<unk>  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 ml of blood from the thorax, with a decrease in systolic and diastolic blood pressures. The overall effect is insufficient blood perfusion in the upper part of the body. 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.
Orthostatic hypotension
Reflex syncope  Reflex syncope  1
Carotidynia
Reflex bradycardia
Oscillopsia
Cardiovascular drift
Cyclospasm
Hyperpathia