Blood pressure is measured using a medical instrument called a sphygmomanometer. A rubber cuff is wrapped around your upper arm and inflated. When the cuff is inflated, it compresses a large artery in your arm, momentarily stopping the blood flow.

Next, air in the cuff is released, and the person measuring the blood pressure listens with a stethoscope. When the blood starts to pulse through the artery, it makes a sound. Sounds continue to be heard until pressure in the artery exceeds the pressure in the cuff.

The person listens and watches the gauge, then records two measurements. Systolic pressure is the pressure of the blood flow when the heart beats (the pressure when the first sound is heard). Diastolic pressure is the pressure between heartbeats (the pressure when the last sound is heard). Blood pressure is measured in millimeters of mercury, which is abbreviated mm Hg.