Blood is composed of red cells, white cells, platelets, and a number of other substances such as clotting factors, electrolytes, proteins, and hormones, all floating around in a liquid called plasma. The main constituent of blood is the red blood cell, or erythrocyte. In a normal, healthy individual, red blood cells make up between 35 and 55 percent of the total volume of blood.
One of the unique properties of mature blood cells is that they are unable to divide as other types of cells do. In fact, red blood cells lose their nucleus before they enter the blood stream. So, if they can't divide to make more of themselves, where do they come from? They are produced by stem cells in the bone marrow through a process called hematopoiesis. Hematopoiesis is a general term for the production of all types of blood cells. The specific type of hematopoiesis that makes red blood cells is called erythropoiesis.
Erythropoiesis is stimulated when the body senses that there isn't enough oxygen being carried by the blood to properly supply all of the tissues. When this happens, the kidney produces a stimulating factor called erythropoietin, or EPO. You may have heard of this growth factor. Professional athletes illegally use a synthetic version of it to increase their bloodstream's ability to carry more oxygen. EPO binds to a special type of progenitor stem cell called a Colony Forming Unit, or CFU. This causes these CFUs to divide, with one of the daughter cells remaining as a CFU, and the other becoming a precursor cell called a pronormoblast. The CFU continues to divide in this fashion until the stimulation from the EPO is removed. The pronormoblast, however, no longer has the capability to divide. It begins a path of development on its way to becoming a mature red blood cell.
While in the pronormoblast stage, the cell begins to synthesize hemoglobin, the molecule responsible for carrying oxygen. It also begins to take up iron, which is a cofactor needed by hemoglobin to carry the oxygen. The cell then moves from the pronormoblast stage to the next stage of development called the polychromatophilic normoblast stage. From there, it becomes an orthochromic normoblast, then a reticulocyte, and finally a mature red blood cell. As the cell passes through each stage, it gets smaller and the nucleus condenses until it is finally ejected at the reticulocyte stage. The reason the nucleus is ejected is that it is no longer needed, since the cell has already produced all of the proteins that it will use during its lifespan of approximately four months.
Since red blood cells only live for about four months they must continuously be replaced. As they near the end of their lifespan, they don't function as well as they should, and an organ called the spleen removes them from circulation. New red blood cells from the bone marrow replace them at a rate of more than one billion cells per hour!
The body is truly amazing in its ability to maintain homeostasis. The production of red blood cells is a good example of just how complex our bodies really are.