ANEMIA - A Complete MBBS First Year Account

DEFINITION

PART 1 - ANATOMY

1.1 Anatomy of the Bone Marrow - Primary Site of Erythropoiesis

• In adults, red (active) bone marrow is found in: flat bones (sternum, ribs, vertebrae, skull, pelvis, sacrum) and the proximal ends of long bones (femur, humerus).
• Yellow bone marrow (fat) fills the shafts of long bones and is inactive, but can be reconverted to red marrow under conditions of chronic anemia or increased demand.

• In severe chronic anemia (e.g., thalassemia, sickle cell disease), there is marked expansion of the erythropoietic tissue. This leads to:
  • Widening of medullary cavities in long bones (seen on X-ray)
  • "Hair-on-end" appearance of skull on X-ray (perpendicular trabeculae due to marrow expansion)
  • "Crew-cut" appearance on skull X-ray in thalassemia
  • Frontal bossing and prominence of malar eminences due to marrow expansion into facial bones

• When the bone marrow cannot meet demand, hematopoiesis reverts to fetal sites: liver, spleen, and lymph nodes.
• In the fetus: yolk sac (first 8 weeks) → liver and spleen (8 weeks to birth) → bone marrow becomes the sole site by 8-10 weeks postnatally.
• In adults with severe anemia (thalassemia, myelofibrosis): EMH causes hepatosplenomegaly, a key clinical sign.

1.2 Anatomy of the Spleen - Relevant to Hemolytic Anemia

• Located in the left hypochondrium, between ribs 9-11, measuring approximately 12 cm × 7 cm × 4 cm (weight ~150 g)
• The red pulp (sinusoids and cords of Billroth) is where old RBCs are destroyed by macrophages (extravascular hemolysis)
• The white pulp houses lymphoid tissue
• In hemolytic anemias, the spleen enlarges due to hyperfunction (work hypertrophy) - this is called congestive splenomegaly or work hypertrophy
• In thalassemia, spleen may become massively enlarged due to both EMH and RBC destruction

1.3 Anatomy of Relevant Blood Supply and Lymphatics

• The portal circulation is relevant because iron absorbed from the duodenum enters the portal blood and reaches the liver for storage as ferritin.
• The duodenum and proximal jejunum are the primary sites of iron absorption - the anatomy of the GI tract is therefore directly relevant to iron-deficiency anemia.
• Lymph nodes can be enlarged in hemolytic anemia due to hyperplastic red pulp of lymphoid tissue.

PART 2 - PHYSIOLOGY

2.1 Normal Red Blood Cell Values

• MCV (fL) = Hct × 10 / RBC count (in millions/µL)
• MCH (pg) = Hb × 10 / RBC count
• MCHC (g/dL) = Hb × 100 / Hct

• Macrocytes: MCV > 95 fL
• Microcytes: MCV < 80 fL
• Hypochromic: MCHC < 25 g/dL

2.2 Erythropoiesis - The Physiology of RBC Production

Pluripotent Stem Cell
    ↓
CFU-GEMM (Colony Forming Unit - Granulocyte, Erythroid, Monocyte, Megakaryocyte)
    ↓
BFU-E (Burst-Forming Unit - Erythroid)
    ↓
CFU-E (Colony Forming Unit - Erythroid)
    ↓
Pronormoblast (Proerythroblast) ← First identifiable erythroid precursor
    ↓
Basophilic Normoblast (4 mitotic divisions occur during normoblast stages)
    ↓
Polychromatophilic Normoblast
    ↓
Orthochromatic Normoblast
    ↓ (Nucleus extruded here)
Reticulocyte (released to circulation; circulates 1-2 days; retains ribosomes and mRNA; 
matures in SPLEEN where ribosomes/mRNA are lost)
    ↓
Mature Red Blood Cell (lifespan ~120 days)

