Blood pressure is determined by the force of the blood acting on the walls of the blood vessels. Two factors determine the size of this force. One is the volume of blood being pumped through the vessel. The other is the size of the vessels. Changes in blood pressure can be caused by either a change in the amount of blood being pumped or by a change in the size of the blood vessels. Feedback mechanisms, described in this animation, will alter heart rate and blood vessel dilation to maintain blood pressure at appropriate levels.