Blood pressure is the force exerted upon the wall of the arteries when blood is pumped out of the heart. It’s measured in millimeters of mercury (abbreviated as mm Hg) and the measurement consists of two numbers. One number, usually given first, is the pressure when the heart contracts. That’s called the systolic pressure. The second number is the pressure when the heart is at rest. That’s called the diastolic pressure. Your doctor may say or present them, for example, as “120 over 80” or 120/80 mm Hg