Blood pressure refers to the force exerted by circulating blood on the walls of blood vessels, and constitutes one of the principal vital signs. The pressure of the circulating blood decreases as blood moves through arteries, arterioles, capillaries, and veins. The term blood pressure generally refers to arterial blood pressure, i.e., the pressure in the larger arteries.

Blood pressure is most commonly measured via a sphygmomanometer, which uses the height of a column of mercury to reflect the circulating pressure (see non-invasive measurement). Although many modern blood pressure devices no longer use mercury, blood pressure values are still universally reported in millimetres of mercury (mmhg).

The systolic pressure is defined as the peak pressure in the arteries, which occurs near the beginning of the cardiac cycle; the diastolic pressure is the lowest pressure (at the resting phase of the cardiac cycle). The average pressure throughout the cardiac cycle is reported as mean arterial pressure; the pulse pressure reflects the difference between the maximum and minimum pressures measured.

Typical values for a resting, healthy adult human are approximately 120 mmhg systolic and 80 mmhg diastolic (written as 120/80 mmhg, and spoken as "one twenty over eighty"), with large individual variations. These measures of blood pressure are not static, but undergo natural variations from one heartbeat to another and throughout the day (in a circadian rhythm); they also change in response to stress, nutritional factors, drugs, or disease. The states in which blood pressure is abnormally high or low are called hypertension and hypotension, respectively.

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