The bottom number in a blood pressure reading is the diastolic blood pressure. It measures the pressure in the arteries when the heart rests between beats. This is the time when the heart fills with blood and gets oxygen. In a blood pressure reading, the systolic number always comes first and then the diastolic number. For example, a blood pressure reading of "120 over 80" means that the systolic pressure is 120 and the diastolic pressure is 80. Knowing both numbers is important in diagnosing and treating high blood pressure.