Pathophysiology Erythrocyte Physiology

A review of RBC physiology is important to understand the pathophysiology of SCD. Pediatric RBCs possess normal hemoglobin concentrations between 9 and 18.5 g/dL (90-185 g/L or 5.6-11.5 mmol/L), depending on the age of the child.7 Adult RBCs contain 12 to 15.5 g/dL (120-155 g/L or 7.4-9.6 mmol/L) hemoglobin. The predominant form of hemoglobin is adult hemoglobin or HbA (96%). Other forms of hemoglobin include HbA2 and fetal hemoglobin (HbF). HbA2 makes up less than 1% of hemoglobin in newborns. In adults, HbA2 constitutes approximately 1.6% to 3.5%. Fetal hemoglobin (HbF) is present primarily in fetal RBCs (60-90%), whereas adult RBC's contain less than 1% HbF. HbF is the primary oxygen transport protein in the fetus. After birth, only a small proportion of red cell clones remain to produce HbF. The elasticity of young erythroid cells enables them to deform and squeeze through capillaries (Fig. 68-2). As RBCs age, mean corpuscular hemoglobin concentration (MCHC) increases, deformability decreases, and the cells are removed by the reticuloendothelial system. Impaired circulation, destruction of RBCs, and vascular stasis are three known problems that are primarily responsible for the clinical manifestations of SCD (Fig. 68-3).

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