Blood pressure is the force of blood inside an artery. A blood pressure reading measures the force of blood against the walls of an artery.
Blood pressure is measured by temporarily stopping the flow of blood in an artery, usually by wrapping a cuff around the upper arm and pumping air into the cuff. As the air is released from the cuff, blood begins to flow through the artery again. When the blood begins to flow, the sound of blood flowing through the artery can be heard through a stethoscope placed on the skin over the artery inside the elbow.
WebMD Medical Reference from Healthwise