Why does blood pressure decrease when you stand up from a lying down position?

[scilearner]

I know there is venous pooling due to gravity but how does this exactly affect pressure exerted on the walls of blood vessels. Does diastolic or sytolic blood pressure decrease? Thanks!!

Edited February 9, 2010 by scilearner

[Sisyphus]

Does overall blood pressure decrease, or is it just in the upper portions of the body? Because from a lying down position, at least, it's probably just a matter of raising the height of the head relative to the heart, and the body needing a couple moments to compensate. I'm not sure about going from sitting upright to standing, but perhaps it's just the acceleration draining blood downwards. That's a physics explanation, anyway. I don't know much about anatomy.

[Mr Skeptic]

I don't think the acceleration has much to do with it, so much as the change in pressure (both an increase and a decrease). At the most extreme, there is about 2 meters of water (blood) pressure against your feet that wasn't there before, whereas for the head the heart now needs to pump against a few centimeters of pressure.

 

I don't know much about this, but I would guess the body would compensate by constricting and relaxing arteries.

[Sisyphus]

Oh I agree, but that alone doesn't account for a change when standing up from a sitting upright position, because the relative heights of the head and heart don't change. Or is the idea that the increase in pressure to the lower extremities results in a corresponding decrease in the upper?

[Mr Skeptic]

Any increase in pressure in one part comes at the cost of a decrease in pressure in other parts, assuming a constant overall pressure. If you had a person-shaped balloon filled with water, moving it from a sitting position to a standing position would move water from above to the legs.

[John Cuthber]

Any increase in pressure in one part comes at the cost of a decrease in pressure in other parts, assuming a constant overall pressure. If you had a person-shaped balloon filled with water, moving it from a sitting position to a standing position would move water from above to the legs.

 

I dare you to put in a research grant for "a person shaped balloon".

[evan7777]

perhaps gravity maybe!daaaaa:doh: