sympathetic stimulation and vasoconstriction

[frochocinco]

lets say a fight-or-flight situation arises and a persons ANS is activated...there is significant sympathetic stimulation, leading to a whole bunch of consequences: bronchiodialation, vasoconstriction, redirecting of blood flow, etc...

however, if vasoconstriction is the primary response to sympathetic ANS stimulation, wouldnt that decrease blood flow to vital areas of the body such as the brain (quick thinking in a stressful enviornment, therefore needing lots of oxygen) and the muscles (to either "fight" or "flight").

I'm just a little confused as to how this is physiologically beneficial.

thanks

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[FeinMS]

The statement that sympathetic stimulation causes vasoconstriction is partially correct. While it constricts blood vessels in unnecessary organs, it dialates blood vessels in necessary organs. By unnecessary, I mean unnecessary to the fight-or-flight situation such as digestive organs. By necessary, I mean the opposite such as muscle.

 

[cheetoh]

Vasoconstriction doesn't stop or resist blood flow; constriction acts to increase blood pressure so that you're pumping more efficiently. A sympathetic response wants to help the brain function more acutely like you said, and constriction is a way to go about it by increasing the rate of blood flow to the brain.

(Blood) Pressure = Force/area

If I'm constricting, I'm decreasing the area of whatever vessel I'm currently in - thereby increasing pressure.

Vasoconstriction - Increase BP
Vasodilation - Decrease BP

At least, this is the way I was led to understand it.

 

[Intraleukin-6]

Vasoconstriction doesn't stop or resist blood flow; constriction acts to increase blood pressure so that you're pumping more efficiently. A sympathetic response wants to help the brain function more acutely like you said, and constriction is a way to go about it by increasing the rate of blood flow to the brain.

(Blood) Pressure = Force/area

If I'm constricting, I'm decreasing the area of whatever vessel I'm currently in - thereby increasing pressure.

Vasoconstriction - Increase BP
Vasodilation - Decrease BP

At least, this is the way I was led to understand it.

Vasoconstriction really decreases efficiency a lot and does restrict blood flow...like a thumb on a hose. But it works well because blood can be shunted to where it's needed. I think you've gotten the pressure part a little bit off.

Constant=P + 1/2pv^2 + pgh

Increase in P leads to decreased velocity. It's when P goes back down that the velocity increases (think aortic valve closed vs just after it opens).

however, if vasoconstriction is the primary response to sympathetic ANS stimulation, wouldnt that decrease blood flow to vital areas of the body such as the brain (quick thinking in a stressful enviornment, therefore needing lots of oxygen) and the muscles (to either "fight" or "flight").

Vasodilation (at least in the muscles) during Sympathetic stimulation (so after vessels have been constricted) occurs humorally. Metabolites from muscle contractions lead to local vasodilation. Vasoconstriction occurs even to the brain, but blood is being circulated much quicker, so the brain can have it's normal levels of blood flow while other tissues are getting much more.

 

[frochocinco]

regardless of the physics involved, while they are certainly equally important, i just wanted to confirm that although it may seem counter-intuitive to a degree, sympathetic stimulation will cause vasoconstriction?

i guess looking at Q=Av, given that Q is always constant, is it correct to state that by vasoconstriction decreasing A, v must increase as a response?

I suppose that makes sense but if flow rate is constant, youre still going to be exchanging total fluid at the same rate, so it doesnt seem to show how this helps during a fight or flight response. it would appear you would need to increase BOTH area and velocity, but then again, if Q is constant, then that may not be possible. sorry for the blabbering, i'm just hoping to find a train of thought that makes sense that someone can confirm

 

[Intraleukin-6]

Vasoconstriction allows other tissues' blood flow to remain stable despite the heart pumping like crazy. Hopefully the chart helps a bit.

 

[Gauss44]

lets say a fight-or-flight situation arises and a persons ANS is activated...there is significant sympathetic stimulation, leading to a whole bunch of consequences: bronchiodialation, vasoconstriction, redirecting of blood flow, etc...

however, if vasoconstriction is the primary response to sympathetic ANS stimulation, wouldnt that decrease blood flow to vital areas of the body such as the brain (quick thinking in a stressful enviornment, therefore needing lots of oxygen) and the muscles (to either "fight" or "flight").

I'm just a little confused as to how this is physiologically beneficial.

thanks

Vasoconstriction apparently reduces blood flow to the kidneys: http://forums.studentdoctor.net/showthread.php?t=984414&highlight=kidneys+constrict

 

[Gauss44]

The statement that sympathetic stimulation causes vasoconstriction is partially correct. While it constricts blood vessels in unnecessary organs, it dialates blood vessels in necessary organs. By unnecessary, I mean unnecessary to the fight-or-flight situation such as digestive organs. By necessary, I mean the opposite such as muscle.

I think you have this right. I suspect what is meant by "sympathetic stimulation causes vasoconstriction" is that it causes the arteries and arterioles to constrict in general circulation to provide more blood to the dialated areas, which would be the ones you mentioned.

 

[gettheleadout]

The statement that sympathetic stimulation causes vasoconstriction is partially correct. While it constricts blood vessels in unnecessary organs, it dialates blood vessels in necessary organs. By unnecessary, I mean unnecessary to the fight-or-flight situation such as digestive organs. By necessary, I mean the opposite such as muscle.

This is correct. Sympathetic stimulation causes cutaneous vasoconstriction, for example, but definitely causes vasodilation among the resistance arteries leading to major muscle groups.

 

[mehc012]

I think you have this right. I suspect what is meant by "sympathetic stimulation causes vasoconstriction" is that it causes the arteries and arterioles to constrict in general circulation to provide more blood to the dialated areas, which would be the ones you mentioned.

Isn't this is where the whole α vs ß adrenergic receptors thing comes into play? Yes, the sympathetic nervous system all uses catecholamines, but different tissues can have different receptors with widely varying effects...constricting peripheral vessels while dilating those supplying skeletal muscles, for example.

Also keep in mind that sympathetic/parasympathetic innervation is used to maintain BP on a normal basis, which might be the more relevant application of the Sympathetic = vasoconstriction rule, as lax baroreceptors on the aortic arch and/or carotid will stimulate the sympathetic nervous system, constricting systemic vessels and increasing BP.

Edit: here's a Wiki table on the receptors/effect of NE on the various blood vessels!

 

[GomerPyle]

After going through this thread, I am still confused. Someone mentioned bernoullis equation where K = P + 1/2pv^2 + pgh. If you vasoconstrict, you are increasing pressure which decreases velocity, but if you look at Q = Av, you are decreasing area which increases velocity. So which one is it?

 

[gettheleadout]

After going through this thread, I am still confused. Someone mentioned bernoullis equation where K = P + 1/2pv^2 + pgh. If you vasoconstrict, you are increasing pressure which decreases velocity, but if you look at Q = Av, you are decreasing area which increases velocity. So which one is it?

The continuity equation applies to the sum of the possible paths of blood flow; the circulatory system branches, so when one path increases its resistance to flow due to local vasoconstriction, the flow rate through the entire system, must stay constant. The flow rate through that particular path, intuitively, decreases. To compensate, flow rate increases through the other possible paths.

See here and related pages: http://hyperphysics.phy-astr.gsu.edu/hbase/ppois2.html#bl