Blood pressure represents the force (pressure) exerted by blood against the arterial walls during a cardiac cycle (heart beat) which consists of heart muscle contraction (systole) and heart muscle relaxation (diastole).
Systolic blood pressure, the higher of the two pressure measurements, occurs as the heart muscles contract pumping blood into the aorta. Heart muscles then relax allowing the heart to refill with blood and the lowest pressure reached represents the diastolic blood pressure.
Normal systolic blood pressure in an adult varies between 110 and 140 mm Hg, and diastolic pressure varies between 60 and 90 mm Hg.
The following figures is the classification of blood pressure levels. (Note: "mm Hg" means millimeters of mercury)
Systolic(mm Hg) Diastolic(mm Hg) Classification
<130 <35 Normal
130-139 85-89 High Normal
140-159 90-99 Hypertension (stage 1)
160-179 100-109 Moderate Hypertension (stage 2)
>180 >110 Severe Hypertension (stage 3)