Erythropoietin Mechanism: (See Figure 18.6) Erythropoietin is secreted in maintenance amounts constantly, but hypoxia (reduced oxygen in blood or tissues) stimulates an increase in erythropoietin secretion by the kidney. This triggers more of the pleuripotential stem cells (hemocytoblasts) to produce rbc (See Figure 18.5) and to do so at a faster rate. More rbc will carry more oxygen and thus raise blood oxygen levels, reducing erythropoietin secretion by negative feedback.