Rice University
Rice University: Intro. To Heart Rate Monitors
This article, entitled "Intro. to Heart Rate Monitors," discusses how your heart rate is maintained by your body, and how heart rate monitors work.
University of Utah
University of Utah: Cardiovascular Pathology Index
This site features a detailed index of photographs for cardiovascular pathology. You can see what a normal human heart looks like and what a sick one looks like too. Come and check it out.
BBC
Bbc: Gcse Bitesize: Cellular Respiration and Transport
The circulatory system transports substances between the exchange surface and cells. It delivers oxygen and glucose to the tissues for respiration, which is the release of energy to cells.