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, using a cuff placed around the upper arm. Air is added to the cuff until blood flow stops. As the air is released, blood begins to flow through the artery again. Blood pressure is measured as the blood begins to flow again.
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).
Systolic blood pressure 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.
Diastolic blood pressure 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.