The two numbers that measure your blood pressure are written like a fraction; one number on top and one on the bottom. For example, 128 over 82. The number on top is called the "systolic pressure". It measures the pressure inside your blood vessels at the moment your heart beats. The number on the bottom is your diastolic pressure. It measures the pressure in your blood vessels BETWEEN heartbeats, when your heart is resting.
Because your blood pressure changes so often throughout the day, it is better to use more than one reading to figure out your "average" blood pressure. This is because, from day to day, a person's blood pressure usually goes up and down within a certain range. Posture, exercise, tension and nicotine are some things that may make your blood pressure change within a few minutes.
Blood pressure can also run high or low in families. So, it's important to look at your overall health, lifestyle, diet and family history when comparing your blood pressure to what's considered "normal."