Normal blood pressure is characterised by a systolic reading of less than 120 and a diastolic reading of less than 80, according to the American Heart Association. This is expressed as a ratio, with the systolic measure on top and the diastolic measure on the bottom.
The systolic reading measures the pressure in the arteries when the heart is beating, while the diastolic reading measures the pressure between heartbeats. An elevated blood pressure reading may indicate that a person has high blood pressure, or hypertension, advises the American Heart Association. A low blood pressure reading is generally only a concern if the person is experiencing symptoms such as dizziness, lightheadedness, fainting, dehydration or blurred vision.