Red blood cells are responsible for transporting oxygen throughout the body. These floating cells in your blood begin their voyage in the lungs, where they absorb oxygen from the air you breathe. They next proceed to the heart, which pumps blood throughout your body, supplying oxygen to every cell. Finally, when its duty is done, a red blood cell collapses and dies, being replaced by another cell. New red blood cells are created constantly from stem cells. The process of creating new red blood cells is called erythropoiesis. New red blood cells are released into the bloodstream from the bone marrow. They then travel through the venous system to reach their final destination-the small capillaries of our organs. It takes about 100 days for the body to replace all red blood cells that are lost through bleeding or even normal wear and tear. However, during certain conditions such as malaria or AIDS, this life cycle is altered so that people do not recover their losses. Instead, they become anemic - lacking sufficient red blood cells to transport oxygen. Under the microscope, red blood cells appear red because of hemoglobin, the molecule that transports oxygen. Each red blood cell contains a single layer of membrane covered with proteins and sugars that determine its type. Inside the cell, hemoglobin is packed together with iron molecules. As red blood cells pass through smaller vessels, they lose some of their content including some of the hemoglobin.