erythrocytes

Erythrocytes are the most abundant blood cells, lacking a nucleus and mitochondria in their mature form, which allows maximal space for hemoglobin (Hb) but limits their metabolism to anaerobic glycolysis. Their biconcave shape increases surface area for gas exchange and allows deformability to pass through narrow capillaries. Hemoglobin, composed of four globin chains each bound to a heme group with iron (Fe2+), reversibly binds oxygen; the oxygen-hemoglobin dissociation curve describes this relationship and shifts with pH, CO2, temperature, and 2,3-BPG (Bohr effect). Erythropoiesis occurs in the bone marrow and is stimulated by erythropoietin, produced by peritubular cells in the kidney in response to hypoxia. RBCs require vitamin B12, folate, and iron for proper maturation; deficiencies cause characteristic anemias (megaloblastic anemia with B12/folate deficiency; microcytic anemia with iron deficiency). Senescent RBCs are removed by macrophages in the spleen (extravascular hemolysis), releasing heme that is metabolized to bilirubin, contributing to the bilirubin/heme catabolism pathway. Clinically, RBC indices (MCV, MCHC, RDW) and peripheral smear morphology are used to classify anemias (microcytic, normocytic, macrocytic) and identify hemolytic processes (schistocytes, spherocytes, sickle cells). Erythrocyte disorders are high-yield topics including sickle cell disease (HbS mutation, vaso-occlusion), hereditary spherocytosis (membrane defect), G6PD deficiency (oxidative hemolysis), and thalassemias (globin chain imbalance). The reticulocyte count reflects marrow response to anemia, useful in distinguishing hypoproliferative vs. hemolytic/blood loss anemias.