The heart pumps blood through the system of arteries and veins (blood vessels) throughout the body. This is called the circulatory system. Since the heart is actively pumping blood through these vessels, the entire system is under pressure. We can measure the pressure of this system using a device called a sphygmomanometer. You have seen this device; it is wrapped around a person's upper arm and a squeeze bulb is used to pressurize the system to measure the pressure during and after a heartbeat. Blood pressure is given as a fraction with the top number being a measure of the systolic pressure and the bottom number being the diastolic pressure. Both numbers have units of millimeters of mercury (mm Hg) which is a classic measure of pressure. The systolic pressure is the higher number and measures the pressure during the contraction of a heartbeat while the diastolic pressure is the lower number and measures the pressure between heartbeat contractions.