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Old 02-09-2016
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Why do you get an increase in venous return if you undergo venoconstriction?
You could say that if the radius decreases the resistance increases. so if you use Q=delta P/R the flow should decrease.
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Old 02-09-2016
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Quote:
Originally Posted by Ewaa View Post
Why do you get an increase in venous return if you undergo venoconstriction?
You could say that if the radius decreases the resistance increases. so if you use Q=delta P/R the flow should decrease.
Veins have pooled blood. They are what's called the capacitance vessels (as opposed to arteries which are conductance vessels). The majority of the blood volume is in the veins, just hanging around. When you venoconstrict the pressure in the veins goes up and since right atrial pressure remains the same, pressure gradient increases between the peripheral veins and the right atrium=more flow (more venous return).
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Quote:
Originally Posted by Nodo View Post
Veins have pooled blood. They are what's called the capacitance vessels (as opposed to arteries which are conductance vessels). The majority of the blood volume is in the veins, just hanging around. When you venoconstrict the pressure in the veins goes up and since right atrial pressure remains the same, pressure gradient increases between the peripheral veins and the right atrium=more flow (more venous return).
But what about the resistance? Doesn't that also go up when you constrict the veins becouse the radius dicreases?
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Originally Posted by Ewaa View Post
But what about the resistance? Doesn't that also go up when you constrict the veins becouse the radius dicreases?
Yes, the resistance increases. But as I said veins already contain a huge portion of the total blood volume which will move towards the heart. Veins already have blood, it does not have to come from the arteries. The increase in venous return comes from the pooled blood volume.

The decrease in flow across the capillary bed is probably negligible. The biggest determinant of how much blood will flow across the capillaries is arteriolar constriction.
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Quote:
Originally Posted by Nodo View Post
Yes, the resistance increases. But as I said veins already contain a huge portion of the total blood volume which will move towards the heart. Veins already have blood, it does not have to come from the arteries. The increase in venous return comes from the pooled blood volume.

The decrease in flow across the capillary bed is probably negligible. The biggest determinant of how much blood will flow across the capillaries is arteriolar constriction.
Ok. But what I mean with the increase in resistance is not that of the arteries. I mean that becouse you have venoconstriction you get decreased radius of the veins so thereby an increase in resistance of the veins.

But what I understand from you is(if I understood it right) : you get increased resistance in the veins
but also increase in pressure
and apparently the increase in resistance is negligible so this leads to more venous return.
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Quote:
Originally Posted by Ewaa View Post
Ok. But what I mean with the increase in resistance is not that of the arteries. I mean that becouse you have venoconstriction you get decreased radius of the veins so thereby an increase in resistance of the veins.

But what I understand from you is(if I understood it right) : you get increased resistance in the veins
but also increase in pressure
and apparently the increase in resistance is negligible so this leads to more venous return.
The increase in the resistance of the veins is only going to affect the blood coming from the arteries. It is the arteries that will have to overcome the increased venous resistance. Blood going through the capillaries will have a harder time getting to the venous system. However, my guess is that this increase in resistance is negligible since the mechanism by which the body controls flow through the capillaries is through arteriolar constriction/dilation.

Venoconstriction decreases venous compliance. Meaning that a given amount of blood will produce a higher pressure. Higher pressure in the veins will drive blood towards the right atrium.

It is the same mechanism by which skeletal muscle contraction drives the venous flow. Walking, for example, compresses the veins in the legs that shoot up blood up to the heart. That is the same mechanism why immobility produces venous stasis. This is the same mechanism why alpha-1 blockers have crazy orthostatic hypotension. It all ties in to the fact that the pressure difference is the driving force for blood flow. If you decrease venous pressure (ex. alpha-1 blockade, immobility) you will decrease venous return, if you increase venous pressure (ex. alpha-1 agonists, skeletal muscle contraction) you will increase the amount of blood going to the heart.
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Quote:
Originally Posted by Nodo View Post
The increase in the resistance of the veins is only going to affect the blood coming from the arteries. It is the arteries that will have to overcome the increased venous resistance. Blood going through the capillaries will have a harder time getting to the venous system. However, my guess is that this increase in resistance is negligible since the mechanism by which the body controls flow through the capillaries is through arteriolar constriction/dilation.

Venoconstriction decreases venous compliance. Meaning that a given amount of blood will produce a higher pressure. Higher pressure in the veins will drive blood towards the right atrium.

It is the same mechanism by which skeletal muscle contraction drives the venous flow. Walking, for example, compresses the veins in the legs that shoot up blood up to the heart. That is the same mechanism why immobility produces venous stasis. This is the same mechanism why alpha-1 blockers have crazy orthostatic hypotension. It all ties in to the fact that the pressure difference is the driving force for blood flow. If you decrease venous pressure (ex. alpha-1 blockade, immobility) you will decrease venous return, if you increase venous pressure (ex. alpha-1 agonists, skeletal muscle contraction) you will increase the amount of blood going to the heart.
Thank you for the great explenations!
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Originally Posted by Ewaa View Post
Thank you for the great explenations!
Always glad to help.

I just wanted to clarify that venous return means the amount of blood leaving the venous system (and entering the right atrium) not the amount of blood entering the venous system (from the arterial side of the circulation).
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You could think of the venous system as a container and venous return is what overflows when the container is filled.
So when there is venoconstriction, you are effectively reducing the size of the container and hence more overflows i.e more venous return.

The formula u provided is applicable for the arterial system only and not the venous system because like Nodo said, they are capacitance vessels.
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