2.2 Erythrocyte Physiology, Hemoglobin & Anemias

Erythrocyte Physiology, Hemoglobin & Anemias

Quick Answer: Erythrocytes (RBCs) transport oxygen via hemoglobin. Anemias are classified morphologically by MCV (microcytic, normocytic, macrocytic) and pathophysiology. Wright-stained peripheral smears offer critical clues through RBC inclusions and variations in shape (poikilocytosis).

Hemoglobin Structure and Function

Hemoglobin is a specialized transport tetramer comprised of four heme groups (each containing an iron atom in the ferrous Fe2+ state capable of reversibly binding oxygen) and four globin (protein) chains. The type of globin chains determines the hemoglobin variant:

• HbA: Normal adult hemoglobin (α2β2), making up ~95-98% of hemoglobin in a healthy adult.
• HbA2: Minor adult hemoglobin (α2δ2), making up ~2-3%.
• HbF: Fetal hemoglobin (α2γ2), making up <2% in adults, but it is the predominant form in fetuses and newborns. HbF has a higher affinity for oxygen, allowing the fetus to extract oxygen from maternal blood.

Hemoglobin function is depicted by the oxygen dissociation curve. A "Right Shift" indicates decreased oxygen affinity (meaning hemoglobin more readily releases oxygen to the tissues). This occurs under conditions of high metabolic demand: high body temperature, high 2,3-DPG, high pCO2, and low pH (the Bohr effect). Conversely, a "Left Shift" means increased affinity (holds onto oxygen tighter), occurring with low temperature, low 2,3-DPG, and high pH, as well as with HbF or carbon monoxide poisoning.

RBC Inclusions and Wright Stain Morphology

When evaluating a peripheral blood smear, observing variations in RBC size (anisocytosis), shape (poikilocytosis), and intracellular inclusions provides essential diagnostic clues. Standard hematology utilizes the Romanowsky-based Wright stain or Wright-Giemsa stain.

Classification of Anemias

Anemias represent a reduction in the oxygen-carrying capacity of the blood. They are classically categorized morphologically by MCV (size) and MCHC (color), and pathophysiologically by mechanism (decreased production vs. increased destruction/loss).

1. Microcytic, Hypochromic Anemias (MCV < 80 fL, MCHC < 32 g/dL)

These conditions involve defective hemoglobin synthesis. Since hemoglobin is composed of iron, heme, and globin, a defect in any of these components leads to microcytosis.

• Iron Deficiency Anemia (IDA): The most common anemia worldwide, caused by chronic blood loss or dietary deficiency. Iron stores are depleted before anemia manifests.
  • Lab Profile: Low serum iron, Low ferritin (the best and earliest indicator of depleted stores), High TIBC (Total Iron Binding Capacity, reflecting increased transferrin production by the liver), and a low % saturation.
• Thalassemia: An inherited defect in the rate of globin chain synthesis (alpha or beta).
  • Morphology: Prominent target cells (codocytes) and basophilic stippling.
  • Lab Profile: Iron studies are typically normal to high (normal/high ferritin, normal TIBC). Diagnosis requires Hemoglobin Electrophoresis or HPLC, which may show elevated HbA2 or HbF in beta-thalassemias.
• Sideroblastic Anemia: A defect in the protoporphyrin synthesis pathway (heme synthesis), often acquired via lead poisoning or drugs, or inherited.
  • Diagnostic Hallmark: Ringed sideroblasts in the bone marrow (iron trapped in the mitochondria surrounding the nucleus, visualized with Prussian Blue stain).
• Anemia of Chronic Disease (ACD): Can be normocytic or microcytic. Characterized by a block in iron release from macrophages due to the inflammatory cytokine hepcidin.
  • Lab Profile: Low serum iron, High ferritin (iron is trapped in storage), and a normal/low TIBC.

2. Macrocytic Anemias (MCV > 100 fL)

Macrocytic anemias are subdivided into megaloblastic (impaired DNA synthesis) and non-megaloblastic.

• Megaloblastic Anemia: Caused by Vitamin B12 or Folate deficiency. The lack of these vitamins impairs DNA replication, causing nuclear-cytoplasmic asynchrony (the cytoplasm matures and expands while the nucleus lags behind).
  • Morphology: Oval macrocytes, hypersegmented neutrophils (finding neutrophils with 6 or more lobes is highly specific), and Howell-Jolly bodies.
  • Pernicious Anemia: A specific autoimmune type of B12 deficiency caused by autoantibodies destroying gastric parietal cells, leading to a lack of Intrinsic Factor, which is required for B12 absorption in the ileum.
• Non-Megaloblastic Anemia: Associated with liver disease, alcoholism, and hypothyroidism.
  • Morphology: Round macrocytes and target cells. Neutrophils have normal segmentation.

3. Normocytic, Normochromic Anemias (MCV 80-100 fL)

These often stem from acute blood loss, bone marrow failure, or increased RBC destruction (hemolysis).

• Aplastic Anemia: A life-threatening bone marrow failure syndrome leading to pancytopenia (decreased RBCs, WBCs, and platelets). The marrow is hypocellular and replaced by fat.
• Hemolytic Anemias (Increased Destruction):
  • Hereditary Spherocytosis: A structural defect in the RBC membrane cytoskeleton (spectrin/ankyrin). Cells lose their biconcave shape, become fragile spheres, and are destroyed by the spleen. Labs: Increased MCHC, increased osmotic fragility.
  • Sickle Cell Anemia: A qualitative hemoglobinopathy caused by a point mutation where valine is substituted for glutamic acid at the 6th position of the beta globin chain (HbS). Under low oxygen tension, HbS polymerizes, distorting the cell into a sickle shape (drepanocyte).
  • G6PD Deficiency: An X-linked enzyme defect. Exposure to oxidative stress (fava beans, antimalarial drugs, infections) causes hemoglobin to denature, forming Heinz bodies and bite cells.
  • Microangiopathic Hemolytic Anemias (MAHA): Conditions like Disseminated Intravascular Coagulation (DIC), Thrombotic Thrombocytopenic Purpura (TTP), and Hemolytic Uremic Syndrome (HUS). Fibrin strands in the microvasculature sheer RBCs as they pass through.
  • Diagnostic Hallmark: Schistocytes (RBC fragments) and helmet cells on the peripheral smear.