Autonomic Nervous System. Vessels, heart rate and venous return

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Robpattinsonxxx
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Sorry if wrong part of forum.

I'm totally confused..

Does dilation or constriction of blood vessels increase or decrease heart rate?

Does dilation or constriction of respiratory vessels increase or decrease heart rate?

'Veins constrict so more blood returns to the heart.' Why is this the case? Is it due to the increase in pressure? Another analogy would be to think that dilating the vessels will allow for less resistance and hence more blood flow to the heart? Which one is true?

"Parasympathetic Nervous System does not constrict blood vessels.' ??????????//

Thanks for your help!
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Dynamo123
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(Original post by Robpattinsonxxx)
Sorry if wrong part of forum.

I'm totally confused..

Does dilation or constriction of blood vessels increase or decrease heart rate?

Does dilation or constriction of respiratory vessels increase or decrease heart rate?

'Veins constrict so more blood returns to the heart.' Why is this the case? Is it due to the increase in pressure? Another analogy would be to think that dilating the vessels will allow for less resistance and hence more blood flow to the heart? Which one is true?

"Parasympathetic Nervous System does not constrict blood vessels.' ??????????//

Thanks for your help!
Let us start with this at a basic level:

Sympathetic nervous system mediates its effects (we'll come to what they are later) through epinephrine and norepinephrine.

Parasympathetic system mediates its effects via Acetylcholine.

Now, let's come to blood vessels. Before dealing with effect of ANS on blood flow, let us consider a basic point:
--> Blood flows from heart --> arteries --> arterioles --> capillaries --> veins --> back to heart.

Now think of your veins as huge pipes, compared to your arteries which are smaller pipes.

If you constrict an artery (or more correctly, an arteriole) the blood pressure increases. Think of it as pinching a straw filled with coke: the pressure would increase.

If you constrict a vein, you are basically just trying to force a large volume of blood into a smaller area. And since veins move blood to heart, so constricting veins will INCREASE blood flow to heart.

So the summary:
Vasoconstriction--> low blood flow through arterioles.
Venoconstriction--> increased blood return to heart.

Now on a generalized note, the effect of ANS is such:

Sympathetic system: constricts blood vessels
Parasympathetic system: dilates blood vessels.

Coming to your questions:

1. There is perhaps no direct relationship between vasoconstriction and heart rate. So the answer to your question is really indirect. You might say that sympathetics increase heart rate, and they cause vasoconstriction as well.

2. Again, I think there is very likely an indirect relation between pulmonary vascular changes and heart rate. A narrowing of pulmonary artery leads to a rapid heart rate (as in PAS). I do not know the exact mechanism for this though.

3. I tried to answer that above,

4. Exactly. As I mentioned above, except in some cases we won't discuss, generally sympathetic constricts and parasympathetic dilates blood vessels. We saw earlier that Parasympathetic mediates via Acetylcholine. This binds to certain receptors called muscarinic receptors on blood vessels, and causes release of NO. NO then causes vasodilation via a long, long pathway which I'm sure you do not need to know.

I hope this helps. If you have any questions, ask away!
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Robpattinsonxxx
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Thankyou, you're a star!
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Lethorio
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Note that in the UK, it's adrenaline and noradrenaline rather than epinephrine and norepinephrine.
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