, due to the stasis of blood in peripheral regions (usually in the legs, in case of humans).

Edited January 17, 2010 by Rickdog

[Mokele]

Capilaries don't contract - they have no smooth muscle lining.

[Rickdog]

Very true, my friend (my error in not pointing it out correctly on my first post). So they aren´t considered here, only Arterial and Venous systems, blood vessels are. ( to not get misunderstood, when I said that different elements will stay in the surrounding tissue, it´s not refered to the main vessel(vein) but to the area from where it comes). In the case of the Venous Syndrome, edema (tissue with infiltrated liquid) will get formed in the lower limbs, due to the failure of the venous valves.

Edited January 17, 2010 by Rickdog