Bend to select a book from the lowest shelf, then rise quickly. Chances are, you'll feel a little lightheaded for a few seconds. The reason is a drop of blood pressure caused by the change in position. To maintain normal blood pressure levels, the heart and circulatory system must make frequent minor adjustments as we move, sit, stand, and lie down. Other factors besides position and movement can affect blood pressure. The amount of blood in the system, the strength of the heart's flexing, and the pliancy of artery walls all play a part. Emotions such as fright, excitement, or worry increase blood pressure. Blood pressure can fall in people who are depressed, lonely, or grieving.

Blood pressure might better be called heart pressure, for the heart's pumping action creates it. To measure blood pressure, health workers determine how hard the blood is pushing at two different times: when the heart contracts, called systole; and when the heart relaxes, called diastole. Blood pressure is measured in millimeters of mercury. The blood pressure of a healthy adult might be recorded as 115/70 (read one-fifteen over seventy). The normal range is about 100 to 120 systolic pressure and 60 to 80 diastolic. Readings above that level indicate prehypertension or hypertension--the familiar 'high blood pressure' that increases risks of heart attack or stroke.

Another, lesser-known condition, is orthostatic hypotension. It's diagnosed in people who--when rising from lying down to standing--experience a drop in systolic blood pressure of 20 millimeters or more. The drop may also occur after the person has been standing for several minutes. This disorder occurs most often in elderly people and, if severe, can be incapacitating. Its symptoms may include dimming or loss of vision, lightheadedness, dizziness, pale skin, nausea, sweating, and weakness. It can have a variety of causes, including cardiac pump failure, pooling of blood in the veins of the legs, reduced blood volume, diabetes, and various disorders of the nervous system. It can also be a side effect of some medications. The disorder can be treated with drugs that increase blood volume and accelerate heart rhythms.

About the Author

Faith Brynie, Ph D

Faith BrynieFaith Brynie holds a B.A. in Biology from West Virginia University and an M.A. and Ph.D in science curriculum and instruction from the University of Colorado. She writes books and articles on science and health topics for children, teens, and non-scientist adults. Some of her books have won awards, including two 'Best Book of the Year' citations from the American Association for the Advancement of Science.