Blood pressure(Bp) is measured in millimeters of mercury (mmHg) with an instrument called sphygmanometer . The device consists of an inflatable cuff attached to a mercury meter (there are digital ones but doctors trust this one) The doctor wraps the cuff around the arm and inflates it with air from a squeeze bulb. This drives the mercury column up toward the top of the gauge while shutting off blood flow through the brachial artery (a major artery in the arm). With a stethoscope just placed below the cuff, the doctor releases the air and listens for the first thudding sound that signal the return of blood flow as blood pressure on the wall of the artery equals the air pressure in the cuff. He records this mercury meter reading. This number represents the systolic pressure, the force developed by the heart when it contracts.
By continuing to let air out, the physician reaches a point where he can no longer hear the pulsing sounds of flowing blood. He marks the gauge reading as diastolic pressure, the pressure on the heart when the heart is relaxing between beats. Thus two numbers are used to record blood pressure the systolic followed by the diastolic (e.g. 140/90)
The reading is taken when patient is relaxed and at rest. Normal systolic pressure for most adults is between 100-140 and diastolic pressure is between 60-90. Factors such as age and sex is responsible for this variation in blood pressure readings. Normally Bp goes up during periods of exercise and excitement. Therefore, hypertension is the diagnosis when Bp measurements show persistent elevated pressure of 140 or higher for systolic and 90 or more for diastolic.
Written by Opatola Olufolarin Oyemuyiwa
Reference: The new complete medical and health encyclopedia (volume 2)
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