Red blood cells are produced inside the bone marrow through a process called erythropoiesis. The marrow contains stem cells that can develop into different blood cells. When the body needs more oxygen carrying cells, some stem cells commit to becoming red blood cells and begin maturing step by step. As they develop, these cells produce large amounts of hemoglobin, the protein that binds oxygen. At the same time, they shrink and eventually lose their nucleus. This loss creates more space for hemo