Transport of Oxygen & Carbon Dioxide

Red Blood Cells, Haemoglobin & Oxygen

• Oxygen Transport: Most oxygen is transported bound to haemoglobin in red blood cells (erythrocytes).
• Haemoglobin Structure:
  • Made up of four subunits, each with a haem group.
  • Each haem group can bind one oxygen molecule, allowing one haemoglobin molecule to carry four oxygen molecules.
  • Oxygen binding forms oxyhaemoglobin.
• Cooperative Binding:
  • Binding of the first oxygen molecule changes haemoglobin's shape, facilitating easier binding of subsequent oxygen molecules.
  • The process is reversible during oxygen dissociation in tissues.
• Example Calculation:
  • 150 g of haemoglobin in 1 dm³ of blood can carry 195 cm³ of oxygen.

The Chloride Shift

• Definition: Movement of chloride ions into red blood cells accompanies the formation of hydrogen carbonate ions.
• Formation Process:
  • CO₂ diffuses into red blood cells.
  • Carbonic anhydrase catalyzes CO₂ and H₂O to form carbonic acid.
  • Carbonic acid dissociates into hydrogen carbonate and hydrogen ions.
• Ion Transport:
  • Hydrogen carbonate ions exit red blood cells, chloride ions enter to prevent electrical imbalance.

Plasma & Carbon Dioxide

• CO₂ Transport:
  • 85% as hydrogen carbonate ions in plasma.
  • 5% dissolves in plasma.
  • 10% as carbaminohaemoglobin.
• Reaction Process:
  • CO₂ + H₂O forms carbonic acid in red blood cells via carbonic anhydrase.
  • Carbonic acid dissociates, hydrogen ions buffered by haemoglobin to prevent pH drop.
  • Hydrogen carbonate ions diffuse into plasma.

The Oxygen Dissociation Curve

• Description:
  • Shows haemoglobin's affinity for oxygen at different partial pressures of oxygen (pO₂).
  • High affinity means easy binding, low affinity means easy dissociation.
• Curve Interpretation:
  • Low pO₂: Slow binding, low saturation.
  • Medium pO₂: Rapid binding, quick saturation increase.
  • High pO₂: Easy binding, high saturation; limited effect of higher pO₂.
  • Cooperative Binding: Initial slow binding speeds up with subsequent oxygen molecules, leading to sigmoidal curve.

The Bohr Shift

• Effect: High CO₂ levels reduce haemoglobin's oxygen affinity, aiding oxygen release in respiring tissues.
• Mechanism:
  • CO₂ forms carbonic acid, which dissociates, lowering blood pH.
  • Hydrogen ions promote oxygen release from haemoglobin.
• Graph Shift: Oxygen dissociation curve shifts right with increased CO₂, indicating reduced oxygen saturation at a given pO₂.