Orthostatic hypotension, also known as postural hypotension,, and colloquially as head rush or dizzy spell, is a form of hypotension in which a person's blood pressure suddenly falls when standing up or stretching. The symptom is caused by blood pooling in the lower extremities upon a change in body position. It is quite common and can occur briefly in anyone, although it is particularly prevalent among the elderly, and those with low blood pressure.

When orthostatic hypotension is present, the following symptoms can occur after sudden standing or stretching (after standing):

They are consequences of insufficient blood pressure and cerebral perfusion (blood supply). Occasionally, there may be a feeling of warmth in the head and shoulders for a few seconds after the dizziness subsides.

Orthostatic hypotension is primarily caused by gravity-induced blood pooling 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. It can also be noted that there is a decreased systolic blood pressure and a decreased diastolic blood pressure. The overall effect is an insufficient blood perfusion in the upper part of the body.