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What causes it?
The causes of high blood pressure can vary, and most of the time, the cause isn't known. This form of the condition is called "essential hypertension." It might be due to a narrowing of the arteries, more blood than normal, or the heart beating more forcefully or faster than it usually should. Any of these conditions can raise the force of the blood against the artery walls.

Sometimes high blood pressure can be caused by another medical problem, such as kidney disease. When this happens, the condition is called "secondary hypertension." As the name indicates, treating the main problem makes the blood pressure go down.

This section © National Heart, Lung, and Blood Institute

Systolic and diastolic
Your heart can beat more than 100,000 times a day. Each time it beats, a surge of blood is pumped from your heart into your arteries. This increases the pressure in your arteries. In between heartbeats the pressure in your arteries decreases.

That is why blood pressure is reported as two numbers, e.g., 120/80. The first, higher number (systolic) is the pressure of the blood against the artery walls when the heart contracts (e.g., 120). The second, lower number (diastolic) is the pressure against the artery walls when the heart relaxes between beats (e.g., 80).