When you have your blood pressure taken it will be given as one number over another. The first number is the systolic blood pressure, which is the pressure produced when the ventricles of your heart are contracting. The systolic pressure is when your blood is at its maximum pressure. The second number is your diastolic blood pressure, which is the pressure of the blood when your heart's ventricles are relaxing. The diastolic blood pressure is the lowest pressure the blood in your arteries reaches.