REDUCING YOUR RISK OF CORONARY ARTERY DISEASE: WHAT IS BLOOD PRESSURE?
The pumping action of the heart pushes blood into the arteries with enough pressure to keep it following forward. The amount of blood pumped out of the heart (cardiac output) and the tone of the arteries (peripheral resistance) determine the amount of tension pushing against the walls of the arteries.
The standard way to measure blood pressure is in millimeters of mercury (mm Hg). This unit of measurement refers to how high the pressure inside your arteries is able to raise a column of mercury. Each blood pressure measurement has two numbers. The top number is your systolic blood pressure, or the highest pressure within your arteries that occurs during systole, when your heart is contracting. The bottom number is your diastolic blood pressure, or the lowest pressure within your arteries that occurs during diastole, when your heart is relaxing and filling with blood.
Although a typical blood pressure is considered to be 120/70 mm Hg, your blood pressure is not constant. Normal blood pressure refers to your blood pressure when you are resting comfortably. But your blood pressure varies with exercise, strong emotion, or stress. Even a change in position from lying down to standing to standing can change your blood pressure. Therefore, it is necessary to take more than one reading to determine whether you have high blood pressure. About 35 percent of people who have high blood pressure on a single reading will not have an elevated value when the blood pressure is measured again. If your blood pressure is elevated during three separate measurements, it requires medical evaluation and treatment.