Editing Orthostatic hypotension (section)

==Mechanism==
Orthostatic hypotension happens when gravity causes blood to pool in the lower extremities, which in turn compromises venous return, resulting in decreased [[cardiac output]] and subsequent lowering of arterial pressure. For example, changing from a lying position to standing loses about 700 [[liter|ml]] of blood from the [[human thorax|thorax]], with a decrease in [[systolic blood pressure|systolic]] and [[diastolic blood pressure]]s.<ref>{{EMedicine|article|1154266|Idiopathic Orthostatic Hypotension and other Autonomic Failure Syndromes}}</ref> The overall effect is insufficient blood perfusion in the upper part of the body.{{citation needed|date=March 2017}}

Normally, a series of cardiac, vascular, neurologic, muscular, and neurohumoral responses occurs quickly so the blood pressure does not fall very much.  One response is a [[vasoconstriction]] ([[baroreceptor reflex]]), pressing the blood up into the body again. (Often, this mechanism is exaggerated and is why [[diastolic blood pressure]] is a bit higher when a person is standing up, compared to a person in the horizontal position.) Therefore, some factor that inhibits one of these responses and causes a greater than normal fall in blood pressure is required. Such factors include low blood volume, diseases, and medications. {{citation needed|date=October 2016}}