"The blood in our bodies is composed of three types of cells (red blood cells, white blood cells, and platelets) that circulate throughout the body. Red blood cells contain hemoglobin (Hb), a red, iron-rich protein that carries oxygen from the lungs to all of the body's muscles and organs. Oxygen provides the energy the body needs for all of its normal activities. Anemia occurs when the number of red blood cells (or the Hb in them) falls below normal and the body gets less oxygen and therefore has less energy than it needs to function properly. "
This explains better:http://www.anemia.com/overview/overviewwhatis.html