Blood pressure is a measure of the force with which blood pushes against artery walls. Systolic blood pressure (the top number) refers specifically to the pressure when the heart beats and pushes blood through the heart and into the arteries. The pressure between heartbeats, when the heart is at rest, is called the diastolic pressure (the bottom number). According to the JNC7 “normal” blood pressure for an adult is 120 systolic and 80 diastolic or 120/80 mmHg.

High blood pressure, also known as hypertension, occurs when arteries have contracted making it more difficult for blood to flow, which puts extra stress on the heart and places excessive pressure on the artery walls usually causing damage to the endothelium (the lining in the arteries). The danger lies in the extra stress on the heart, often causing the heart to enlarge over time, and the increased likelihood for narrowed, stiff blood vessels that are easily blocked. There are a variety of known reasons for why the arteries contract, although there are instances in which there is no explanation found.