Blood pressure is the force that blood exerts on the walls of the arteries, the vessels that carry blood away from the heart to the body. Blood pressure is usually reported as two numbers, e.g. 120 over 80 or 120/80. The units of pressure are millimeters of mercury or mm Hg. The first number represents the force on the arteries while the heart is contracting and is called systole (SIS-tuh-lee). The systole number is always higher than the second number, which represents the blood pressure when the heart is at rest and is called diastole (di-AS-tuh-lee).

High blood pressure, or hypertension, occurs when the blood pressure is higher than normal. If the blood pressure is too high and it stays high for a long period of time, it can damage the arteries and lead to a stroke, heart attack, heart failure, or kidney failure. In general, a resting blood pressure of 120/80 or lower is considered to be normal for adults, while 140/90 or higher is considered high blood pressure. A systolic number between 120 and 139 or a diastolic number between 80 and 89 is called “prehypertension.” There are several circumstances under which different definitions of what is “normal” blood pressure are used.

According to the American Heart Association, about one in three U.S. adults has high blood pressure and since high blood pressure does not always have obvious symptoms, as many as one-third of those people may not even know it.