Blood pressure is the force of blood against the walls of the arteries. It is measured in a ratio of two numbers: systolic - recorded during a heartbeat (when the heart muscle is contracted) and diastolic recorded between beats (when the heart rests and refills with blood). Blood pressure is measured in a numerical reading of millimeters of mercury, abbreviated as mm Hg. Ideal blood pressure is less than 120/80 mm Hg. In general, the lower your blood pressure, the better. A sistolic blood pressure between 120-139 and a diastolic pressure between 80-89 is considered prehypertension. A reading of 140/90 or higher is hypertension, or high blood pressure. This puts the person at increased risk of heart attack, angina pectoris, stroke, kidney failure and peripheral arterial disease.