Both are important. Different actions make your blood pressure go up or down. For example, if you run for a bus, your blood pressure goes up. When you sleep at night, your blood pressure goes down. These changes in blood pressure are normal.
Some people have blood pressure that stays up all or most of the time.
High blood pressure harms the arteries by making them thick and stiff. This speeds the build up of cholesterol and fats in the blood vessels like rust in a pipe, which prevents the blood from flowing through the body. In time this can lead to a heart attack or stroke. Heart Attack Blood carries oxygen to the body. When the arteries that bring blood to the heart muscle become blocked, the heart cannot get enough oxygen.