Production of red blood cells (erythropoiesis)
Red cells are produced continuously in the marrow of certain bones. As stated above, in adults the principal sites of red cell production, called erythropoiesis, are the marrow spaces of the vertebrae, ribs, breastbone, and pelvis. Within the bone marrow the red cell is derived from a primitive precursor,
or erythroblast, a nucleated cell in which there is no hemoglobin. Proliferation occurs as a result of several successive cell divisions. During maturation, hemoglobin appears in the cell, and the nucleus becomes progressively smaller. After a few days the cell loses its nucleus and is then introduced into the bloodstream in the vascular channels of the marrow.
Almost 1 percent of the red cells are generated each day, and the balance between red cell production and the removal of aging red cells from the circulation is precisely maintained. When blood is lost from the circulation, the erythropoietin activity of marrow increases until the normal number of circulating cells has been restored.
In a normal adult the red cells of about half a litre (almost one pint) of blood are produced by the bone marrow every week. A number of nutrient substances are required for this process. Some nutrients are the building blocks of which the red cells are composed. For example, amino acids are needed in abundance for the construction of the proteins of the red cell, in particular of hemoglobin.
Iron also is a necessary component of hemoglobin. Approximately one-quarter of a gram of iron is needed for the production of a pint of blood. Other substances, required in trace amounts, are needed to catalyze the chemical reactions by which red cells are produced.
Important among these are several vitamins such as riboflavin, vitamin B12, and folic acid, necessary for the maturation of the developing red cell; and vitamin B6 (pyridoxine), required for the synthesis of hemoglobin. The secretions of several endocrine glands influence red cell production.
