A person's blood pressure is determined by several different forces: how hard the heart is pumping, how much blood is in the body, and the width of the smallest blood vessels.
If the heart is not pumping with normal force, or if the smallest blood vessels open up wide, or if blood has been lost because of a hemorrhage or the fluid in the blood has been reduced by dehydration, the blood pressure will be low.
For example, a heart attack can injure the wall of the heart and reduce the force with which the heart pumps blood around the body. The width of the smallest blood vessels is controlled by nerve endings; those nerve endings respond to signals that begin in the brain.
Many people just naturally have a low blood pressure. If your blood pressure is below 115/75 (either or both numbers), you can feel and be just fine -- if that is normal for you.
I get concerned when patients say they feel okay, but their blood pressure is abnormally low -- for them. This could be a sign of impending trouble from poor blood flow to the heart because of blockages in coronary arteries. It can also be a side effect of a medication like nitroglycerin that people take for angina. But if a person normally runs a low blood pressure, and feels fine, then I feel fine, too.
Many patients with the illness called chronic fatigue syndrome have a tendency for their blood pressure to drop when they are upright for a period of time. This appears in some people to be caused by abnormalities of the nerves that control the width of the smallest blood vessels. In other people, there appears to be less fluid than normal in the blood. The cause of both conditions is unknown.
