Describe the three routes by which carbon dioxide is transported in human blood, with their approximate percentage contributions, and explain the chloride shift.

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Concept understanding — Transport Of Carbon Dioxide

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.

Note

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.

Tip

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 …

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Carbon dioxide is transported by three parallel routes of unequal size. The largest, approximately 70%, travels as bicarbonate ions (HCO3-): CO2 diffusing into a red blood cell is rapidly converted by the enzyme carbonic anhydrase, together with water, into carbonic acid, which dissociates into a hydrogen ion (buffered within the cell by haemoglobin) and a bicarbonate ion, which diffuses out into the plasma for transport. A second route, approximately 20-25%, is carbaminohaemoglobin, in which CO2 binds directly to the globin (protein) portion of haemoglobin, at a site distinct from the haem group that binds oxygen, so the two gases do not compete for the same binding site. The remaining, smallest share, approximately 7%, is simply dissolved directly in blood plasma. The chloride shift (Hamburger shift) keeps the red blood cell electrically neutral as the bicarbonate route proceeds: because the outward diffusion of negatively charged HCO3- would otherwise leave the cell with a net positive charge, a chloride ion (Cl-) simultaneously diffuses in from the plasma to …

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Q.Describe the three routes by which carbon dioxide is transported in human blood, with their approximate percentage contributions, and explain the chloride shift.

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