<P> Blood pressure that is too low is known as hypotension . Hypotension is a medical concern if it causes signs or symptoms, such as dizziness, fainting, or in extreme cases, shock . </P> <P> When arterial pressure and blood flow decrease beyond a certain point, the perfusion of the brain becomes critically decreased (i.e., the blood supply is not sufficient), causing lightheadedness, dizziness, weakness or fainting . </P> <P> Sometimes the arterial pressure drops significantly when a patient stands up from sitting . This is known as orthostatic hypotension (postural hypotension); gravity reduces the rate of blood return from the body veins below the heart back to the heart, thus reducing stroke volume and cardiac output . </P> <P> When people are healthy, the veins below their heart quickly constrict and the heart rate increases to minimize and compensate for the gravity effect . This is carried out involuntarily by the autonomic nervous system . The system usually requires a few seconds to fully adjust and if the compensations are too slow or inadequate, the individual will suffer reduced blood flow to the brain, dizziness and potential blackout . Increases in G - loading, such as routinely experienced by aerobatic or combat pilots' pulling Gs', greatly increases this effect . Repositioning the body horizontally largely eliminates the problem . </P>

A medical term that means decrease in blood pressure is