Transport of Carbon Dioxide in Blood
Being readily soluble in water, carbon dioxide diffuses rapidly into the cytoplasm, through the cell membrane, through the capillary walls and then into the blood plasma.
In the blood, carbon dioxide is carried in three forms:
1.
As the dissolved gas. About 10% of the carbon dioxide is carried in the blood in the dissolved state.
2.
Dissolved carbon dioxide reacts with water to give carbonic acid. This in turn dissociates into hydrogen ions and bicarbonate ions. Though spontaneous, the reaction is slow. In the red blood cell the reaction is hastened through the presence of an enzyme called carbonic anhydrase. The resulting bicarbonate ions diffuse out into the plasma, while the hydrogen ion is buffered by the hemoglobin. Around 70% of the carbon dioxide is carried in this form
3.
Carbon dioxide has the ability to react with the amino groups of the amino acids making up the proteins. In this way carbon dioxide forms carbamino compounds:
Though carbamino can be produced by many proteins, the main protein in the red blood cell is hemoglobin and thus it is carbaminohemoglobin, the main carbamino product produced in the RBC. Around 20% of carbon dioxide is carried in this form.
Apart from carbon dioxide transport, the formation of carbaminohemoglobin has also the advantage that it reduces the affinity of hemoglobin for oxygen and thus helps in the unloading of oxygen at the tissue level. The opposite occurs at the level of the lungs.
