how vasodilation decreases the preload ?

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mayaar

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I have a question

I read that Nitrates help patients with angina due to several facts

1- it decreases the after load by dilating the arterioles which reduces the peripheral vascular resistance ( that I do understand )

2- it decreases the preload by dilating the veins which decreases Venus return

Here comes my question

Whenever we dilate a vein that decreases the pressure but increases the blood flow am I right ???

If that's right how come we increase the blood flow but still decreasing the venous return ?


Thank you

Looking forward to your answers

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The premise of venous return is due to increased pressure on the veins. For example, when walking, the muscles contract and compress the veins which propels the blood forward (because of the one-way valves found in veins) towards the heart.

Now if you give a patient nitrates, which dilate the veins, there will be a decrease in the total compression of these veins, which results in a decreased forward blood flow (and by forward I mean centrally, towards the heart), thus decreasing pre-load.
 
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The gross oversimplification is that blood that is in the venous system is simply "dead space". It's not perfusing organs, and it does not contribute to systemic pressure. With that logic, venodilation = more blood in the venous sytem = less blood effectively circulating = ↓ preload. Similarly, venoconstriction = less blood in the venous system = more blood effectively circulating = ↑preload.

In terms of the cardiac vascular return curves, notice that the effects of venoconstriction are identical to the effects of administering fluids.
 
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The gross oversimplification is that blood that is in the venous system is simply "dead space". It's not perfusing organs, and it does not contribute to systemic pressure. With that logic, venodilation = more blood in the venous sytem = less blood effectively circulating = ↓ preload. Similarly, venoconstriction = less blood in the venous system = more blood effectively circulating = ↑preload.

In terms of the cardiac vascular return curves, notice that the effects of venoconstriction are identical to the effects of administering fluids.

While this analogy makes sense. I want to point out that, physiologically, the reason for decreased preload is a decrease in central venous pressure.

ΔCVP = ΔV / Cv

by vasodilating you are increasing venous compliance (Cv), thereby decreasing CVP, which is the major determinant of filling pressure of the heart and therefore the preload of the right ventricle.

check out: CV Physiology | Central Venous Pressure

his website is amazing for cardio phys and pharm.
 
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