A device for measuring blood pressure. The most common instrument consists of an upright glass column filled with mercury and calibrated in millimeters. To this is attached, by a length of plastic or rubber tubing, an inflatable cuff designed to be wrapped around the patient's upper arm. Inflation of the cuff by compression of a rubber bulb applies pressure to the arm and the pressure in the cuff is recorded by the height to which the mercury rises in the calibrated glass tube (manometer).

The stethoscope is applied over the main artery of the arm just below the cuff. The cuff then is inflated to about 180 mm., as shown on the manometer. The pressure in the cuff now exceeds the highest pressure generated in the arteries with each heartbeat. No blood will pass through the artery and no sound will be heard. Lowering the pressure slowly, a point is reached at which the blood begins to flow through the vessel with each heartbeat but the vessel collapses between beats. The coming together of the vessel walls results in a sharp tapping noise, heard by the examiner. This point is described as the systolic pressure---the highest pressure generated by each heartbeat.

Lowering the pressure still further, a point is reached where the blood flow becomes continuous and no sound can be heard. This point is described as the diastolic pressure. It represents the steady pressure remaining in the arteries between heartbeats. The final result of the measurement is recorded in millimeters of mercury pressure---often expressed, for example, as 120/80 (indicating a systolic pressure of 120 millimeters of mercury and a diastolic pressure of 80).

Modern variants of the instrument use direct reading manometers of the pressure may be electrically recorded, but the underlying principle remains the same.

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