Hypertension is high blood pressure, or an above normal force of blood pushing against the walls of arteries as it flows through them. The more pressure the blood exerts on the artery walls, the higher the blood pressure will be. The size of small arteries also affects the blood pressure.

Blood pressure is highest when the heart beats to push blood out into the arteries: this is called systolic pressure. When the heart relaxes to fill with blood again, the pressure is at its lowest point and is called diastolic pressure. When blood pressure is measured, the systolic pressure is stated first and the diastolic pressure second. Blood pressure is measured in millimeters of mercury (mm Hg). For example, if a person's systolic pressure is 120 and diastolic pressure is 80, it is written as 120/80 mm Hg. A normal reading is considered as less than 120 over less than 80.
Many different actions or situations can normally raise blood pressure, such as physical activity or stressful situations. . These temporary increases in blood pressure are not considered hypertension. A diagnosis of hypertension is made only when a person has multiple high blood pressure readings over a period of time.
The cause of hypertension is not known in 90 to 95 percent of the people who have it. Hypertension without a known cause is called primary or essential hypertension.

Hypertension is a major health problem, especially because it has no symptoms. In the United States, about 50 million people age six and older have high blood pressure. Hypertension is more common in men than women and in people over the age of 65: more than half of all Americans over the age of 65 have hypertension. It also is more common in African-Americans than in white Americans.
Hypertension is serious because people with the condition have a higher risk for heart disease and other medical problems than people with normal blood pressure. Serious complications can be avoided by getting regular blood pressure checks and treating hypertension as soon as it is diagnosed.

Even though the cause of most hypertension is not known, some people have risk factors that give them a greater chance of getting hypertension. Many of these risk factors can be changed to lower the chance of developing hypertension or as part of a treatment program to lower blood pressure.