When your doctor says you have high blood pressure, the medical name for your condition is "hypertension." Blood pressure is created when the heart beats, propelling blood throughout the body. Blood pressure occurs in two distinct phases, corresponding to the contraction and relaxation of the heart muscle. When the heart contracts, it ejects a certain volume of blood out from its right side into the body's largest artery, the aorta. This initial thrust causes "systolic" blood pressure, which is the upper number of your blood pressure reading. As the heart relaxes, the blood presses against the walls of the arteries as it circulates, causing "diastolic" blood pressure, which is the lower number in your reading. Systolic blood pressure is a measure of the heart's blood output, while diastolic is determined by the resistance of arteries in the extremities to the flow of blood. If your blood pressure reading is "120 over 80," this means your systolic pressure is 120 and your diastolic is 80.

Blood pressure tends to be lower in women than men, and higher in blacks than whites. It rises with age. Blood pressure generally stays within a fairly narrow range. But it does not stay the same at all times. Blood pressure fluctuates, rising during physical and emotional stress and falling when we are relaxed or asleep. Just because your blood pressure goes up temporarily when you are stressed, this does not mean you have hypertension.

Hypertension is a type of cardiovascular disease characterized by elevation of blood pressure above the level considered normal for people of similar racial and environmental backgrounds.(1) Because it affects the entire circulatory system, hypertension can be detrimental to all the major organs, including the heart, brain, and kidneys. It may contribute to death from heart failure, heart attacks, stroke, and even kidney failure.(2)

Hypertension has many possible causes. Hypertension that results from another disease is called "secondary hypertension." Fewer than 5 percent of people who suffer from hypertension have secondary hypertension, and in the majority of those, kidney disease is the cause. Other disorders known to elevate blood pressure include thyroid disorders and Cushing's disease, which is a disorder of the adrenal glands. Certain medications— nervous system stimulants, synthetic estrogen, and NSAIDs (nonsteroidal anti-inflammatory drugs), to name a few— sometimes contribute to hypertension.

Most people with chronic high blood pressure have "primary" hypertension. Another name for this is "essential" hypertension. The exact cause of primary hypertension remains a mystery, although medical scientists are searching hard for answers. Research studies on hypertension have revealed detailed information about alterations in physiology that may lead to chronically elevated blood pressure. Researchers have zeroed in on the kidneys and their role in maintaining the right amount of sodium in the blood. By controlling retention of sodium and water, the kidneys play a major role in maintaining normal blood pressure.