carbon dioxide transport

what are the 3 ways that carbon dioxide is transported from the tissues to the lungs? (incl. % of each)

• 5% dissolves in the plasma

• 15% combines with Hb to form carbaminohaemoglobin

• 80% is converted to HCO3^–  ions in erythrocyte cytoplasm

Describe how carbon dioxide is transported from respiring tissues in the blood, including the role of erythrocytes and haemoglobin

• At respiring tissues, CO₂ diffuses into erythrocytes

• CO₂ + H₂O → H₂CO₃ (carbonic acid) in cytoplasm, catalysed by carbonic anhydrase

• H₂CO₃ dissociates into H⁺ and HCO₃⁻ ions

• HCO₃⁻ diffuses out into plasma; Cl⁻ diffuses into RBC to maintain electrical balance (chloride shift)

• Hb binds H⁺ to form HHb (haemoglobonic acid), acting as a buffer and preventing pH changes

• Binding of H⁺ causes Hb to release O₂, which diffuses to respiring tissues

• Continuous removal of CO₂ (as HCO₃⁻) maintains a steep diffusion gradient into RBCs

Explain how carbon dioxide is released from the blood in the lungs, including the role of haemoglobin and carbonic anhydrase

• HCO3- ions diffuse back into rbc

• HHb gives up the H+ which react with HCO3- ions to from H2CO3

• Carbonic anhydrase catalyses the reverse reaction: H2CO3 to CO2 + H2O

• This produces CO2 which diffuses out of rbc into the alveoli

what is this HCO3−​

hydrogen carbonate ions