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MLAB 1415: Hematology Keri Brophy-Martinez Chapter 5: Erythrocytes Part Two Red Blood Cell Membrane • Structure ▫ Trilaminar, threedimensional Outermost layer: glycolipids, glycoproteins Central layer: cholesterol, phospholipids Inner layer: cytoskeleton Cytoskeleton of the RBC Membrane • Components ▫ Spectrin Composed of alpha & beta chains Join to form a matrix which strengthens the membrane against sheer force and controls biconcave shape ▫ Ankyrin Binding site for spectrin Red Blood Cell Membrane • Function ▫ Shape Provides the optimum surface to volume ratio for respiratory exchange AND is essential to deformability ▫ Provide deformability, elasticity Allows for passage through microvessels • Provides permeability ▫ Allows water and electrolytes to exchange via cation pumps ▫ RBC controls volume and H2O content primarily through control of sodium and potassium content Metabolic Pathways • Metabolism ▫ Limited ▫ Energy required for: Maintenance of cation pumps Maintenance of hgb in reduced state Maintenance of reduced sulfhydryl groups in hgb and other proteins Maintenance of RBC integrity and deformability Key Metabolic Pathways for the Erythrocyte • • • • Glycolysis or Embden-Meyerhof pathway Hexose Monophosphate Shunt Methemoglobin reductase pathway Rapoport- Luebering Shunt • Key actions: ▫ Use enzymes to supply energy for the system ▫ Reduce oxidants in the system Glycolysis or Embden-Meyerhof Pathway Generates 90- 95% of energy needed by RBC’s Glucose is metabolized and generates two molecules of ATP (energy). Functions in the maintenance of RBC shape, flexibility and the cation pumps Hexose monophosphate shunt Metabolizes 5-10% of glucose. NADPH is end product Protects the RBC from oxidative injury. Most common defect is deficiency of the enzyme glucose-6-phosphate dehydrogenase (G-6PD). If the pathway is deficient, intracellular oxidants can’t be neutralized and globin denatures then precipitates. The precipitates are referred to as Heinz bodies Methemoglobin Reductase pathway Maintains iron in the ferrous (Fe++) state. In the absence of the enzyme (methemoglobin reductase), methemoglobin accumulates and it cannot carry oxygen. Rapoport –Leubering Shunt Allows the RBC to regulate oxygen transport during conditions of hypoxia or acid-base imbalance. Permits the accumulation of 2,3-DPG which is essential for maintaining normal oxygen tension, regulating hemoglobin affinity Red Blood Cell Metabolism: Summary • Three areas of RBC metabolism are crucial for RBC survival and function. ▫ RBC membrane ▫ Hemoglobin structure and function ▫ RBC metabolic pathways= cellular energy Erythrocyte Destruction • Breakdown of the RBC ▫ Toward the end of 120 day life span of the RBC, it begins to break down. The membrane becomes less flexible. The concentration of cellular hemoglobin increases. Enzyme activity, especially glycolysis, diminishes Removal ▫ Aging RBC’s or senescent RBC’s are removed from the circulation by the reticuloendothelial system (RES) which is a system of fixed macrophages. These cells are located all over the body, but those in the spleen are the most efficient at removing old RBC’s. Erythrocyte Destruction • Two Paths ▫ Extravascular ▫ Intravascular Extravascular Destruction • The RES cells lyse the RBC’s and digest them. Components of the RBC are recycled. ▫ Iron is transported by transferrin to the bone marrow to be recycled into hemoglobin. ▫ Amino acids from globin are recycled into new globin chains. ▫ The protoporphyrin ring from heme is broken and converted into biliverdin ▫ Biliverdin is converted to unconjugated bilirubin and carried to the liver by albumin, a plasma protein. ▫ Bilirubin is conjugated in the liver and excreted into the intestine, where intestinal flora convert it to urobilinogen. ▫ Most urobilinogen is excreted in the stool, but some is picked up by the blood and excreted in the urine. ▫ Conjugated (indirect) and unconjugated (direct) bilirubin can be used to monitor hemolysis. FIGURE 5-6 Most hemoglobin degradation occurs within the macrophages of the spleen. The globin and iron portions are conserved and reutilized. Heme is reduced to bilirubin, eventually degraded to urobilinogen, and excreted in the feces. Thus, indirect indicators of erythrocyte destruction include the blood bilirubin level and urobilinogen concentration in the urine. Intravascular Destruction ▫ The free hemoglobin α and β dimers that are released into the bloodstream is picked up by a protein carrier called haptoglobin. ▫ The haptoglobin-hemoglobin complex is large and cannot be excreted in the urine. It is carried to the liver where the RES cells are and the breakdown process occurs as in extravascular destruction. ▫ If there is an increase in intravascular destruction, the haptoglobin is used up and free hemoglobin is excreted in the urine (hemoglobinuria). FIGURE 5-7 When the erythrocyte is destroyed within the vascular system, hemoglobin is released directly into the blood. Normally, the free hemoglobin quickly complexes with haptoglobin, and the complex is degraded in the liver. In severe hemolytic states, haptoglobin can become depleted, and free hemoglobin dimers are filtered by the kidney. Additionally, with haptoglobin depletion, some hemoglobin is quickly oxidized to methemoglobin and bound to either hemopexin or albumin for eventual degradation in the liver. References • Diggs, L., Strum, D., & Bell, A. (1975). The Morphology of Human Blood Cells. North Chicago: Abbott laboratories. • http://tiny.cc/lwgtg • McKenzie, S. B., & Williams, J. L. (2010). Clinical Laboratory Hematology . Upper Saddle River: Pearson Education, Inc.