Blood pressure is a measurement of the pressure inside the arteries as blood is pushed through the circulatory system. This pressure is highest when the venticles in the heart contract to push blood into the system (measured as the systolic reading, or the top number of blood pressure), and lowest when the venticles are relaxed (measured as the diastolic reading, or the bottom number of blood pressure). Both of these pressures are measured in millimetres of mercury. Normal blood pressure is 120/80. A reading of 140/90 or higher is considered to be high blood pressure (also referred to as hypertension).