It is said as "140 over 90," for example. The top number, which is called the systolic pressure, shows the pressure in your arteries when your heart is forcing blood through them. The bottom number, called the diastolic pressure, shows the pressure in your arteries when your heart relaxes. The top number can be anywhere from 90 to 240 and the bottom number can be anywhere from 60 to 140. Blood pressure is measured in millimetres of mercury, which is written down as: mmHg