Blood Pressure Explained
Blood pressure is the force of blood against the walls of arteries. Blood pressure is recorded as two numbers—the systolic pressure (as the heart beats) over the diastolic pressure (as the heart relaxes between beats). The measurement is 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 measurement of 120/80 mm Hg (millimeters of mercury) is expressed verbally as “120 over 80.”
Normal blood pressure is less than 120 mm Hg systolic and less than 80 mm Hg diastolic.
When systolic and diastolic blood pressures fall into different categories, the higher category should be used to classify blood pressure level. For example, 160/80 mm Hg would be stage 2 hypertension (high blood pressure).
NOTE: < means less than; >= means greater than or equal to.
Source: National Heart, Lung, and Blood Institute.