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

[sameeralord]

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

 

[RazorRose]

uhhhh normally your blood pressure would stay the same, or maybe increase just a smigden.
but if your blood pressure does decrease. its called postural hypotension. and you might be dizzy sometimes when you stand up and you should probably consult your doctor about it.
p.s. normally if you were having your blood pressure taken you'd have it in a lying down position then sitting up and then standing up.