Lecture 5: Erythrocytes- function and normal/abnormal destruction

What are the three major functions of erythrocytes?

Transport of oxygen, transport of carbon dioxide, and buffering of hydrogen ions.

By how much does hemoglobin increase the oxygen-carrying capacity of blood compared to plasma?

Approximately 70-fold.

What three factors determine blood oxygen content?

Hemoglobin content, pO2, and hemoglobin oxygen affinity (P50).

What shape is the hemoglobin oxygen dissociation curve and why is this physiologically important?

Sigmoid-shaped; allows efficient O2 loading in the lungs and unloading in tissues.

What is the function of 2,3-DPG in RBCs?

Decreases hemoglobin oxygen affinity, promoting O2 release to tissues.

How is 2,3-DPG produced and how quickly can its levels change in RBCs?

Produced in a side pathway of glycolysis; concentration changes slowly compared to rapid changes in CO2, H+, and temperature.

Why does fetal blood have a lower P50 than maternal blood?

Fetal Hb has higher O2 affinity, advantageous in the low O2 intrauterine environment and facilitates O2 transfer from mother to fetus.

What are the three main forms of carbon dioxide transport in blood and their approximate percentages?

Bicarbonate (~70%), carbaminohemoglobin (~25%), dissolved CO2 (~5%).

How does the bicarbonate buffer system enhance CO2 transport capacity?

Increases CO2 carrying capacity of blood about 17-fold via carbonic anhydrase in RBCs.

What role does deoxyhemoglobin play in CO2 transport compared to oxyhemoglobin?

Deoxyhemoglobin binds ~2x more CO2 than oxyhemoglobin.

What is the primary protein buffer of H+ in blood?

Hemoglobin.

Compare acid strengths of oxyhemoglobin and deoxyhemoglobin.

Deoxyhemoglobin is a weaker acid than oxyhemoglobin.

How does hemoglobin contribute to isohydric CO2 transport?

Buffers protons from carbonic acid, allowing simultaneous transport of CO2 and H+ without major pH change.

Besides carbonic acid buffering, what other acids can hemoglobin buffer?

Organic acids produced by metabolism.

Describe the coupled transport of O2 and CO2 in tissues.

Increased CO2 and H+ promote O2 release from oxyHb; resulting deoxyHb binds more CO2 and H+.

Describe the coupled transport of O2 and CO2 in lungs.

High O2 promotes oxyHb formation, releasing CO2 and H+; H+ reacts with HCO3- to form H2CO3, which dissociates into CO2 and H2O for exhalation.

What are the major lipid components of the erythrocyte membrane and their functional roles?

Phospholipid bilayer (fluidity, asymmetry), unesterified cholesterol (intercalates with FA chains), and glycolipids (outer layer, some as blood group antigens).

What are the two major categories of erythrocyte membrane proteins?

Integral membrane proteins (transmembrane glycoproteins, receptors/transporters like Band 3, and antigens) and skeletal membrane proteins (cytoskeletal lattice providing shape, deformability, and durability).

Why can mammalian erythrocytes not carry out protein or lipid synthesis?

They lack nuclei, ribosomes, mitochondria, and ER, preventing DNA/RNA synthesis, protein synthesis, lipid synthesis, and Krebs cycle metabolism.

What is the primary substrate for erythrocyte energy metabolism and what alternative exists in pigs?

Glucose; pigs can also utilize inosine supplied by the liver.

What percentage of glucose metabolism in erythrocytes occurs via glycolysis (Embden-Meyerhof pathway) vs. the pentose phosphate pathway?

~90-95% glycolysis, ~5-10% pentose phosphate pathway.

What is the function of the diphosphoglycerate (DPG) pathway in RBCs?

Produces 2,3-DPG (important for O2 affinity regulation) but generates no net ATP.

In which species are high concentrations of 2,3-DPG particularly important?

Dogs, horses, and pigs.

What are the major erythrocyte energy requirements?

Operation of Na⁺-K⁺ ATPase, Mg²⁺-ATPase (shape/deformability), limited synthetic activities (e.g., glutathione), maintenance of high [2,3-DPG], methemoglobin reduction, and antioxidant protection.

How is methemoglobin reduced in RBCs?

By NADH generated in glycolysis.

How does the pentose phosphate pathway protect erythrocytes against oxidant injury?

Produces NADPH, which helps regenerate reduced glutathione and defends against oxidative damage.

What are blood group antigens and how are they genetically determined?

Alloantigens (isoantigens), usually carbohydrates (glycolipids or glycoproteins), produced mainly by erythroid cells; encoded at a single chromosomal locus with two or more allelic genes.

