Why Is Blood Pressure Two Numbers?

Blood pressure cuff measures systolic and diastolic pressure.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. The contraction of the ventricles during systole gives the blood a strong push, like the rush of water through a hose when the spigot is turned on. The force propels the blood through the arteries; it also pushes against artery walls. The first number in a blood pressure reading is the systole number. It is a larger number because the pressure of blood against artery walls is greater with the push of the heart's contraction behind it. Diastole is the relaxation phase of heartbeat. Pressure diminishes within the relaxing ventricles. The pressure that blood exerts on artery walls decreases, too. This is the second number in a blood pressure measurement. It is always smaller than the first.

Blood pressure is not the same in all parts of the body, so to make comparisons meaningful, blood pressure is usually measured in the main artery of the upper arm. Also, blood pressure increases with exercise, stress, or exertion, so its readings are most accurate when the subject is lying down and relaxed. Other factors besides position and exercise 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 is measured in millimeters of mercury. A blood pressure of less than 120 mm. over 80 mm. is considered a normal reading for adults. Until recently, pressures below 140/90 were thought acceptable. Now, new guidelines for physicians define pressure that stays between 120-139 systolic or 80-89 diastolic as 'prehypertension.' For people with pressures in that range, the risk of developing hypertension, or blood pressure high enough to require treatment, is elevated.

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.