Measuring Blood Pressure

A hollow inflatable cuff attached to a pressure gauge is wrapped around the  upper arm. A stethoscope is placed over the brachial artery,  just below the cuff.
The cuff is inflated with air to a pressure above the highest pres-sure of the cardiac cycle, when ventricles contract. Above this pressure, you will not hear sounds through the stethoscope, because no blood is flowing through the vessel.
Air in the cuff is slowly released until the stethoscope picks up soft tapping sounds. Blood flowing into the artery under the pres-sure of the contracting ventricles—the systolic pressure—causes the sounds. When these sounds start, a gauge typically reads about 120 mm Hg. That amount of pressure will force mercury (Hg) to move up 120 millimeters in a glass column of a standardized diameter.
More air is released from the cuff. Eventually the sounds stop. Blood is now flowing continuously, even when the ventricles are the most relaxed. The pressure when the sounds stop is the lowest during a cardiac cycle, the diastolic pressure, which is usually about 80 mm Hg.
Right, compact monitors are now available that automatically record the systolic/diastolic blood pressure.