vasoconstriction & vasodilation...

[strangelight]

Ok, I figured everyone is asking last minute questions to clear up a few concepts, so I thought I should add on to it...

So I know this is stupid, but I keep getting confused about vasoconstriction/dilation: when it happens, whether pressure or blood flow increases/decreases, etc. The only thing I am clear cut on is that your body restricts blood flow when you're cold to retain heat, & increases blood flow to cool you down when you're hot, right? (Why though?) I don't even want to try explaining the rest b/c I might confuse someone...somebody wanna help?

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[OHare]

Vasoconstriction increased BP because you have less space for the blood, thus pressure goes up. Vasodilation occurs when you want to increase bloodflow to a certain tissue (or want to decrease blood pressure). This differs from laminar hydraulics where larger cross sectional area=larger pressure.

 

[katnapper]

It always helps me to think of pictures. But that's just me. The reason that your body turns red when you're hot is because there is MORE blood flow to that area which would mean you would be vasodialated. Dialation allows for more blood flow.

If your feet are blue in the winter, you may be vasoconstricted because your body is working to keep your core temperature up, important organs need the warmth and the blood more than your extremities do.

 

[strangelight]

Vasoconstriction increased BP because you have less space for the blood, thus pressure goes up. Vasodilation occurs when you want to increase bloodflow to a certain tissue (or want to decrease blood pressure). This differs from laminar hydraulics where larger cross sectional area=larger pressure.

Aha, I think that's why I was confused b/c I knew in physics, for example, smaller area = increased velocity = less pressure, so I didn't understand why vasoconstriction = decreased blood flow = more pressure...

Oh that is a good question, why ARE they different?

 

[strangelight]

It always helps me to think of pictures. But that's just me. The reason that your body turns red when you're hot is because there is MORE blood flow to that area which would mean you would be vasodialated. Dialation allows for more blood flow.

If your feet are blue in the winter, you may be vasoconstricted because your body is working to keep your core temperature up, important organs need the warmth and the blood more than your extremities do.

Oh yeah, I think of pictures too, heh. Thanks for that though...it added more aspects to my visuals.

 

[MDtoBe777]

Aha, I think that's why I was confused b/c I knew in physics, for example, smaller area = increased velocity = less pressure, so I didn't understand why vasoconstriction = decreased blood flow = more pressure...

When you vasoconstrict you are esentially forcing your heart to pump harder (blood pressure). This is why the pressure is increasing. Yeah, it is kind of counterintuitive when you think about Bernoulli's equation and all that stuff from physics. Also remember when you vasoconstrict you will be increasing velocity (versus in the capillaries where there is barely any resistance and a large cross-sectional area...this is why the velocity is so low here).

 

[VPDcurt]

An important thing to remember about vasoconstriction/vasodilation in the skin. The blood vessels of the skin have dense sympathetic innervation. Therefore, during things like exercise, sympathetic tone is essentially nonexistent due to inhibition. This produces vasodilation so that warm blood can be shunted to the skin for dissipation of heat. The parasympathetic nervous system plays essentially no role in this respect.

 

[Monkey12]

what exactly is "tone"

 

[VPDcurt]

Increased tone basically means increased sympathetic activity - more input from the sympathetic nervous system. The same applies to the PNS.

 

[murphomatic]

Aha, I think that's why I was confused b/c I knew in physics, for example, smaller area = increased velocity = less pressure, so I didn't understand why vasoconstriction = decreased blood flow = more pressure...

Oh that is a good question, why ARE they different?

Remember for an enclosed fluid if the volume decreases pressure must increase
PV=nRT is true for all fluids to some extent, including blood

bernuli's equatoin tells you that where the cross sectional area is low the speed is high and the pressure is low, but only in comparison to other areas of the system. When you vasoconstrict the BP increases everywhere, but burnulli's still applies to individual areas of the system.

and yes I know i spelled bernuli wrong, I am saving all the words i know how to spell for the writen section.

 

[strangelight]

Okay, so let me try & summarize this...

Vasoconstriction:
- decreases the amount of blood flow
- used to retain heat when cold
- increase in blood pressure

Vasodilation:
- increases amount of blood flow
- used to cool down when hot (ie during exercise, which is why some people get really red)
- decrease in blood pressure

Okaaaaay, I think I'm getting it. So it's the pressure that's counterintuitive, right? Also, I think I kept confusing the amount of blood flow w/velocity...which was really stupid of me. But regarding velocity (which may not be important, but still), the same concepts in physics would still apply, yeah? Like an increase in velocity during vasoconstriction b/c of the reduced area...

