Sympathetic NS effects

[IH8ColdWeath3r]

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I know that the "fight or flight" response involves the sympathetic NS but I had a question regarding the effects of this response. When i think fight-or-flight, I think about if I were to run into a snake/mountain lion unexpectedly. I know that following things would occur

1) pupils dilate
2) increased respiratory rate & bronchodilation
3) increased HR due to NE effects on the heart
3) Decreased gastric mobility (blood flow directed away from digestive tract and towards muscles)
4) Increased in BP

what I don't quite get is how can blood pressure be increased during a fight-or-flight response (this would require vasoconstriction) but yet at the same time vasodilation is what results in blood being diverted away from the gastrointestinal tract and towards the muscles?

this is taken directly from one of the responses on the AAMC exams- "Vasodilation increases both blood flow to muscles during exercise and skin during blushing".

I always though that vasoconstriction is what resulted in elevated BP during the fight for flight response (or exercise) and vasoconstriction is what lead to more blood flow to the muscles? is this wrong?

 

[Ssina]

by vasoconstriction, less blood goes to GI tract and now you have more blood (increasing the bp), that now can be diverted to the muscles via vasodilatation, so more blood in the muscles increasing the pressure there.

that's how I think about it!

 

[rjosh33]

I know that the "fight or flight" response involves the sympathetic NS but I had a question regarding the effects of this response. When i think fight-or-flight, I think about if I were to run into a snake/mountain lion unexpectedly. I know that following things would occur

1) pupils dilate
2) increased respiratory rate & bronchodilation
3) increased HR due to NE effects on the heart
3) Decreased gastric mobility (blood flow directed away from digestive tract and towards muscles)
4) Increased in BP

what I don't quite get is how can blood pressure be increased during a fight-or-flight response (this would require vasoconstriction) but yet at the same time vasodilation is what results in blood being diverted away from the gastrointestinal tract and towards the muscles?

this is taken directly from one of the responses on the AAMC exams- "Vasodilation increases both blood flow to muscles during exercise and skin during blushing".

I always though that vasoconstriction is what resulted in elevated BP during the fight for flight response (or exercise) and vasoconstriction is what lead to more blood flow to the muscles? is this wrong?

Not every blood vessel is constricted, just like not every artery or capillary is dilated. The arteries leading to the GI tract are constricted upon sympathetic stimulation, and those leading to the skeletal muscles are dilated, thereby shunting the blood to where it's needed most. The increase in blood pressure is due to the sharp increase in heart rate.

Remember: heart rate x stroke volume = cardiac output. By increasing the heart rate, you increase the cardiac output, and therefore increase your blood pressure.

 

[muhali3]

+1^

 

[MedPR]

Not every blood vessel is constricted, just like not every artery or capillary is dilated. The arteries leading to the GI tract are constricted upon sympathetic stimulation, and those leading to the skeletal muscles are dilated, thereby shunting the blood to where it's needed most. The increase in blood pressure is due to the sharp increase in heart rate.

Remember: heart rate x stroke volume = cardiac output. By increasing the heart rate, you increase the cardiac output, and therefore increase your blood pressure.

Sounds about right to me.

 

[PharmD for Me]

This is probably more in-depth than you need to know, but maybe it will help explain it better.

A sympathetic response stimulates beta-1 adrenoceptors which results in...
- Increasing rate (SA node)
- Increase contractile force of atria and ventricles

... which basically boils down to increased cardiac output, as said before.

A sympathetic response also leads to stimulated beta-2 adrenoceptors, which dilate coronary vessels and skeletal muscle blood vessels.

When you think of constriction of blood vessels from sympathetic responses, that's generally from the stimulation of alpha-1 adrenoceptors on other arterioles and veins.

Physio final studying FTW... hopefully I can remember that tomorrow morning .

 

[IH8ColdWeath3r]

Not every blood vessel is constricted, just like not every artery or capillary is dilated. The arteries leading to the GI tract are constricted upon sympathetic stimulation, and those leading to the skeletal muscles are dilated, thereby shunting the blood to where it's needed most. The increase in blood pressure is due to the sharp increase in heart rate.

Remember: heart rate x stroke volume = cardiac output. By increasing the heart rate, you increase the cardiac output, and therefore increase your blood pressure.

makes perfect sense, thanks.