Hypertension, also known as high blood pressure, is a persistent elevation in blood pressure that taxes the heart. Over time, it can cause serious damage to the heart as well as other organs, such as the kidneys, brain, and eyes.

Blood pressure (BP) is the force that blood exerts on the walls of the arteries. It depends on the strength and rate of the heart's contraction as it pumps blood and on the resistance to the flow of blood through the arteries. The amount of resistance depends on the elasticity and diameter of the blood vessels and the volume of blood flowing through them. The narrower the arteries and the more blood pumping through them, the higher the blood pressure will be. Maintaining a healthy lifestyle can help delay or prevent hypertension.

Blood pressure rises and falls during the day depending on a person's level of activity and physical and emotional stress. Largely controlled by the autonomic nervous system (the part of the nervous system that controls involuntary actions), BP is also affected by several different hormones. When one or more of these hormones is out of balance, it can cause high blood pressure or hypertension.

The hormones include:

  • Angiotensin II - this hormone causes arteries to contract, making them temporarily narrower and increasing the pressure of the blood flowing through them.
  • Aldosterone - this hormone is produced by adrenal glands (mainly stimulated by Angiotensin II) and helps regulate the amount of sodium, potassium, and fluids excreted by the kidneys.
  • Catecholamines - this group of hormones, including, for example, dopamine and epinephrine (adrenaline), are produced by the brain and adrenal glands in response to stress. They increase heart rate and resistance in blood vessels.

Measuring BP takes into account two pressures, measured in millimeters of mercury (mm Hg). The first, systolic pressure, is the force exerted on the blood vessel walls when the heart is pumping blood. Diastolic pressure reflects the force present when the heart relaxes between beats. They are written as systolic over diastolic pressure. For instance, a blood pressure of 120/80 mm Hg or 120 over 80 corresponds to a systolic pressure of 120 and a diastolic pressure of 80.

A single measurement of blood pressure is not diagnostic. Typically, multiple readings are taken on different days and, if measurements are consistently high, a diagnosis of high blood pressure is made.

Usually, diastolic pressures will mirror systolic pressures, but as people age, the diastolic pressure tends to level out. Then, the form of hypertension that involves primarily the systolic pressure (called isolated systolic hypertension) becomes more common. In general, the greater the blood pressures for extended periods of time, the greater the potential for damage.

The classifications of blood pressure based on recommendations of the National Heart Lung and Blood Institute (NHLBI) Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (JNC 7) are summarized in the table below.

Blood Pressure Classifications

Category Systolic mm Hg (top number-force when heart is pumping)   Diastolic mm Hg (bottom number-force when heart is at rest)
Normal Less than 120 and Less than 80
Pre-Hypertension 120-139 or 80-89
Hypertension Stage I 140-159 or 90-99
Hypertension Stage II Equal to or greater than 160 or Equal to or greater than 100