Red blood cells, which transport oxygen to the tissues, grow from immature cells in the bone marrow and are released into the bloodstream when they reach maturity. This maturation process is controlled by a hormone known as erythropoietin, the level of which is controlled by the amount of oxygen in the blood. After reaching the bloodstream, red cells usually survive for about 120 days before being destroyed. Astronauts, when they return from space, have been found to be anemic (too few red cells) as a result of red cell destruction. During space travel, however, instead of destroying the oldest red cells as it usually does, the body selectively destroys the youngest red cells, a process called “neocytolysis.”