• Produced by peritubular interstitial cells of the kidney (90%) and liver (10%) in response to hypoxia
• Acts on CFU-E and BFU-E through the JAK2/STAT5 pathway
• Stimulates proliferation, differentiation, and prevents apoptosis of erythroid precursors
• In anemia → tissue hypoxia → kidneys produce more EPO → increased erythropoiesis
• In chronic renal disease → EPO deficiency → normocytic normochromic anemia (anemia of renal disease)

2.3 Classification of Anemia

Classification 1 - By MCV (Wintrobe's Morphological Classification)

1. Iron deficiency anemia (most common worldwide)
2. Thalassemia (defects in Hb chain synthesis)
3. Anemia of chronic disease (iron sequestration)
4. Sideroblastic anemia

1. Acute blood loss
2. Hemolytic anemia (early)
3. Aplastic anemia
4. Anemia of renal disease (EPO deficiency)
5. Anemia of chronic disease (some cases)

1. Megaloblastic - Vitamin B12 deficiency, Folate deficiency (defective DNA synthesis)
2. Non-megaloblastic - Liver disease, Hypothyroidism, Drugs (hydroxyurea, methotrexate)

Classification 2 - By Mechanism (Reticulocyte-Based)

• Nutritional deficiency (iron, B12, folate)
• Bone marrow failure (aplastic anemia)
• Marrow replacement (infiltration by tumor, granulomas, infections)
• EPO deficiency (renal disease, chronic inflammation)
• Stem cell defects (myelodysplasia, leukemia)

• Intrinsic RBC defects: Hereditary spherocytosis, G6PD deficiency, Sickle cell disease, Thalassemia
• Extrinsic causes: Autoimmune hemolytic anemia, microangiopathic hemolytic anemia, infections (malaria)

• Acute: normocytic normochromic initially; reticulocyte count rises in 5-7 days
• Chronic: leads to iron deficiency → microcytic hypochromic anemia

2.4 Iron Deficiency Anemia - Physiological Perspective

1. Inadequate dietary intake
2. Malabsorption (celiac disease, gastrectomy)
3. Chronic blood loss (GI bleeding, menorrhagia, hookworm)
4. Increased demand (pregnancy, growth spurts in adolescents)

• A serum ferritin < 30 µg/L = body iron stores depleted.
• Ferritin is an acute-phase reactant; in inflammation it may be falsely normal.
• Serum ferritin > 200 µg/L = adequate iron stores even in inflammation.

1. Pre-latent: Depletion of iron stores (↓ ferritin); no anemia
2. Latent: ↓ serum iron, ↑ TIBC, ↑ transferrin receptor; no anemia
3. Overt IDA: ↓ Hb, ↑ RDW, microcytic hypochromic anemia; symptoms appear

• Fatigue, weakness, pallor (conjunctival, palmar, nail bed)
• Dyspnea on exertion (reduced O2 carrying capacity)
• Palpitations, tachycardia (compensatory increase in cardiac output)
• Pallor of mucous membranes

• Koilonychia (spoon-shaped nails)
• Angular stomatitis, glossitis (atrophic tongue)
• Pica (craving for non-food items - chalk, mud)
• Plummer-Vinson syndrome (post-cricoid dysphagia + IDA + angular stomatitis)

2.5 Hemolytic Anemia - Physiological Points

• High LDH (enzyme abundant in RBCs, released on destruction)
• Low haptoglobin (serum protein that salvages free Hb; gets consumed)
• High indirect bilirubin (from Hb catabolism → jaundice)
• Increased urobilinogen in urine
• Elevated reticulocyte count (compensatory erythropoiesis)

• Intravascular hemolysis: destruction within vessels (e.g., G6PD, transfusion reactions) → hemoglobinuria
• Extravascular hemolysis: destruction by macrophages in spleen/liver (e.g., hereditary spherocytosis)

