When your heart pumps blood into the rest of your body, the blood that comes in is different in one key way from the blood that was there before: it contains more oxygen. When oxygen binds to hemoglobin in your red blood cells, it changes the color of your blood it makes it more red. This being the case, it is also true that the fresh blood will transmit less red light than the bluer blood that it replaced. The heart rate monitor uses this property of your blood to measure your pulse. The device consists of a little clip that goes over a finger or perhaps an ear lobe or even a toe. On one side of the clip is a light; on the other is a light sensor. The sensor measures the light that is transmitted through your finger.
