Question

For each item A–E related to carbon dioxide transport, select the most appropriate option from the list below. 

1. Dissolved carbon dioxide. 

2. Carbon dioxide combined with 

3. Carbon dioxide in red cells. 2. hemoglobin. 

4. Carbon dioxide in plasma. 

5. Carbamino hemoglobin. 

6. