Blood Pressure

Blood pressure is simply a measure of the force the blood puts against the walls of the blood vessels as it is push through the body. Pressure is measured by the formula P=FA. Blood pressure is most commonly measured with an electronic blood pressure cuff or a cuff with a stethoscope. In the process the doctor is cutting off the blood and measuring when the pressure when blood begins to flow again, this indicates the systolic. This measures the optimum pressure the heart can push the blood through the body. The next number indicates the diastolic  pressure which shows when the heart is at rest. A normal blood pressure has a systolic pressure 90-140 and a diastolic pressure of 60-90. Atherosclerosis is a problem that occurs in blood vessels which leads to a higher blood pressure.

http://health.howstuffworks.com/medicine/tests-treatment/question146.htm