Constriction and Dilation of Blood Vessels..

[ravupadh]

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I'm a little confused here for which type of cases constriction and dilation are used for. So dilation of a blood vessel near an organ will deliver more blood/nutrients to that organ? If so what does constriction do?

And how is pressure affected by each of the two processes?

Also which process is controlled by the sympathetic nervous system and which one is controlled by the parasympathetic system?

Sorry for the barrage of questions lol.

 

[Funky]

I'm a little confused here for which type of cases constriction and dilation are used for. So dilation of a blood vessel near an organ will deliver more blood/nutrients to that organ? If so what does constriction do?

And how is pressure affected by each of the two processes?

Also which process is controlled by the sympathetic nervous system and which one is controlled by the parasympathetic system?

Sorry for the barrage of questions lol.

Constriction is the opposite of dilation. If you constrict the arterioles going to the organ then you get less flow of blood which means less nutrients and oxygen going to that area.

Pressure increases with constriction and dilation is the opposite. I forgot the formula with flow and pressure so look in your book for that.

Blood vessel vasoconstriction/dilation is controlled by a lot of different factors. Both parasympathetic and sympathetic nervous systems do both. Sympathetics would vasodilate your skeletal muscles but vasoconstrict the arterioles going to the gut. Parasympathetics would vasodilate your gut and i think it vasoconstricts the CNS.

Blood vessels are also controlled by autoregulation and local mediators like endothelin and nitric oxide. One system does not control blood vessels but a combination of things do.

 

[Axon hillock]

I'm a little confused here for which type of cases constriction and dilation are used for. So dilation of a blood vessel near an organ will deliver more blood/nutrients to that organ? If so what does constriction do?

And how is pressure affected by each of the two processes?

Also which process is controlled by the sympathetic nervous system and which one is controlled by the parasympathetic system?

Sorry for the barrage of questions lol.

I'm not understanding the first question. Constriction generally increases blood pressure, dilation decreases blood pressure. Sympathetic nervous system uses epinephrine to constrict blood vessels.

Edit: Funky answers your questions better.

 

[salim271]

To give you the math and equations behind dilation and constriction I've copied part of a post I made for a similar question:

vasoconstricted blood vessels are experiencing a higher blood pressure because just as you've increased the amount of water using ADH, by decreasing the radius of the blood vessel by say, half, you've essentially quadrupled (slightly less because blood is not an ideal fluid) the velocity of the blood, which behaves like any fluid. The mass flow rate will be increased by the same amount.

I = mass flow rate
p = density
Q = volume flow rate
A = Area
v = Velocity

I = pQ
Q = Av
I = (m/V)(Av)
A = πr^2
π = pi

Hence

I = (m x πr^2 x v)/V

The radius is directly proportional to I, and is inversely proportional to velocity. Its simple math that I'm not going to go through, but if you increase the radius, the velocity will decrease and vice versa. The mass flow rate will increase with increased radius, its easier to pump, and therefore the heart has to do less work/lower BP.

Hope that helps.

 

[indianjatt]

If i'm not mistaken, dilation has a larger area and this a smaller velocity, which would imply a higher pressure according to bernoulli's equation, but if there is a lot of resistance then it might not be true.....


I'm still not seeing why vasoconstriction results in higher pressure (does it not follow bernoulli's?)

 

[salim271]

If i'm not mistaken, dilation has a larger area and this a smaller velocity, which would imply a higher pressure according to bernoulli's equation, but if there is a lot of resistance then it might not be true.....


I'm still not seeing why vasoconstriction results in higher pressure (does it not follow bernoulli's?)

Bernoulli's is a restatement of the law of conservation of energy, what it solves for is some constant K for a certain fluid. If you had a lower velocity, a higher pressure would be needed to get that same constant K for the same fluid.

Looking at vasoconstriction and dilation, imagine that the pressure from the heart is constant (its not, but for simplicity's sake.) So when the radius/area of the blood vessel decreases, the flow rate and velocity increase based on the equations I've provided above.

Bernoulli's K constant isnt really a constant, its just a certain value that is gotten when pressure, density, and velocity are at certain values.

When the sympathetic nervous system increases vasoconstriction of BVs, at the same time the vagus nerve lays off its modulation of the heart's contractions and the SA node sends off action potentials to its fullest and contractions/pressure increase as well. This must be taken into account as well when you look at blood flow throughout the body.

 

[indianjatt]

Bernoulli's is a restatement of the law of conservation of energy, what it solves for is some constant K for a certain fluid. If you had a lower velocity, a higher pressure would be needed to get that same constant K for the same fluid.

Looking at vasoconstriction and dilation, imagine that the pressure from the heart is constant (its not, but for simplicity's sake.) So when the radius/area of the blood vessel decreases, the flow rate and velocity increase based on the equations I've provided above.

Bernoulli's K constant isnt really a constant, its just a certain value that is gotten when pressure, density, and velocity are at certain values.

When the sympathetic nervous system increases vasoconstriction of BVs, at the same time the vagus nerve lays off its modulation of the heart's contractions and the SA node sends off action potentials to its fullest and contractions/pressure increase as well. This must be taken into account as well when you look at blood flow throughout the body.

Thanks. I just looked online. Bernoulli's doesn't account for viscosity so it doesn't work in the case of the heart/vessels. It only works for laminar or stsady flow.

 

[Funky]

In terms of blood pressure it makes intuitive sense if you have a systemic vasoconstriction, you'll have higher arterial BP because there is more blood in the elastic arteries. The vasoconstriction happens in the muscular arteries. When you check blood pressure it's in an elastic artery.