• Normal RBCs begin hemolyzing at 0.5% NaCl; 50% lysis at 0.40-0.42% NaCl; complete lysis at 0.35% NaCl.
• In hereditary spherocytosis, osmotic fragility is increased (hemolysis occurs at higher NaCl concentrations) because spherocytes have less membrane reserve than biconcave discs. (Ganong's Physiology)

2.6 Vitamin B12 and Folate Deficiency - Megaloblastic Anemia

• MCV > 95 fL (macrocytes)
• Hypersegmented neutrophils (5+ lobes) - pathognomonic
• Pancytopenia in severe cases
• Megaloblastic changes in bone marrow

• Subacute Combined Degeneration of Spinal Cord (demyelination of posterior and lateral columns)
• Neurological: loss of vibration sense, ataxia, paraesthesias

PART 3 - BIOCHEMISTRY

3.1 Structure of Hemoglobin

• Globin portion: 4 polypeptide chains (protein)
• Heme portion: porphyrin ring (protoporphyrin IX) + Fe²⁺ (non-protein)

• O₂ binds to Fe²⁺ in heme (ferrous form; oxidation to Fe³⁺ → methemoglobin - cannot carry O₂)
• O₂ affinity is affected by: pH (Bohr effect), temperature, 2,3-BPG concentration
• 2,3-BPG binds to deoxyHb, stabilizes the T (tense/deoxy) state, reduces O₂ affinity
• At high altitude or in anemia: 2,3-BPG rises → RBC delivers more O₂ to tissues

3.2 Heme Biosynthesis - The Complete Pathway

• Mitochondria: Steps 1, 6, 7, 8
• Cytoplasm: Steps 2, 3, 4, 5

Pathway:

• Enzyme: ALA Synthase (ALAS) - rate-limiting, committed step
• Cofactor: Pyridoxal Phosphate (PLP / Vitamin B6)
• Two isoforms: ALAS1 (all tissues, regulated by heme via feedback inhibition) and ALAS2 (erythroid-specific, regulated by iron)
• Hemin (heme with Fe³⁺) inhibits ALAS1 by: repressing gene transcription, increasing mRNA degradation, reducing mitochondrial import of ALAS protein

• Enzyme: ALA Dehydratase (PBGS)
• This enzyme is inhibited by lead (Pb²⁺) → accumulation of ALA → lead poisoning causes anemia + neurological symptoms

• Enzyme: PBG Deaminase (Uroporphyrinogen I Synthase)
• Deficiency → Acute Intermittent Porphyria (AIP)

• Enzyme: Uroporphyrinogen III Synthase
• Deficiency → Congenital Erythropoietic Porphyria (CEP)

• Enzyme: Uroporphyrinogen Decarboxylase
• Deficiency → Porphyria Cutanea Tarda (PCT) - most common porphyria

• Enzyme: Coproporphyrinogen Oxidase
• Deficiency → Hereditary Coproporphyria (HCP)

• Enzyme: Protoporphyrinogen Oxidase
• Deficiency → Variegate Porphyria (VP)

• Enzyme: Ferrochelatase (Heme Synthase)
• Inhibited by lead (another mechanism of lead-induced anemia)
• Requires Fe²⁺ (not Fe³⁺)
• In iron deficiency: ferrochelatase cannot incorporate iron → red cell protoporphyrin IX accumulates (diagnostic marker)

Summary Table: Porphyrias

3.3 Iron Metabolism - Biochemistry

1. Dietary non-heme Fe³⁺ is reduced to Fe²⁺ by Duodenal Cytochrome b (Dcytb) on brush border membrane, or by dietary Vitamin C
2. Fe²⁺ enters enterocytes via DMT1 (Divalent Metal Transporter 1 / SLC11A2)
3. Heme iron enters via separate heme transporter; Heme Oxygenase releases Fe from heme intracellularly
4. Inside enterocyte: iron is either stored as ferritin OR exported via Ferroportin (basolateral membrane)
5. On export, Fe²⁺ is oxidized to Fe³⁺ by Hephaestin (a copper-containing ferroxidase)
6. Fe³⁺ binds to transferrin in plasma for transport (each transferrin binds 2 Fe³⁺)
7. Transferrin receptor 1 (TfR1) on erythroid precursors and other cells endocytoses the Fe-transferrin complex

