Transport Of Carbon Dioxide
The Problem CO₂ Poses
Active tissues, such as muscles during exercise, constantly produce carbon dioxide as a waste product of burning fuel, and it must be carried away — a build-up is toxic. Unlike sugar dissolving in tea, carbon dioxide does not simply dissolve in blood in bulk; it travels using three different methods, the most important of which converts it into an entirely different chemical form.
The Three Forms of Transport
At rest, blood carries carbon dioxide from the tissues to the lungs roughly as follows:
| Form | Share of total | How it works |
|---|
| Bicarbonate ion | About 70% | Carbon dioxide reacts with water to form carbonic acid, which splits apart |
| Bound to haemoglobin (carbamino compound) | About 20–25% | Carbon dioxide binds directly onto the haemoglobin protein |
| Dissolved in plasma | About 5–10% | Simply dissolved as gas |
Bicarbonate — the main route
Inside red blood cells, an enzyme called carbonic anhydrase speeds up the otherwise slow reaction of carbon dioxide with water to form carbonic acid, which instantly splits into a hydrogen ion and a bicarbonate ion. The bicarbonate diffuses out into the plasma, while the hydrogen ion stays inside the cell.
To keep electrical balance, chloride ions move into the red blood cell as bicarbonate leaves — this exchange is called the chloride shift. The hydrogen ions produced are acidic and get buffered by haemoglobin, which is therefore not just an oxygen carrier but also a pH buffer.
Carbamino compounds
Carbon dioxide can also bind directly to the amino groups on the globin part of haemoglobin, at a site different from where oxygen binds, forming a carbamino compound. This reaction is reversible: favoured when oxygen levels are low, as in the tissues, and reversed when oxygen levels are high, as in the lungs.
This is the Haldane effect: deoxygenated haemoglobin binds carbon dioxide more readily than oxygenated haemoglobin does, so releasing oxygen in the tissues enhances carbon dioxide loading there, while binding oxygen in the lungs helps unload carbon dioxide.
Dissolved CO₂
A small amount simply dissolves in plasma and travels as gas. It is the least significant by quantity but is the form that directly sets the partial pressure of carbon dioxide in blood.
From Tissues to Lungs …