Your blood pressure is measured in units of millimeters of mercury, and your results will have a numbered pair followed by "mmHg." The systolic value is the upper number and shows the pressure in your blood vessels when the heart beats to contract and send blood into your body. The bottom number is the diastolic reading and reflects the pressure on your blood vessels when the heart relaxes. This number is typically lower. Follow your physician's recommendations when monitoring your blood pressure to know when a reading is too high or low.