Blood, Lymphoreticular & Immune

The pluripotent hematopoietic stem cell (HSC), which gives rise to common myeloid and common lymphoid progenitors. HSCs are CD34+ and capable of self-renewal.

Fetal: yolk sac (3–8 wk) → liver (6 wk–birth, the dominant fetal site) and spleen → bone marrow (begins ~28 wk). Adult: red marrow of the axial skeleton and proximal long bones. Restarting hepatic/splenic hematopoiesis in adults = extramedullary hematopoiesis.

Erythropoietin (EPO), produced by peritubular interstitial (fibroblast-like) cells of the renal cortex in response to hypoxia (via HIF-1α). It stimulates erythroid colony-forming units in the marrow.

~120 days. Senescent RBCs are removed by macrophages of the reticuloendothelial system, chiefly in the spleen (extravascular hemolysis). Platelets live ~8–10 days; neutrophils only hours to a few days in circulation.

They lack mitochondria, so they cannot perform oxidative phosphorylation. ATP comes from anaerobic glycolysis; the HMP shunt supplies NADPH (glutathione defense), and the Rapoport–Luebering shunt makes 2,3-BPG.

HbA = α2β2 (~97%). HbA2 = α2δ2 (~2%). HbF = α2γ2 (fetal, <1% in adults). Each globin chain binds one heme (Fe2+–protoporphyrin IX); one Hb tetramer carries up to 4 O2 molecules.