Optimal blood pressure is less than 120/80 mm Hg, according to the National Heart, Lung and Blood Institute (NHLBI). The exact level that marks low blood pressure differs from person to person, although the official definition of low pressure is a blood pressure below 90/60, according to the NHLBI. The American Heart Association (AHA) goes further, stating that low blood pressure is blood pressure that is low enough to cause symptoms of lightheadedness or fainting. The body uses several mechanisms to regulate blood pressure. It can expand or constrict blood vessels; change the amount of blood pumped from the heart; or change the amount of blood pumped through blood vessels. These mechanisms help blood pressure return to normal after changes brought on by eating, exercise, sleep, or position. Although chronic low pressure may be a sign of disease, it also can indicate good physical conditioning. Athletes often have low blood pressure.

In addition to these medications, other drugs that can cause hypotension include medications for heart disease and diuretics, medications for erectile dysfunction, and narcotics. Depending on how long a medication is taken, the low blood pressure can be short-term or chronic.

Damage to the nervous system, which can occur in diabetes and Parkinson disease, can interfere with the body's normal methods of regulating blood pressure, the AHA says. A heart valve disorder can decrease the amount of blood the heart pumps. Other medical problems such as hypothyroidism, parathyroid disease, and Addison's disease can also cause low blood pressure.

Fainting, or syncope, occurs with low blood pressure because with this condition, the heart isn't able to pump enough oxygen to the brain. The brain is the organ highest up in the body, where it is hardest for blood to overcome the effect of gravity, so the brain is the first to feel the effect of low blood pressure. When a person faints from low blood pressure, he or she falls to the floor, putting the person's head on the same level as his or her heart. Blood flow then can increase to the brain.

The NHLBI says that a heart rate that is too slow can be corrected by surgically implanting a pacemaker, an electronic device that stimulates heartbeats. A heart rate that is too rapid can be slowed by using medications such as beta blockers.

This is called "postprandial hypotension.” Older adults with Parkinson disease or high blood pressure should be especially careful when standing up after eating.