Question

The transport of oxygen to tissue is relatively straight forward but the transport of carbon dioxide to the lungs is more complicated. Provide a description of this process. Be sure to discuss carbaminohemoglobin, the role of carbonic anhydrase and the significance of the chloride shift.

Answer 1

Ans : Carbon dioxide is produced as a product of many metabolic processes in the mitochondria. Transport of carbon dioxide to the lungs takes place through any one of the following three pathways :

1. Dissolved in solution/blood

2. In association with CO2 carrying protein

3. Carried as bicarbonate ion

1. Dissolved in solution/ blood :

Carbon dioxide is 20 times more readily soluble than oxygen in blood. Arterial blood carries about 2.5 ml of dissolved carbon dioxide per 100ml while venous blood carries about 3ml dissolved CO2 per 100ml blood. This is because of the difference in partial pressure of CO2 in venous(6.1 kPa) and arterial (5.3 kPa) blood. According to Henry's Law, the partial pressure of a molecule in a liquid is proportional to its concentration in the liquid.

5 litre of blood therefore contains 150ml of dissolved CO2 out of which, 25ml is exhaled.

2. In association with CO2 carrying protein :

Hemoglobin(Hb) is the major transport protein for CO2 and carries about 23% of CO2 exhaled. It is a globular protein bound to a heme prosthetc group. CO2 binds to amino group of N-terminal amino acid of the alpha and beta chains of hemoglobin to form carbaminogen.

R-NH2 + CO2    RNH-CO2 + H+

Hemoglobin needs to be in its reduced form in order to bind with CO2. This is accomplished by binding of hydrogen ions to the imidazole group of histidine amino acid groups of hemoglobin. Reduced Hb is 3 to 5 times more effective in combining with CO2 than Oxyhemoglobin.

3. As bicarbonate ion :

Carbon dioxide combines with water in red blood cell to form carbonic acid which is unstable and easily dissociates to form bicarbonate ion and hydrogen ion. This forms the bicarbonate buffer system which helps to maintain the PH of blood even with the release of large amount of H+ ions during CO2 transport. About 70% of CO2 is transported by bicarbonate buffer system.

CO2 + H2O   H2CO3   HCO3- + H+

Carbon dioxide diffuses into red blood cell within which, an enzyme called Carbonic anhydrase coverts it into carbonic acid by addition of water molecule. Carbonic acid readily dissocistes into bicarbonate ions and hydrogen ions. The free H+ released is used to reduce hemoglobin and the PH is thus maintained.

The bicarbonate ion is transported out of the cell, into the blood stream by exchange with Cloride ion. This is called Cloride shift.

The bicarbonate ions released into the blood are taken up by red blood cells in the pulmonary capillaries in lungs where they are combined back with H+ ions released by Hemoglobin to form carbonic acid. Carbonic anhydrase enzyme then coverts this carbonic acid to CO2 which is exhaled through the nose.