If high blood pressure is bad for your health, that means that low blood pressure should be good, right? Not exactly. There’s a normal range for an adult’s blood pressure. Results that are too far above or too far below the normal range can both be unhealthy. To determine whether a patient’s blood pressure reading puts them at risk, doctors use absolute values.
Blood pressure measurements yield two numbers, the systolic and diastolic pressures. The systolic pressure is the pressure on your blood vessels when your heart is pumping blood. The diastolic pressure is the pressure on your veins when your heart is resting. Doctors will give you your blood pressure reading with the systolic number on top and the diastolic number on the bottom.
See for yourself: http://www.youtube.com/watch?v=qXlwcC9x9zE