Why are blood group antigens clinically significant?

They determine transfusion compatibility; mismatched transfusions may stimulate antibody formation or hemolysis.

After a mismatched transfusion, how long does it typically take for antibodies to appear in plasma?

3-5 days.

What is the purpose of crossmatching before transfusion?

Detects antibodies not identified or predicted by blood typing, preventing transfusion reactions.

What is the major blood group antigen of clinical importance in dogs?

DEA 1.

What are the major blood group antigens of clinical importance in cats?

The AB system and the Mik antigen.

What are the major blood group antigens of clinical importance in horses?

A and Q factors.

What are natural blood group antibodies?

Antibodies against blood group antigens present in plasma without prior transfusion or pregnancy.

Are natural blood group antibodies clinically important in dogs?

No, they are typically absent or not clinically significant.

Are natural blood group antibodies clinically important in cats?

Yes, they are naturally present and can cause severe transfusion reactions.

What natural antibodies are present in type B cats?

Strong anti-A antibodies, present in high amounts and highly hemolytic.

What natural antibodies are present in type A cats?

Weak, variable anti-B antibodies.

Why is the Mik antigen clinically important in cats?

Cats lacking Mik may have natural anti-Mik antibodies, but no commercial test exists for Mik typing.

What are the main sources of oxidants that can cause erythrocyte injury?

Environmental exposures, normal metabolic processes, and certain drugs/compounds that generate high oxidant levels.

What cellular system provides reducing power to protect RBCs from oxidative damage?

NADPH from the pentose phosphate pathway.

What is the role of reduced glutathione (GSH) in protecting erythrocytes?

Functions as a free radical scavenger, donates electrons for reductive enzyme reactions (e.g., glutathione peroxidase), and is regenerated from oxidized GS-SG by NADPH-dependent glutathione reductase.

Which enzymes protect erythrocytes against oxidant injury?

Superoxide dismutase (SOD), glutathione peroxidase (GPx), and glutathione reductase (GR).

What type of oxidative injury occurs to erythrocyte enzymes?

Oxidation of sulfhydryl (-SH) groups, impairing enzyme activity.

What type of oxidative injury occurs to hemoglobin?

Formation of methemoglobin (oxidized Fe³⁺ Hb) and Heinz bodies (denatured Hb precipitates).

What type of oxidative injury occurs to erythrocyte membranes?

Formation of eccentrocytes, increased susceptibility to hemolysis, and enhanced phagocytosis by macrophages.

How does erythrocyte lifespan relate to body size across species?

Lifespan increases with body weight; in domestic mammals it's ~2-5 months.

What age-related process describes programmed death of erythrocytes?

Eryptosis—an apoptosis-like pathway triggered by cumulative oxidant injury.

What membrane changes accumulate with erythrocyte aging?

Oxidant-induced damage with altered phospholipids and carbohydrates.

What is the "senescent antigen" on aging RBCs and how does it form?

Band 3 clustering on the membrane forms a neo-epitope recognized as senescent.

What is the predominant physiologic fate of aged erythrocytes?

Erythrophagocytosis—removal by macrophages, especially in the spleen.

During physiologic turnover, how much intravascular lysis occurs?

Essentially none within the systemic circulation.

What do macrophage receptors recognize on aged erythrocytes?

Damage-associated membrane changes signaling cells for clearance.

Which scavenger-receptor ligand reflects altered phospholipids on aged RBCs?

Increased externalized phosphatidylserine (PS).

Which carbohydrate alteration marks senescent RBCs for clearance?

Desialation of sialoglycoproteins on the outer membrane.

Which protein alteration on aged RBCs promotes recognition by scavengers?

Partially degraded Band 3 protein.

How do antibodies participate in removal of senescent erythrocytes?

Antibodies bind the senescent (altered Band 3) antigen, opsonizing cells.

Which macrophage receptors mediate opsonized RBC clearance?

Fc receptors and C3b receptors facilitate phagocytosis.

Why is methemoglobin unable to transport oxygen?

Its iron is in the oxidized Fe³⁺ state, which cannot bind O₂.

What are the main sources of oxidants that convert hemoglobin to methemoglobin?

Normal metabolic processes and environmental oxidants.

Approximately what percentage of hemoglobin is oxidized to methemoglobin daily?

~3%.

By which enzyme and cofactor is methemoglobin reduced back to functional hemoglobin in erythrocytes?

Cytochrome b5 reductase (Cb5R), using NADH as the electron donor.

What is the normal concentration of methemoglobin in blood as a percentage of total hemoglobin?

Less than 1.5%