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.

• Blood pressure readings consist of an upper and lower number (such as 120 over 90 or 120/90). Blood pressure readings are measured in units called millimeters of mercury (mm Hg).
• The reading on the gauge when blood flow is first heard is called the systolic pressure. It is the first or upper number in a blood pressure reading. Systolic pressure is the pressure of blood against the artery walls when the heart has just finished contracting or pumping.
• The reading on the gauge when blood flow is no longer heard is the diastolic pressure. It is the second or lower number in a blood pressure reading. Diastolic pressure is the pressure of blood against the artery walls between heartbeats, when the heart is relaxed and filling with blood.
• Blood pressure readings usually increase as a person ages and with a variety of medical conditions.