Clinical Hematology - Blood's Cellular Elements - RBC, HGB

What is the primary role of mature Erythrocytes?

To transport oxygen (O₂) from the lungs to tissues via hemoglobin, assist in returning carbon dioxide (CO₂) to the lungs, and buffer blood pH via carbonic anhydrase.

What key structural and cytoplasmic changes occur during erythroid maturation?

Progressive decrease in cell size, nuclear chromatin condensation, loss of nucleoli, nuclear extrusion (at the orthochromatic stage), and cytoplasm shift from basophilic (RNA-rich) to acidophilic (hemoglobin-rich).

What are the 6 stages of erythroid maturation in order?

1. Pronormoblast (Rubriblast)

2. Basophilic Normoblast (Prorubricyte)

3. Polychromatic Normoblast (Rubricyte)

4. Orthochromatic Normoblast (Metarubricyte)

5. Reticulocyte (Polychromatophilic Erythrocyte)

6. Mature Erythrocyte

Which erythrocyte precursor is the last stage capable of mitosis?

Polychromatic Normoblast (Rubricyte).

What are the main lipid components of the erythrocyte membrane?

Phospholipids (40%, asymmetric outer/inner leaflets), Unesterified Cholesterol (30%, determines fluidity), and Glycolipids (10%, form blood group antigens).

What proteins form the erythrocyte membrane cytoskeleton, and what are their functions?

Spectrin (α/ß dimers): Forms elastic meshwork.

Ankyrin & Band 3 / Protein 4.2: Form vertical linkages to the lipid bilayer.

Protein 4.1 & Actin / Glycophorin C: Form horizontal linkages between spectrin tetramers.

What structural defects cause Spherocytes and Elliptocytes?

• Spherocytes: Vertical link defects (loss of spectrin, ankyrin, or Band 3) leading to membrane microvesicle loss.

• Elliptocytes: Horizontal link defects (disruption of spectrin self-association or protein 4.1).

What membrane changes cause Acanthocytes and Target Cells?

• Acanthocytes: Excess outer leaflet cholesterol relative to phospholipids.

• Target Cells: Excess total membrane surface area relative to intracellular hemoglobin volume.

What is the main energy-producing pathway in erythrocytes, and why is ATP necessary?

Embden-Meyerhof Glycolytic Pathway (90%): Generates net 2 ATP to power active membrane cation pumps (Na⁺/K⁺ -ATPase and Ca²⁺ -ATPase) to maintain cell shape, volume, and flexibility.

What is the function of the Hexose Monophosphate (HMP) Shunt in RBCs?

Uses G6PD to generate NADPH, which maintains reduced glutathione to protect hemoglobin and membrane proteins from oxidative denaturation (preventing Heinz body formation).

What is the function of 2,3-BPG generated by the Luebering-Rapoport Pathway?

Binds to the central cavity of deoxyhemoglobin, decreasing oxygen affinity and promoting $O_2$ release to hypoxic tissues.

What is the function of the Methemoglobin Reductase Pathway?

Uses NADH and cytochrome b5 reductase to reduce non-functional ferric iron (Fe3⁺) back to functional ferrous iron (Fe²⁺).

What is the difference between Extravascular and Intravascular Hemolysis?

• Extravascular (~90%): Macrophages in splenic red pulp phagocytose aged/rigid RBCs and recycle iron, globin, and bilirubin.

• Intravascular (~10%): RBCs break down inside circulating vessels; free cell-free Hb is scavenged by plasma haptoglobin.

What is the structural composition of adult Hemoglobin A (Hb A)

A tetramer composed of 4 globin polypeptide chains (2α and 2ß), 4 heme groups (each with a central Fe²⁺ iron), and 1 central cavity for 2,3-BPG binding.

What are the subunit compositions of Hb A, HbA₂, and Hb F

• Hb A: α₂ß₂ (>95% of adult Hb)

• Hb A₂: α₂δ₂ (1.5-3.5% of adult Hb)

• Hb F: α₂γ₂ (<1-2% of adult Hb, predominant fetal Hb)

Where does heme synthesis occur, and what is its rate-limiting step?

Occurs in the mitochondria and cytoplasm. The rate-limiting step is the condensation of Glycine + Succinyl-CoA to form δ-ALA via δ-ALA synthase (requires Vitamin B₆).

How is iron incorporated into protoporphyrin IX to complete heme synthesis?

The enzyme Ferrochelatase (Heme Synthase) inserts ferrous iron (Fe²⁺) into Protoporphyrin IX inside the mitochondria.

How does Heme regulate hemoglobin synthesis?

High free heme inhibits δ-ALA synthase (preventing excess porphyrin production) and stimulates globin mRNA translation on ribosomes.

What factors cause a RIGHT SHIFT in the Oxygen Dissociation Curve?

CADET, face Right: Increased CO₂, Acidosis (↑H⁺/↓pH - Bohr Effect), Increased DPG/2,3-BPG, Exercise, Increased Temperature. (Decreases O₂ affinity; increases tissue delivery).

What factors cause a LEFT SHIFT in the Oxygen Dissociation Curve?

Decreased CO₂, Alkalosis (↑ pH), Decreased 2,3-BPG, Decreased Temperature, Hb F (Fetal Hb), Carboxyhemoglobin, and Methemoglobin. (Increases O₂ affinity; decreases tissue delivery).

How is carbon dioxide (CO₂) transported in the blood?

1. Bicarbonate ions (HCO₃^- ~70%): Generated via carbonic anhydrase and transported in exchange for Cl^- (Chloride Shift).

2. Carbaminohemoglobin (~20%): Bound to globin chains.

3. Dissolved gas (~10%): In plasma solution.

What are the breakdown products of Hemoglobin during extravascular destruction?

• Globin: Broken into amino acids for reuse.

• Iron: Stored as ferritin/hemosiderin or carried by transferrin.

• Protoporphyrin IX: Converted to biliverdin → unconjugated bilirubin → excreted in bile.

What is Methemoglobin, what causes it, and how is it treated?

Hemoglobin with oxidized ferric iron (Fe³⁺) that cannot bind O₂. Caused by oxidant exposure (nitrates, benzocaine). Blood turns chocolate brown; treated with IV methylene blue.

What is Carboxyhemoglobin and how is it treated?

Hemoglobin bound to carbon monoxide (CO), which has a 200–250x higher affinity than O₂. Blood turns cherry red; treated with 100% high-flow oxygen or hyperbaric oxygen.

What is Sulfhemoglobin and why is it unique among nonfunctional hemoglobins?

Hemoglobin with an incorporated sulfur atom caused by sulfur-containing drugs or phenazopyridine. Blood turns brownish-green. It is irreversible and cannot be treated chemically (persists for the life of the RBC).