Blood pressure is the force exerted by the blood against the walls of blood vessels, and the magnitude of this force depends on the cardiac output and the resistance of the blood vessels.

Hypertension is having a blood pressure higher than 140 over 90 mmHg, a definition shared by all the medical guidelines.

This means the systolic reading (the pressure as the heart pumps blood around the body) is over 140 mmHg (millimeters of mercury) or the diastolic reading (as the heart relaxes and refills with blood) is over 90 mmHg.

While this threshold has been set to define hypertension, it is for clinical convenience and because achieving targets below this level brings benefits for patients.

Rather than being marked by a particular cut-off point, the medical expert committees on the condition actually see high blood pressure as having a continuous relationship to cardiovascular health.

They believe that, to a point (down to levels of 115-110 mmHg systolic, and 75-70 mmHg diastolic) the lower the blood pressure the better.

This view has led the American Heart Association (AHA), for example, to define the following ranges of blood pressure (in mmHg):

• Normal blood pressure is below 120 systolic and below 80 diastolic
• Prehypertension is 120-139 systolic or 80-89 diastolic
• Stage 1 high blood pressure (hypertension) is 140-159 systolic or 90-99 diastolic
• Stage 2 high blood pressure (hypertension) is 160 or higher systolic or 100 or higher diastolic
• Hypertensive crisis (a medical emergency) is when blood pressure is above 180 systolic or above 110 diastolic.

Reference: Medical News Today, 2015.