Red Blood Cells (Erythrocytes) - Structure & Function

• Red Blood Cells (RBCs) may also be referred to as Erythrocytes or Red Blood Corpuscles.
• RBCs are biconcave shaped, non-nucleated, flat, disc-shaped cells indented in the middle on both sides.
• The biconcave shape enables a larger surface area for diffusion of oxygen and carbon dioxide across the membrane.
• RBCs have a diameter of 7.5 µm.
• At the outer edges, RBCs are 2.0 µm thick, while in the center they are 1.0 µm thick.
• The thinness of RBC allows oxygen to diffuse more efficiently.
• On average, there are 5 million RBCs per mm³ (µL) of blood.
• Men typically have 5-6 million RBCs/mm³, while women have 4-5 million RBCs/mm³.
• Newborn infants possess higher RBC counts compared to children and adults.
• The lifespan of RBCs is 120 days in circulation.
• RBCs are the most abundant cells in the blood.
• RBCs consist of a plasma membrane-enclosed sac filled with hemoglobin.
• The RBC membrane is lipoprotein in nature and semipermeable.
• RBCs can deform and squeeze through capillaries.
• Red blood cell structure, functions, and abnormalities.

RBC Membrane Functions

• Contains agglutinogens that determine blood groups.
• Keeps hemoglobin inside, preventing loss in urine, which would increase blood viscosity and workload on the heart.
• Maximizes surface area for gas exchange.
• Allows changes in shape and size.
• Allows anions to pass freely, but keeps K+ inside and Na+ outside.

RBC Contents

• Hemoglobin makes up 34%.
• Water makes up 60%.
• Stroma, inorganic, and organic substances (e.g., carbonic anhydrase, glycolytic enzymes) makes up 6%.
• The chief cation is K+ (Na+ and Mg2+ are present in small amounts).
• Anions include Cl-, HCO3-, and phosphate.
• Erythrocytes contain no nucleus, organelles, or ribosomes.
• Single erythrocyte is filled with hundreds of hemoglobin molecules.
• The presence of hemoglobin inside the RBC prevents its rapid loss in urine
• Free hemoglobin in plasma can cause osmotic pressure, interfering with filtration at capillaries.

Hemoglobin (Hb)

• Hemoglobin is a red pigment, appearing red when oxygenated and bluish when deoxygenated.
• It is a chromoprotein with a globular shape and a molecular weight of 64,450.
• The globin portion consists of four highly folded polypeptide chains (2 pairs).
• Each pair consists of 2 subunits representing one type of polypeptide chain.
• The heme portion consists of four iron-containing, non-peptide nitrogenous groups (protoporphyrin).
• Each heme binds one O2 molecule.
• Each of the four iron atoms can combine reversibly with one molecule of O2.
• Each hemoglobin molecule can carry four O2 molecules.
• Approximately 98% of the O2 carried in the blood is bound to hemoglobin.
• The iron remains in the ferrous state.
• One gram of Hb unites with 1.33 ml of O2.
• Carries 02 from lungs to tissues
• Each gram of Hb unites with 1.33 ml of O2.
• 15 gm of Hb are contained in 100 ml blood, so each 100 ml of blood can carry 15 X 1.33 = 19.95 ml of oxygen.
• Helps transport CO2 from tissues to lungs.
• Acts as a blood buffer.

Types of Hemoglobin

• HbA (Normal adult Hb) : α2β2 (2 a-chains, 2 ẞ-chains)
• HbA2 (2.5% of adult Hb) : α2δ2 (2 a-chains, 2 δ-chains)
• HbF (Fetal Hb) : α2γ2 (2 a-chains, 2 y-chains)
  • Replaced by adult Hb 2-3 months after birth
  • Higher oxygen affinity than HbA for better O2 transport across the placenta.

Hemoglobin Abnormalities

• Hemoglobinopathies are abnormal polypeptide chains (e.g., Sickle Cell Anemia - HbS).
• HbS occurs in 10-20% of African populations.
• Under low O2 conditions, HbS forms long crystals inside RBCs, deforming them into a sickle shape.
• This increases blood viscosity, damages RBC membranes, and causes hemolysis.
  • Thalassemias (Mediterranean Anemia):
  • Partial or complete deficiency in α- or β-globin chain synthesis
  • Leads to hypochromic anemia and rapid RBC hemolysis.

RBC Fragility and Hemolysis

• RBCs lyse when exposed to hypotonic saline (osmotic fragility) or mechanical trauma (mechanical fragility).
• Hemolysis is the lysis of RBCs with Hb release into plasma.
• Normal RBCs begin hemolysis at 0.5% NaCl in an osmotic fragility test.
• 50% hemolysis at 0.40-0.42% NaCl.
• Complete hemolysis at 0.35% NaCl.
• Spherocytic RBCs hemolyze earlier (hereditary spherocytosis).

Causes of RBC Hemolysis

• Drugs: Sulfonamides (especially in G6PD deficiency)
• Chemical Poisons: Lead, arsenic, snake venoms
• Infections & Bacterial Toxins
• Antibodies against RBCs (e.g., incompatible blood transfusions)
• Corpuscular Defects:
  • Spherocytosis
  • Abnormal Hbs (Sickle Cell Anemia, Thalassemia)
  • Enzyme deficiencies (e.g., G6PD deficiency)