SHEILA TERRY / SCIENCE PHOTO LIBRARY SHEILA TERRY / SCIENCE PHOTO LIBRARY
Blood pressure measurement. Mercury pressure gauge used in an instrument used to measure blood pressure, or sphygmomanometer, showing a normal blood pressure reading. The sphygmomanometer works by inflating a cuff wrapped around the arm until blood flow is stopped in a main artery. By slowly deflating the cuff and listening for the return of blood flow, the blood pressure during a heartbeat (systolic pressure) is recorded. Further cuff deflation gives the blood pressure between heartbeats (diastolic pressure). Blood pressure is then expressed as a difference between these two measurements, in millimetres of mercury (mm Hg).
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