As the blood flows through the blood vessels it exerts a certain amount of pressure. If the pressure is too low then blood would stop flowing. If the pressure is too high then the heart has to work harder to push the blood through the blood vessels.
High Blood Pressure is sometimes referred to as the silent killer because there are no warning signs to this condition. However, it can seriously contribute to a stroke or a heart attack.
Below are some videos that will explain this a bit more