Vasoconstriction from Sympathetic NS?

[The Head]

I'm confused about what happens during exercise or after sympathetic nervous system stimulation. I've read that the blood vessels going to the skeletal muscles constrict to increase BP and get blood moving faster, but also that the blood vessels going to the GI constrict to shunt blood flow away from these structures and to the skeletal muscles. While I see the logic in both separately, these seem like contradictions. Help?

 

[Stephen Tashi]

I suggest you quote the sources that made these claims and at least distinguish between veins and arteries.

 

[The Head]

I don‘t know if I can distinguish between arteries and veins, but I guess you‘re implying that veins constrict from skeletal and arteries dilate, and then the opposite for GI?

I‘m just generally confused because whether it be on science shows, clips on youtube, etc. I‘ve seen vasoconstriction cited for shunting blood away from non-target areas, while other times it is said to be responsible for speeding up blood flow to target areas. It seems to be a contradiction.

 

[BillTre]

It does seem like a contradiction.
If your sources were available (cited), some misunderstanding might become obvious.

I am guessing that the intent is something like vasoconstriction shunts the blood flow away from one tissue, thereby allowing more flow to the other. Vasoconstriction should not lead to more going through the constricted set of vessels.
Alternatively, one of your sources could just be wrong. It could be a one word mistake.

 

[jim mcnamara]

@The Head
If you want a correct answer please tell us where you read or saw this - like 'I read it in National Geographic'. Otherwise the thread is at an endpoint. Why? Nobody can do anything useful for your problem.

 

[The Head]

It is something I hear quite often so I thought people here might have familiarity the topic. I‘m not trying to prove anything with a source, just understand the truth. But here, this isn‘t the “source,” but one of a dozen sites that came up in a search, and mentions vasoconstriction in skeletal muscles in exercise to get more blood flow.

https://healthyliving.azcentral.com/happens-vessels-exercise-17621.html

 

[BillTre]

At the onset of exercise, the sympathetic nervous system causes heart rate to elevate and blood vessels to constrict

This part of your reference may refer to an initial response to starting exercise (or a fight or flight response).

Generally, to get ready for exercise, blood vessel constriction will cause more blood to be in more rapid circulation.
There is a lot of compliance (flexibility) in blood vessels (especially veins) that can be modulated by vasoconstriction.
A general constriction of the veins (or spleen) will put more blood into more rapid circulation so more oxygen can be carried to where its needed.
For example veins in your legs will normally be somewhat expanded (large diameter) so they will hold more blood and it will flow through the larger diameter vessels more slowly. Constrict those veins and that blood will go elsewhere (presumably where needed) and the blood returning through those veins will pass through them more quickly (smaller diameter) and return to get reoxygenated.
These could also be constriction of arteries going to body parts not needed at the time.
The spleen is also a storage place for blood (in some animals anyway).

In addition, the contraction-relaxation cycle of muscles will provide local high and low pressure on vessels going to and from the contracting muscles. This can help get blood depleated of oxygen out of the muscles so it can be reoxygenated.

 

[Laroxe]

I think the general control of blood flow can be quite complex and depends more on just the sympathetic nervous system. First I haven't come across the idea that the vessels supplying muscles would contract, the body does have to balance the needs of various organs by maintaining blood pressure, so there may be some initial contraction until cardiac output increases but this would rapidly change. The muscles would need increased blood flow and not necessarily at a faster flow rate which would interfere with O2 and nutrient transfer so the vessels would dilate, the dilation in the capillary bed may be under local biochemical control.

Blood vessels supplying parts of the body with no increased demands would contract in order to divert the blood to where its needed. So blood supply to the gut is reduced and indeed the skin, however just to show what a juggling act this can be, as heat production increases the skin then becomes flushed with blood to control the temperature.

This might be helpful.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4551211/