SystoleThe contraction of the heart by which the blood is forced out of the
chambers and into the aorta and pulmonary artery. The first number in a
blood pressure reading is called systolic blood pressure. Systolic
blood pressure measures the pressure in the arteries when the heart
beats.
DiastoleThe relaxation and dilation of the chambers of the heart and especially the
ventricles during which the chambers fill with blood. The second number in a
blood pressure reading is called diastolic blood pressure. Diastolic
blood pressure measures the pressure in the arteries when the heart rests
between beats.