Brent Cornell

    
Carbon dioxide is transported between the lungs and the tissues by one of three mechanisms:

• Some is bound to haemoglobin to form HbCO₂ (carbon dioxide binds to the globin and so doesn’t compete with O₂ binding)
• A very small fraction gets dissolved in water and is carried in solution (~5% – carbon dioxide dissolves poorly in water)
• The majority (~75%) diffuses into the erythrocyte and gets converted into carbonic acid

Transport as Carbonic Acid

• When CO₂ enters the erythrocyte, it combines with water to form carbonic acid (reaction catalysed by carbonic anhydrase)
• The carbonic acid (H₂CO₃) then dissociates to form hydrogen ions (H⁺) and bicarbonate (HCO₃^–)
• Bicarbonate is pumped out of the cell in exchange with chloride ions (exchange ensures the erythrocyte remains uncharged)
• The bicarbonate in the blood plasma combines with sodium to form sodium bicarbonate (NaHCO₃), which travels to the lungs
• The hydrogen ions within the erythrocyte make the environment less alkaline, causing haemoglobin to release its oxygen
• The haemoglobin absorbs the H⁺ ions and acts as a buffer to maintain the intracellular pH
• When the red blood cell reaches the lungs, bicarbonate is pumped back into the cell and the entire process is reversed

Carbon Dioxide Transport in the Bloodstream