Liquid pressure is dependent on the depth of the fluid. Since blood pressure can’t be measured around the heart, and the depth of the fluid must be the same as the heart, doctors and nurses need to find a location at the same depth as the heart. A convenient location at that level is your upper arm. When lying down, however, your blood pressure can be taken just about anywhere, since most of the blood is at the same vertical level as the heart.

The cuff is inflated until no pulse can be heard. It is then slowly lowered. As the pressure falls below the systolic pressure the pulse can be heard. When it’s below the diastolic pressure the pulse gets weaker. The report “120 over 70” means that the systolic pressure is 120 torr, the diastolic pressure 70 torr.