Transport of carbon dioxide

H+

Transport of carbon dioxide

H+

CO2 is carried in three ways:• 5% carried in solution in the plasma as

CO2

• 10% combines with the Hb in the form of carbamino-haemoglobin

• 85% is carried as hydrogen carbonate

Transport of carbon dioxide

The way in which hydrogen carbonate is formed is important and needs to be known in detail.

Transport of carbon dioxide

In tissue :

CO2

plasma

Transport of carbon dioxide

In tissue :

+ H2O H++HCO3-

plasma

enzyme

CO2

Transport of carbon dioxide

In lungs:

HCO3-

plasma

Transport of carbon dioxide

In lungs:

CO2 + H2O H++ HCO3-

plasma

enzyme

Transport of carbon dioxide

In lungs:

CO2 + H2O H++ HCO3-

plasma

enzyme

Formation of hydrogen carbonate

CO2 diffuses from tissues into rbc

Some combines with amino groups to form carbamino-haemoglobin

The RBC contain an enzyme called carbonic anhydrase which catalyses the reaction between CO2 and water to form carbonic acid

Formation of hydrogen carbonate Carbonic acid dissociates into negatively

charged hydrogen carbonate and positive hydrogen ions

The hydrogen ions increase the acidity of the blood

The H+ combine with Hb to give haemoglobinic acid.

This decreases the affinity of Hb to oxygen so it gives it up to the tissues (hence the Bohr effect)

Formation of hydrogen carbonate

The build up of hydrogen carbonate ions causes them to diffuse out of the RBC leaving the inside of the RBC positively charged.

In order to balance this electric charge chloride ions diffuse into the rbc from the plasma – this is known as the chloride shift.

In this reaction Hb is acting as a buffer

Formation of hydrogen carbonate

When blood gets to the lungs, all the reactions are reversed

The hydrogen carbonate and hydrogen ions recombine releasing CO2

The chloride shift is reversed Carbamino-haemoglobin breaks down to

release CO2