Blood pressure is the force of blood against the walls of arteries. Blood pressure is noted as two numbers—the systolic pressure (as the heart beats) over the diastolic pressure (as the heart relaxes between beats). The numbers are written one above or before the other, with the systolic number on top and the diastolic number on the bottom. For example, a blood pressure reading of 120/80 mmHg (millimeters of mercury) is called "120 over 80."