Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. It usually refers to the arterial pressure of the systemic circulation. During each heartbeat, the heart contracts to pump blood through the system. The pressure measurement value for the contraction in is called systolic (maximum value) and for the contraction out is called diastolic (minimum value). So measurement devices measure the pressure rhythmically with the heart contraction.
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