Expansion
Bicarbonate leaves the red cell in exchange for chloride entering
Expansion
Inside the red cell:
- Carbon dioxide is hydrated by carbonic anhydrase to carbonic acid
- This dissociates into bicarbonate and hydrogen ions
- Hydrogen ions are buffered by haemoglobin
- Bicarbonate accumulates and would halt the reaction, so it is exported
The band 3 anion exchanger moves bicarbonate out and chloride in, preserving electrical neutrality without needing energy.
Consequences:
- Plasma bicarbonate rises in venous blood while red cell chloride rises
- The osmotic load draws water in, so venous red cells are slightly larger, and the venous haematocrit is marginally higher
- In the lungs the whole process reverses
Carbonic anhydrase is essential to the speed of this reaction, which is why inhibiting it with acetazolamide impairs carbon dioxide carriage and produces a metabolic acidosis, exploited in altitude sickness prophylaxis and glaucoma.