Erythropoiesis refers to the formation of red blood cells. It is a continuous process. Many nutrients are required for this notably, Vit B12, folic acid, amino acids and iron.
Vit B12 deficiency in humans cause macrocytic hypochromic anemia but in animals, though there is anemia the size of the RBCs are not much altered. Also cobalt is necessary in ruminants as they synthesize vit B12 in the rumen.
The stimulus for erythropoiesis appears to be the tissue need for oxygen. A reduced concentration of oxygen at tissue level results in the secretion of erythropoietin (EPO). The kidney is the major site (only site in dogs) for EPO production in adult mammals. It is produced by peritubular interstitial cells located within the inner cortex and outer medulla of the kidney. Liver is a minor extra-renal source of EPO in adults while in fetus it is a major site. In case of chronic renal failure in dogs, anemia is a common sequela as they cannot produce EPO elsewhere.
In bone marrow, EPO stimulates committed stem cells to increase their mitotic activity for a more rapid production and release of reticulocytes and mature erythrocytes into the circulation. EPO has a life span of less than one day.