How Blood Pressure Is Measured

Blood pressure is the amount of force placed on the artery walls as blood flows through them. With every contraction of the heart, the right ventricle pumps blood into the lungs while the left ventricle pumps blood through the aorta to the arteries in the rest of the body. After each contraction, the heart muscle relaxes and blood rushes in to fill the ventricles again.

Systolic blood pressure—the first and larger number in a blood pressure reading—is the pressure in the arteries while the heart is contracting; diastolic blood pressure—the second and smaller number—is the pressure during the heart’s relaxation.

Blood pressure levels are expressed in millimeters of mercury (mm Hg), because traditionally, blood pressure levels are determined by measuring how high the pressure in the arteries forces a column of mercury to rise in a gauge. A device called a sphygmomanometer is used to measure blood pressure. It consists of a cuff, an inflating bulb, a pressure-reading device (either a mercury column, or a dial or digital gauge) and a stethoscope.

The cuff is wrapped around the upper arm and inflated until the brachial artery—the main artery in the arm—collapses temporarily, restricting blood flow. The cuff is then deflated slowly. At the moment that blood begins to reflow through the brachial artery, a thumping sound is heard through the stethoscope. This is your systolic pressure. Sounds continue until the artery is fully open and blood flows freely and quietly. Diastolic pressure is the reading on the monitor at the moment when the sounds stop.

Your blood pressure fluctuates throughout the day, depending on your activity level and your emotions as well as a number of other factors.