there are two numbers in a blood pressure. the first is the systolic pressure. it measures how hard the blood pushes against the walls of the arteries when the heart is in systole (beating and pushing out blood). this is when the pressure against the walls of the arteries is highest. the second number is the diastolic blood pressure. it measures how hard the blood pushes against the walls of the arteries when the heart is in diastole (resting between beats and not pushing out any blood). for example, in the picture on this page, the pressure on the walls of the arteries is 122 mmhg when the heart is beating, and 65 mmhg when the heart is resting. this blood pressure would be written as "122/65" and read as "122 over 65."