Hypertension is the medical term used to describe high blood pressure. Blood pressure is necessary to help pump the blood to the rest of the body, but high blood pressure can be unhealthy and lead to significant health problems.

In an adult (more than 18 years old), high blood pressure is defined as a systolic blood pressure ("top number") of 140 mm Hg or greater, and a diastolic blood pressure ("bottom number") of 90 mm Hg or greater. Systolic blood pressure is a measure of the force applied on the artery walls when the heart is contracting. Diastolic blood pressure is the force applied on the artery walls when the heart is relaxed or in between beats. Blood pressure is measured in millimeters of mercury (mm Hg).

High blood pressure can occur in people of any age, but is more common in adults over the age of 35. More than 50 million Americans are hypertensive (one out of four). African-Americans, in particular, are at high risk of developing hypertension (one out of three).

Many factors affect blood pressure, such as the volume of water in the body, salt content in the body, hormones, activity level, temperature, emotional state, and condition of the kidneys, nervous system, and blood vessels. If blood pressure is high and is left uncontrolled, the heart and arteries will not function as well as they should.

Essential (primary) hypertension has no identifiable cause. This type of hypertension comprises approximately 95% of all patients having hypertension. Although the cause is unclear, genetic and environmental factors, such as diet and exercise, play important roles.

Secondary hypertension is high blood pressure, which is caused from an underlying disorder. Examples include the following:

• Narrowing of certain arteries
• Adrenal gland disorders
• Kidney disorders
• Use of medications, drugs or other chemicals
• Pregnancy
• Use of oral contraceptives, antihistamines, steroids, antidepressants, or non-steroidal anti-inflammatory drugs
• Other related disorders and syndromes

There are two levels of high blood pressure: stage 1 and stage 2 (see the chart below).

Category	Systolic BP(mm Hg)(top number)	Diastolic BP(mm Hg)(bottom number)
Normal	less than 120	less than 80
Prehypertension	120-139	80-89
High blood pressure
Stage 1	140–159	90–99
Stage 2	160 or higher	100 or higher