Red Blood Cells (Erythrocytes): Structure, Function, and Lifecycle

Red blood cells (RBCs, erythrocytes) are the most abundant cells in blood (~45% hematocrit), specialized for respiratory gas transport.

Structure

• Biconcave disc (7–8 μm): Maximizes surface-area-to-volume ratio for gas exchange; enables deformation through capillaries (3–5 μm).
• Anucleate (mammals): No nucleus, mitochondria, ribosomes → more space for hemoglobin; relies on anaerobic glycolysis.
• Hemoglobin (Hb): ~270 million molecules/cell; each binds 4 O₂. Iron-containing heme groups.
• Membrane: Lipid bilayer + cytoskeleton (spectrin, ankyrin, protein 4.1) → flexibility, durability.

Function

• O₂ transport: Hb binds O₂ in lungs (high pO₂, high pH, low CO₂) → releases in tissues (low pO₂, low pH, high CO₂, high 2,3-BPG, high temp) — Bohr effect.
• CO₂ transport: 70% as bicarbonate (HCO₃⁻) via carbonic anhydrase; 23% carbamino-Hb; 7% dissolved.
• Buffering: Hb buffers H⁺ (imidazole groups).

Production (Erythropoiesis)

• Bone marrow (axial skeleton in adults).
• Stimulated by erythropoietin (EPO) from kidneys (hypoxia sensor).
• Progression: HSC → CFU-E → proerythroblast → basophilic → polychromatophilic → orthochromatic → reticulocyte (1–2 days in marrow) → mature RBC.
• Requires iron, B12, folate, copper, protein.

Lifespan & Clearance

• ~120 days in circulation.
• Senescent RBCs removed by splenic/liver macrophages.
• Iron recycled (transferrin → marrow); heme → bilirubin → bile.

Clinical Relevance

Anemia (low Hb/RBC), polycythemia (high), hemoglobinopathies (sickle cell, thalassemia), hemolysis, transfusion medicine.