The kidney cells make a product known as erythropoietin. It's main function is to stumulate the production of red blood cells. When the kidneys fail, they are no longer able to manufacture this important chemical. The red blood cells carry oxygen from the lungs to the cells of the body. There the oxygen is used to supply the energy reactions needed to keep the body functioning. These cells also the carry carbon dioxide generated by the cells back to the lungs, where it is exhaled. When the red blood cells are low, it is known as anemia. Since the oxygen inside redblood cells forms a complex with iron and a molecule known as hemoglobin, the measurement of the hemoglobin can determine the level of anemia.
The treatment of anemia varies upon the condition and the hemoglobin level. In order for the red cells to deliver oxygen properly they must also have an adequate supply of iron.
Synthetic erythropoietin (ESAs) is now available, and is often used in CKD and ESRD to treat anemia. However, it has been associated with side effects under specific condiiton, and there are special guidelines dictating its use.
For more information about the anemia of chronic kidney disease, please refer to the NIH website on Anemia in KIdney Disease and Dialysis
For more specific information about the risks of using ESAs, refer to the information provided by the FDA.