Blood pressure (BP) is the pressure 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 (Systole (medicine))) and a minimum (diastolic) pressure. The mean BP decreases as the circulating blood (Circulatory system) moves away from the heart through arteries (artery), has its greatest decrease in the small arteries and arterioles, and continues to decrease as the blood moves through the capillaries (capillary) and back to the heart through veins.cite book last = Klabunde first = Richard authorlink = coauthors = title = Cardiovascular Physiology Concepts publisher = Lippincott Williams & Wilkins year = 2005 location = pages = 93-4 url = doi = id = isbn = 978-0781750301

The term blood pressure usually refers to the pressure measured at a persons upper arm (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 115/75.

BP is sometimes measured at other places, for instance at an ankle. The ratio of the BP measured at the main artery of an ankle, to the BP measured at the brachial artery of an upper arm, gives the Ankle Brachial Pressure Index (Ankle brachial pressure index) (ABPI).