I remain amazed at the ignorance people can display in their answers.
Blood pressure is indeed determined by peripheral resistance. When your blood pressure is measured the cuff compresses an artery
. As the "pressure" is released from the cuff there is a specific point at which the first spurt of blood passes through the expanding artery -- this is the "top" number (systolic value). The point at which your artery is again fully expanded during the compression will give you the "bottom" number (diastolic value).
The higher the systolic value, the harder your heart is working as the blood is being forced through earlier, and at a higher value (measured in mm/Hg).
Also, a high diastolic number can indicate hardening of the arteries. The artery is not able to expand as much, and is fully expanded sooner during the measurement, thus giving a higher reading.