text: Erythropoiesis is regulated by erythropoietin, which increases in the presence of hypoxia and regulates RBC production. In most species, the kidney is both the sensor organ and the major site of erythropoietin production, so chronic renal failure is associated with anemia. Erythropoietin acts on the marrow in concert with other humoral mediators to increase the number of stem cells entering RBC production, to shorten maturation time, and to cause early release of reticulocytes. Other factors that affect erythropoiesis are the supply of nutrients (eg, iron, folate, or vitamin B12) and cell-cell interactions between erythroid precursors, lymphoid cells, and other components of the hematopoietic microenvironment. Factors that may suppress erythropoiesis include chronic debilitating diseases and endocrine disorders (such as hypothyroidism or hyperestrogenism).