The Human Body

Blood Pressure

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The pressure generated by the pumping action of the heart propels the blood to the arteries. In order to maintain an adequate flow of blood to all parts of the body, a certain level of blood pressure is needed. Blood pressure, for instance, enables a person to rise quickly from a horizontal position without blood pooling in the legs, which would cause fainting from deprivation of blood to the brain. Normal blood pressure is regulated by a number of factors, such as the contraction of the heart, the elasticity of arterial walls, blood volume, and resistance of blood vessels to the passage of blood.

Blood pressure is measured using an inflatable device with a gauge called a sphygmomanometer that is wrapped around the upper arm. Blood pressure is measured during systole, the active pumping phase of the heart, and diastole, the resting phase between heartbeats. Systolic and diastolic pressures are measured in units of millimeters of mercury (abbreviated mm Hg) and displayed as a ratio. Blood pressure varies between individuals and even during the normal course of a day in response to emotion, exertion, sleep, and other physical and mental changes. Normal blood pressure is less than 120/80 mm Hg, in which 120 describes systolic pressure and 80 describes diastolic pressure. Higher blood pressures that are sustained over a long period of time may indicate hypertension, a damaging circulatory condition. Lower blood pressures could signal shock from heart failure, dehydration, internal bleeding, or blood loss.