Understanding sympathetic and vasoconstriction

[SephirothXR]

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Activation of sympathetic causes arterioles to CONSTRICT and the blood moves faster (and also blood pressure increases) to places like muscles. But the blood vessels in the muscles DILATE. Did I say anything wrong here?

 

[Xivilus1231]

well... as heart rate increases due to whatever triggers sympathetic response, the velocity of blood increases in the vessel ...and according to Bernoulli's principle, as flow increases, pressure decreases relative to the space outside of the vessel causing it to constrict.

i just made that up, does that sound right?

 

[StIGMA]

Activation of sympathetic causes arterioles to CONSTRICT and the blood moves faster (and also blood pressure increases) to places like muscles. But the blood vessels in the muscles DILATE. Did I say anything wrong here?

Blood vessel response to sympathetic stimulation varies based on what the blood vessel perfuses. Arterioles going to muscles dilate during a sympathetic response, allowing greater blood flow to compensate for increased oxygen demand. On the other hand, arterioles to your gut (and a few other organs) constrict to limit unnecessary blood flow to organs that are less pertinent to a flight/fight sympathetic response. The constriction of blood flow to your gut during a sympathetic response allows the overall resistance in the cardiovascular system to remain high enough to adequately perfuse the dilated vessels (eg, to keep blood pressure high). The heart also beats faster to compensate for the dilated vessels, and blood flow in general is accelerated. If all of your vessels dilated at once (eg, during states of shock), then your blood pressure would drop to a physiologically unstable state due to a lack of vessel resistance.

 

[SephirothXR]

So arterioles going to gut would constrict, but arterioles originating from the aorta would constrict and eventually dilate as they reach the muscle (they have to constrict to get it there faster, and then dilate as it's getting there, right?)?

 

[StIGMA]

Arterioles are small diameter arteries. They do not branch off the aorta- they branch off of arteries that diverge from segments of the aorta/other large vessels. Dilation/constriction have nothing to due about getting blood to an organ "faster," it is about the volume of blood to the organ. A dilated artery gets more blood volume to an organ at the same blood pressure than a constricted artery would, because it has a higher flow rate.

Arteries in general do not dilate/constrict--- their branches (the arterioles) dilate/constrict. The pressure in the whole arterial side of the cardiovascular system is very high -- so arteries/arterioles do not need to be regulated to control speed. The pressure is high enough to ensure 'fast' speed. It is all about blood volume per unit time eg, flow rate.

 

[SephirothXR]

To me at least the equation av=AV means a smaller area means faster blood flow. But that's just me. Thanks for your clarification.

 

[inaccensa]

To me at least the equation av=AV means a smaller area means faster blood flow. But that's just me. Thanks for your clarification.

Blood is a nonideal flow. Although its safe to assume according to Q =Av that the volume flow rate is high in constricted vessels, you cant really decipher the pressure. Bernoulli's eq says that pressure is generally high when the area is large and the velocity is low. With blood that is not the case.

 

[Kimchii]

Arterioles will constrict to prevent blood from flowing to areas where it is not needed. When they constrict, the resistance of those vessels increases so the blood will be less likely to go through them. Instead the blood will go to areas where its needed. So say the stomach, vessels constrict so less blood goes ther and more blood goes to muscles. Vasodilation occurs at muscles so that the speed of blood moves slower allowing more exchange between muscle and blood since you'd be using your muscles there would be a lot of waste build up. Since vasodilation occured, its resistance goes down and flow goes up. From a physics perspective its like fluids.

A1V1=A2V2. Since the dilation occured in the muscles, the area is greater so there is lower velocity but the total amount of blood that flows out must be equal. Also, since vasoconstriction occurs in places like the stomach, the speed goes up since area goes down.