1. MLAB 1415: Hematology Keri Brophy-Martinez
Chapter 5: Erythrocytes
Part Two

2. Red Blood Cell Membrane
Structure
Trilaminar, three- dimensional
Outermost layer: glycolipids, glycoproteins
Central layer: cholesterol, phospholipids
Inner layer: cytoskeleton

3. 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

4. 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

5. 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

6. 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

7. Glycolysis or Embden-Meyerhof Pathway
Generates % 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

8. 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

9. 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.

10. 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

11. 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

12. 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.

13. Erythrocyte Destruction
Two Paths
Extravascular
Intravascular

14. 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.

15. FIGURE 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.

16. 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).

17. FIGURE 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.

18. References
Diggs, L., Strum, D., & Bell, A. (1975). The Morphology of Human Blood Cells. North Chicago: Abbott laboratories.
McKenzie, S. B., & Williams, J. L. (2010). Clinical Laboratory Hematology . Upper Saddle River: Pearson Education, Inc.