Everyday Measurements

Blood Pressure Units

Definition

Blood pressure is measured in millimeters of mercury (mmHg), expressed as two values: systolic pressure (heart contracting) over diastolic pressure (heart at rest), e.g., 120/80 mmHg.

Explanation

The unit dates to the mercury manometers used in early sphygmomanometers, where pressure literally raised a column of liquid mercury. Normal adult blood pressure is below 120/80 mmHg; hypertension stage 1 begins at 130/80 mmHg. Although mercury instruments are now largely replaced by electronic sensors for safety reasons, mmHg remains the universal clinical standard because of its century-long reference base.

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