Blood pressure is the force exerted by the blood upon the walls of the arteries. The pressure in the arteries originates in the pumping action of the heart, and pressure waves can be felt at the wrist and at other points where arteries lie near the surface of the body. Since the heart can pump blood into the large arteries more quickly than it can be absorbed and released by the tiny arterioles and capillaries, considerable inner pressure always exists in the arteries. The contraction of the heart (systole) causes blood pressure to rise to it's highest point, and relaxation of the heart (diastole) brings the pressure down to its lowest point.

A sphygmomanometer measures blood pressure in millimeters of mercury. The normal blood pressure readings in young people are about 120 mm for systolic pressure and about 80 mm for diastolic pressure, commonly written as 120/80 and read as one-twenty over eighty. With age, and the constriction of the small arteries and then the large ones, blood pressure increases, so that at 50 years, normal individuals have a systolic pressure between 140 and 150, and a diastolic pressure of about 90.

Factors other than heart action and the condition of the arteries also influence blood pressure. Temporary high blood pressure usually occurs during or following physical activity, nervous strain, and periods of rage or fear. Therapy for persistent high blood pressure, sometimes called hypertension, consists of sufficient rest, a diet low in salt and alcohol, reduction in weight where there is obesity, and increased exercise.

Hypertension or high blood pressure results from an increase in the amount of blood pumped by the heart or from increased resistance to the flow of blood through the small arterial blood vessels (arteries). Hypertension is generally defined as a blood pressure reading greater than 140 over 90. When the cause is unknown, the condition is called primary, or essential, hypertension. When a known cause can be identified (e.g., a disorder of the adrenal glands, kidneys, or arteries), the condition is know as secondary hypertension. Factors such as heredity, obesity, smoking, and emotional stress are thought to play a role.