Blood pressure (BP) is a force exerted by circulating blood on the walls of blood vessels, and is one of the principal vital signs.
During each heartbeat, BP varies between a maximum (systolic) and a minimum (diastolic) pressure. The mean BP, due to pumping by the heart and resistance in blood vessels, decreases as the circulating blood moves away from the heart through arteries.
It has its greatest decrease in the small arteries and arterioles, and continues to decrease as the blood moves through the capillaries and back to the heart through veins. Gravity, valves in veins, and pumping from contraction of skeletal muscles, are some other influences on BP at various places in the body.
The term blood pressure usually refers to the pressure measured at a persons upper arm. It is measured on the inside of an elbow at the brachial artery, which is the upper arms major blood vessel that carries blood away from the heart. A persons BP is usually expressed in terms of the systolic pressure and diastolic pressure, for example 120/80.