the body responds to blood loss by hematopoiesis. This occurs by upregulating transferrin (an iron transporter in the blood), erythropoietin production (a hormone made exclusively in the renal peritubular interstitial cells), and heme synthesis. First step of heme synthesis was actually alluded to in another question on this test: condensation of glycine and succinyl CoA into delta-aminolevulinate. This is rate-limiting step of heme synthesis.