Red Blood Cells & Blood Typing – Key Vocabulary

Red Blood Cell (Erythrocyte) Overview

• Smallest human cells; approx. 7–8 µm diameter.
• Lack nucleus & all membrane-bound organelles (no mitochondria, ER, Golgi, etc.).
  • Creates extra intracellular space (≈ 97 % of cytoplasmic volume) for hemoglobin (Hb).
• Shape: biconcave disc (dent on both sides).
  • ↓ diffusion distance → rapid O₂ loading/unloading.
  • ↑ surface-area-to-volume ratio.
  • Flexible “tortilla” – folds & twists through narrow capillaries without rupturing (vs. fragile spherical bubbles).
• Primary job: optimized O₂ transport; secondary: facilitates CO₂ transport.

Hemoglobin (Hb)

• “Super protein” (Power-Ranger analogy).
  • 4 sub-units (globin chains): α₁, α₂, β₁, β₂.
  • Each sub-unit has 1 heme group.
• Heme
  • Porphyrin ring + central Fe²⁺ (iron).
  • Fe²⁺ reversibly binds 1 O₂ molecule.
  • 4 heme sites → 1 Hb can carry 4 O₂.
• Binding characteristics
  • O₂–Hb bond intentionally weak.
  • High-O₂ milieu of lungs drives binding; low-O₂ milieu of tissues drives release (affinity shift).
• Color change
  • Oxyhemoglobin (O₂ bound): bright cherry-red (arterial blood).
  • Deoxyhemoglobin: dark red/ bluish hue (venous blood appears blue through skin).
  • Color difference due to conformational change altering light reflection.
• Transports ~10 % of CO₂ directly; remainder as $\text{HCO}_3^-$ in plasma.

Cellular Respiration Link

• Overall reaction:
  $\text{C}<em>6\text{H}</em>{12}\text{O}<em>6 + 6\,\text{O}</em>2 \;\longrightarrow\; 6\,\text{CO}<em>2 + 6\,\text{H}</em>2\text{O} + \text{ATP}$
• Produced CO₂ and H₂O are expelled by lungs: inhale O₂, exhale CO₂ + water vapor (window-fog example).

Erythrocyte Homeostasis (Erythropoiesis)

• Stimulus: hypoxemia (↓ arterial O₂).
  • “Hypo-” = low; “-emia” = in blood.
• Kidney sensors release erythropoietin (EPO).
• EPO → red bone marrow → ↑ erythrocyte production.
• Requirements for RBC synthesis
  • Iron (heme).
  • Amino acids (globin).
  • Vitamin B₁₂ & folate (B₉) for DNA synthesis.
• Baseline turnover
  • ≈ 1 % of RBC mass replaced daily (~3 million new RBC ⋅ s⁻¹).

Life Span & Destruction

• Functional life ≈ 120 days (no DNA → no self-repair machinery).
• Senescent RBCs phagocytosed by macrophages in:
  • Spleen (main “RBC graveyard”).
  • Liver.
  • Red bone marrow.
• Phagocytosis sequence
  1. Macrophage engulfs RBC → phagosome.
  2. Phagosome fuses with lysosome → digestion.

Hemoglobin Recycling

• Globin chains → hydrolyzed into amino acids → reused for new proteins (albumin, clotting factors, etc.).
• Heme split into:
  1. Iron (Fe²⁺)
  • Transported in blood bound to transferrin ("iron in transit").
  • Stored in liver as ferritin.
  • Mobilized back to marrow for erythropoiesis.
  1. Biliverdin → bilirubin.
  • Liver absorbs bilirubin → conjugates → excretes into bile.
  • Pathways:
    • Part reabsorbed enterohepatically.
    • Excess → feces (bacterial action → brown color).
    • Small amount → urine (yellow color).
• Disruptions
  • Gallstones blocking bile duct → bilirubin back-up → jaundice.
  • Neonatal jaundice: immature liver cannot conjugate bilirubin; treated with blue-light phototherapy (isomerizes bilirubin for renal excretion).

Bile & Fat Digestion

• Bile acts as detergent (micelle formation).
  • Molecule has hydrophilic head (interfaces with watery chyme) & hydrophobic tail (interfaces with lipid).
  • Emulsifies fats → ↑ surface area for pancreatic lipase.
• Produced continuously by liver; stored & concentrated in gallbladder.
• Absence of gallbladder
  • No storage reservoir → limited bile surge.
  • High-fat meals → bloating, cramping, steatorrhea/diarrhea.

Gas Exchange Dynamics

• In Pulmonary Capillaries (high plasma O₂): Hb binds O₂.
• In Systemic Capillaries (low plasma O₂): Hb releases O₂.
• Rule of thumb:
  • "High plasma O₂ ⇒ loading; low plasma O₂ ⇒ unloading."

ABO Blood Group System

Key Terms

• Antigen = molecule capable of triggering immunity.
• Antibody = plasma protein that binds specific antigen; drives response.
• A & B antigens reside on RBC membrane glycoproteins.
• Pre-formed antibodies to absent antigens appear within first year of life (no prior transfusion needed).

Antigen–Antibody Matrix

Compatibility Logic (Dinner-Party Analogy)

• Recipient’s antibodies attack donor antigens they perceive as “foreign.”
• Summaries (ignoring Rh for now):
  • A recipients ACCEPT A or O.
  • B recipients ACCEPT B or O.
  • AB recipients ACCEPT A, B, AB, or O (no antibodies).
  • O recipients ACCEPT only O (have antibodies versus A & B).
• Donor perspective (who they can give to):
  • O → A, B, AB, O.
  • A → A, AB.
  • B → B, AB.
  • AB → AB only.

Common Misconceptions

• “Universal donor/recipient” & Rh (±) factor not covered in this section; must evaluate separately.

Quantitative & Miscellaneous Facts

• 1 RBC carries ≈ 250 million Hb molecules → ≈ 1 billion O₂ molecules per cell.
• Average adult: 4–6 ×10¹² RBC ⋅ L⁻¹ blood.
• Production rate: $\approx 3\times10^6\;\text{cells s}^{-1}$.
• RBC mass replaced daily: ≈ 1 %.
• Hb concentration: males ≈ 13–18 g ⋅ dL⁻¹; females ≈ 12–16 g ⋅ dL⁻¹.

Clinical Correlations

• Anemia: ↓ RBC count/Hb → tissue hypoxia → ↑ EPO.
• Polycythemia: ↑ RBC → ↑ blood viscosity → cardiovascular strain.
• Erythropoietin as performance-enhancing drug (“blood doping”).
• Jaundice assessments: scleral icterus in deeply pigmented individuals.
• Phototherapy for neonates: converts unconjugated bilirubin to water-soluble forms.
• Spleen/liver injury can release large amounts of Hb breakdown products into plasma → hyperbilirubinemia.

Key Take-Home Connections

• Structure dictates function: loss of nucleus/organelles & biconcave shape maximize O₂ transport efficiency.
• Hb simultaneously manages O₂ delivery & partial CO₂ return; color changes reveal saturation.
• Iron/bilirubin recycling highlights body’s material efficiency & links hematology with hepatic & GI physiology.
• EPO-mediated negative feedback maintains erythrocyte count, integrating renal oxygen sensing with marrow output.
• ABO compatibility hinges on antigen–antibody interactions; mismatched transfusions provoke hemolytic reactions.
• Bile chemistry exemplifies amphipathic molecules’ role in lipid digestion and illustrates cross-system recycling of RBC components.