The top number of a blood pressure reading is referred to as systolic, and it measures the pressure in the arteries when the heart beats, notes the American Heart Association. The bottom number is referred to as diastolic, and it measures the pressure in the arteries between heart beats.
The systolic number is higher than the diastolic number, according to the American Heart Association. Normal blood pressure should have a systolic number less than 120 and a diastolic number of less than 80. Emergency care is needed if blood pressure reaches a systolic number of 180 or more and a diastolic number of 110 or higher, as this is a hypertensive crisis.