 

[SanDiegoSOD]

Remember for an enclosed fluid if the volume decreases pressure must increase
PV=nRT is true for all fluids to some extent, including blood

bernuli's equatoin tells you that where the cross sectional area is low the speed is high and the pressure is low, but only in comparison to other areas of the system. When you vasoconstrict the BP increases everywhere, but burnulli's still applies to individual areas of the system.

and yes I know i spelled bernuli wrong, I am saving all the words i know how to spell for the writen section.

PV=nRT is the ideal gas law. It doesnt apply to fluid. The proper equation is P1V1 = P2V2

 

[captain hazel]

Aha, I think that's why I was confused b/c I knew in physics, for example, smaller area = increased velocity = less pressure, so I didn't understand why vasoconstriction = decreased blood flow = more pressure...

Oh that is a good question, why ARE they different?

okay, so first of all PV=nRT applies to gases ONLY. both gas & liquid are fluids, but that law is only for gases.

in blood you *are* assuming laminar (rather than turbulent) flow. I doubt Reynolds # and all that crap is gonna be on the bio section.

As far as the physics of it goes, and why it's different than what Bernouli predicts, like someone said, you can't strictly apply Bernouli's eqn because Bernouli one of Bernouli's assumptions is that you can keep a constant flow rate without any loss of energy; that is, there is no resistance/viscosity. Except blood *is* viscous. There's RBCs and such in it. You need a pressure difference to "push" fluid through at constant flow rate (so Bernouli doesn't fit: it says that pressure is constant w/ constant flow rate through a horizontal pipe).

The law that relates pressure to flow & resistance is the pressure drop (delta P) is equal to the flow rate (f) times the resistance to flow (R). (So it kinda looks like V=IR.)
R is inversely proportional to the radius^4. So if you vasoconstrict, by say 1/2, then the resistance increases by a factor of 16. deltaP=fR, so the pressure difference is gonna increase by 16 if flow stays the same. So there you go: vasoconstrict => increase resistance => increase Pressure.


NOT mcat stuff. But since you asked.

 

[strangelight]

Alright, I think that solves my dilemma. I was just trying to figure out the 'why' to everything rather than strictly memorizing it & I think everyone's input did it.

Thanks so much for your help guys!

 

[MDtoBe777]

An important thing to remember about vasoconstriction/vasodilation in the skin. The blood vessels of the skin have dense sympathetic innervation. Therefore, during things like exercise, sympathetic tone is essentially nonexistent due to inhibition. This produces vasodilation so that warm blood can be shunted to the skin for dissipation of heat. The parasympathetic nervous system plays essentially no role in this respect.

I'm a little confused here. During exercise your parasympathetic system is essentially blocked, right? You need to shunt blood away from your GI tract and other places where it is not necessary to have it. So, the sympathetic nervous system kicks in and allows for increased blood flow to the muscles that need it. Therefore, your skin would become red and have a flush appearance. So does this mean a loss of tone?

 

[Clemson Doc]

When the sympathetic nervous system is activated during times of stress, vasodilation occurs, right? Because extra blood is needed for muscles, extremities, etc. So why does sympathetic nervous system activation cause an increase in blood pressure? We established earlier that vasodilation DECREASES blood presssure, and I'm good with that, but now I'm confused as to why sympathetic activation seems to go against that. I know that this question is probably beyond the scope of the MCAT, but I would still like to know why.

To the OP: I think the main thing we need to know is that vasodilation decreases blood pressure, and vasoconstriction increases blood presssure. I like the strategy of remembering that blood vessels dilate in order to carry more blood, and this is why someone who is hot looks red. Conversely, vasoconstriction occurs when someone is cold in an attempt to conserve body heat, and this is why the feet look purple. Great examples!

 

[BigLazy]

Even though Vasodilation increase the diameter of the blood vessels to your muscles (hence the you would think blood pressure would decrease), the blood vessels to your digestive tract is vasoconstricted, so there's an increase of blood pressure there. Since the heart is pumping fast and harder, and your overall systemic resistance is increasing your overall blood pressure is also increasing (BP=Stroke volume x systemic resistence). It all depends on how you look at it, the blood pressure may be lower in the vessels to your muscles, but its higher in vessels to your organs.

 

[captain hazel]

When the sympathetic nervous system is activated during times of stress, vasodilation occurs, right?

nope. for the most part, sympathetic stim. causes arterioles of vascular beds to vasoconstrict (hence the increase in blood pressure). the important exception: arterioles in skeletal muscle vasodilate. end result: less blood going to other organs, more of the blood ending up at your muscle.

it's cause of the whole alpha & beta adrenergic receptor thing. (the usual arteriole response: alpha-receptors bind nor-epi =>vasoconstrict. but skeletal muscle arterioles have a lot more beta receptors, which bind circulating epinephrine from the adrenal medulla => vasodilation).

just remember for fight or flight your muscle needs more blood. screw everything else and their lousy blood needs.