Carbon dioxide transport

There are 3 ways in which carbon dioxide is transported in the blood:

1. Dissolved carbon dioxide

Carbon dioxide is much more soluble in blood than oxygen.

About 5 % of carbon dioxide is transported unchanged, simply dissolved in the plasma

2. Bound to haemoglobin & plasma protein

Carbon dioxide combines reversibly with haemoglobin to form carbaminohaemoglobin. Carbon dioxide does not bind to iron, as oxygen does, but to amino groups on the polypeptide chains of haemoglobin.

Carbon dioxide also binds to amino groups on the polypeptide chains of plasma proteins

About 10 % of carbon dioxide is transported bound to haemoglobin and plasma proteins

3. Bicarbonate ions

The majority of carbon dioxide is transported in this way. Carbon dioxide enters red blood cells in the tissue capillaries where it combines with water to form carbonic acid. This reaction is catalysed by the enzyme carbonic anhydrase, which is found in the red blood cells. Carbonic acid then dissociates to form bicarbonate ions (HCO3-) and hydrogen ions (H+).

The hydrogen ions, formed from the dissociated carbonic acid, combine with the haemoglobin in the red blood cell. Bicarbonate ions diffuse out of the red blood cell into the plasma whilst chloride ions (Cl-) diffuse in to take their place. This is known as the chloride shift.

The diagram above shows the reversal of the reactions which occurs at the lungs. Bicarbonate ions enter the red blood cells and combine with hydrogen ions to form carbonic acid. This is broken down into carbon dioxide and water. Carbon dioxide diffuses out of the red blood cells and into the alveoli.