ERYTHROPOIETIN is a hormone produced largely by the kidneys. Erythropoietin signals the stem cells in the bone marrow to produce Red Blood Cells (erythrocytes).

Erythropoietin once created, is transported through the blood  to signal the bone marrow.  It keeps things in check when red cells become too low, or when oxygen being circulated in the blood become too low.

See Procrit