With each heartbeat, the heart muscle contracts and pumps blood into the large vessels of the circulatory system. In the process, the blood exerts pressure on the vessel walls. When measuring blood pressure, a distinction is made between two values:
Systolic blood pressure measures the pressure during the heartbeat – that is, when the heart muscle contracts and pumps oxygen-rich blood into the vessels.
Diastolic blood pressure measures the pressure on the vessels when the heart muscle relaxes. Diastolic pressure is lower than systolic pressure.