When the heart is contracting, it pumps out blood. The blood is forced out to the lungs from the right ventricle and to the rest of the body from the left ventricle. This phase of contraction is called systole. After the heart has contracted and the blood has been squeezed out, the heart muscle relaxes and it expands. During this period, the heart fills with blood. This phase is called diastole.
It is these two concepts we use when we measure a person’s blood pressure. A normal blood pressure is ”120 over 80”. We write 120/80. These figures indicate our blood pressure: 120 indicates that the pressure is 120 mmHg during systole, and 80 indicates that the pressure is 80 mmHg during diastole. More on this topic is covered in cardiovascular physiology.