Transport of carbon dioxide
Carrying carbon dioxide
- Carbon dioxide made by respiring cells must be carried to the lungs.
- A little dissolves in the plasma — but most is carried a cleverer way.
- The action happens inside the red blood cells.
The reaction in red blood cells
- the enzyme carbonic anhydrase speeds up: carbon dioxide + water → carbonic acid.
- the carbonic acid splits into hydrogen ions and hydrogencarbonate ions.
- the hydrogencarbonate ions move out into the plasma — this is the main way CO₂ is carried.
Practice
Most carbon dioxide is carried in the blood as:
The largest share is carried as hydrogencarbonate ions in the plasma.
Practice
The enzyme carbonic anhydrase speeds up the reaction of:
Carbonic anhydrase catalyses CO₂ + water → carbonic acid, inside the red blood cell.
The chloride shift
- As negative hydrogencarbonate ions leave, chloride ions move in to keep the charge balanced — the chloride shift.
- The leftover hydrogen ions join haemoglobin to form haemoglobinic acid — this mops them up and keeps the pH steady.
- Some CO₂ also joins haemoglobin directly as carbaminohaemoglobin.
Practice
The chloride shift is when:
As negative hydrogencarbonate ions exit, chloride ions move in to keep the electrical charge balanced.
Practice
The hydrogen ions produced are dealt with by:
H⁺ binds haemoglobin (haemoglobinic acid), buffering the blood so the pH does not fall sharply.
You've got it
Key idea
- most CO₂ is carried as hydrogencarbonate ions in the plasma
- carbonic anhydrase: CO₂ + water → carbonic acid → H⁺ + hydrogencarbonate
- chloride shift: chloride ions move in as hydrogencarbonate moves out (balances charge)
- H⁺ joins haemoglobin (haemoglobinic acid) to buffer pH; some CO₂ → carbaminohaemoglobin