• Ferritin is the primary iron storage protein; found in liver, spleen, bone marrow
• Apoferritin (protein shell) + iron = Ferritin
• Hemosiderin = degraded ferritin aggregates; insoluble iron storage form seen in iron overload (stains blue with Prussian Blue stain)
• Serum ferritin reflects body iron stores - very low in IDA

• 25-amino acid peptide synthesized by liver as an 84-amino acid precursor (prohepcidin)
• Binds to ferroportin → triggers its internalization and degradation
• Effect: blocks iron absorption from intestine AND blocks iron recycling from macrophages
• When iron is high → hepcidin ↑ → ferroportin ↓ → iron absorption ↓ (protective)
• When iron is low → hepcidin ↓ → ferroportin ↑ → iron absorption ↑
• In chronic inflammation: IL-6 stimulates hepcidin production → ferroportin degradation → iron trapped in macrophages → low serum iron despite adequate iron stores → Anemia of Chronic Disease/Inflammation

• IRP (Iron Regulatory Protein) binds IRE (Iron Response Element) on mRNA
• Ferritin mRNA has IRE in 5' UTR; TfR1 mRNA has IRE in 3' UTR
• When iron is LOW: IRP binds IRE on ferritin mRNA (5' UTR) → blocks translation of ferritin (no storage needed); IRP binds IRE on TfR1 mRNA (3' UTR) → protects mRNA from degradation → more TfR1 synthesized (to capture more transferrin)
• When iron is HIGH: Iron binds IRP (forms 4Fe-4S cluster) → IRP released from IREs → Ferritin mRNA translated (iron stored); TfR1 mRNA degraded (no more uptake needed)
• Result: a reciprocal, elegant feedback system maintaining cellular iron balance

3.4 Vitamin B12 and Folate - Biochemistry of Megaloblastic Anemia

• Dietary folate → THF (Tetrahydrofolate) by Dihydrofolate Reductase (DHFR)
• THF is the active coenzyme form; accepts and donates 1-carbon units
• Required for: Purine synthesis, dTMP synthesis (thymidylate synthesis via thymidylate synthase)
• Folate deficiency → impaired dTMP synthesis → impaired DNA replication → megaloblastic anemia

• Required for 2 reactions:
  1. Methylmalonyl-CoA → Succinyl-CoA (enzyme: Methylmalonyl-CoA Mutase) - used in fatty acid and amino acid catabolism
  2. Homocysteine → Methionine (enzyme: Methionine Synthase) - requires methyl-THF as methyl donor
• Methyl Trap Hypothesis: In B12 deficiency, 5-methyl-THF cannot donate its methyl group (because B12-dependent methionine synthase is non-functional) → THF becomes trapped as 5-methyl-THF → functional folate deficiency (even if folate levels are adequate) → megaloblastic anemia
• B12 deficiency also → methylmalonyl-CoA accumulates → incorporated into abnormal odd-chain fatty acids → neurological damage (subacute combined degeneration)
• This is why B12 deficiency causes both megaloblastic anemia AND neurological symptoms, while folate deficiency causes ONLY megaloblastic anemia (no neurological features)

• B12 deficiency: ↑ Serum methylmalonic acid + ↑ homocysteine
• Folate deficiency: ↑ Serum homocysteine only (methylmalonic acid normal)

3.5 Hemoglobin Catabolism - Relevant to Hemolytic Anemia

Aged RBC (120 days) → destroyed by splenic macrophages
    ↓
Hemoglobin → Globin (amino acid pool) + Heme
    ↓
Heme → Fe²⁺ (recycled) + Protoporphyrin IX
    ↓
Protoporphyrin IX → Biliverdin (green) by Heme Oxygenase
    ↓
Biliverdin → Bilirubin (yellow, unconjugated/indirect) by Biliverdin Reductase
    ↓
Bilirubin transported in plasma bound to Albumin → Liver
    ↓
Liver: Bilirubin + 2 Glucuronate → Bilirubin diglucuronide (conjugated/direct) by UDP-Glucuronosyl Transferase
    ↓
Secreted into bile → Intestine
    ↓
Urobilinogen (by gut bacteria) → Urobilin (excreted in feces, gives brown color)
    ↓ some urobilinogen reabsorbed
Portal blood → Liver (re-excreted) + small amount → Kidney → Urine (urobilinogen)

• Excess RBC destruction → excess unconjugated bilirubin (pre-hepatic jaundice)
• Urine: dark urobilinogen (NO bilirubin in urine since unconjugated bilirubin is not water-soluble and cannot be filtered by kidney)
• In intravascular hemolysis: free Hb in plasma → binds haptoglobin (Hb-Hp complex cleared rapidly by liver) → haptoglobin depleted → hemoglobinemia → hemoglobinuria (dark/red urine)

PART 4 - IMPORTANT SPECIFIC ANEMIAS (Integrated)

4.1 Sickle Cell Anemia - AIT Integration

• Chronic hemolytic anemia → splenomegaly in childhood; repeated infarcts cause progressive splenic atrophy ("autosplenectomy") in adults → susceptibility to encapsulated organisms (Streptococcus pneumoniae)
• Avascular necrosis of femoral head (common)
• "Hair-on-end" skull X-ray (marrow expansion)
• Dactylitis (hand-foot syndrome in infants) - painful swelling due to bone infarcts

4.2 Thalassemia - AIT Integration

• α-Thalassemia: gene deletions on chromosome 16 (HBA1/HBA2 genes); 4 α-globin genes per cell
  • 1 deletion: silent carrier
  • 2 deletions: α-thalassemia trait (mild microcytic anemia)
  • 3 deletions: HbH disease (HbH = β₄ tetramers; Hb 7-10 g/dL; splenomegaly)
  • 4 deletions: Hydrops fetalis (fatal in utero; Hb Bart = γ₄)
• β-Thalassemia: mutations in β-globin gene on chromosome 11; splice-junction, promoter, nonsense mutations
  • β-thalassemia minor (trait): mild microcytic hypochromic anemia, HbA₂ > 3.5%
  • β-thalassemia major (Cooley's anemia): severe transfusion-dependent anemia

PART 5 - EXAM-READY QUICK TABLES

Comparison Table: Iron Deficiency vs Megaloblastic vs Hemolytic Anemia

Peripheral Blood Smear - Key Abnormal RBC Morphologies

PART 6 - KEY MNEMONICS FOR EXAMS

• Steps 1 → Mitochondria
• Steps 2-5 → Cytoplasm
• Steps 6-8 → Mitochondria Remember: First and Last 3 in Mitochondria; Middle in Cytoplasm

• T - Thalassemia
• A - Anemia of chronic disease
• I - Iron deficiency
• L - Lead poisoning
• S - Sideroblastic anemia

• Dcytb → reduces Fe³⁺ to Fe²⁺
• DMT1 → transports Fe²⁺ into enterocyte
• Ferroportin → exports Fe from enterocyte
• Hephaestin → oxidizes Fe²⁺ to Fe³⁺ for transferrin binding
• Transferrin → carries iron in plasma

SOURCES

• Harrison's Principles of Internal Medicine, 22nd Edition (McGraw Hill, 2025)
• Harper's Illustrated Biochemistry, 32nd Edition
• Lippincott's Illustrated Reviews: Biochemistry, 8th Edition
• Ganong's Review of Medical Physiology, 26th Edition
• Basic Medical Biochemistry: A Clinical Approach, 6th Edition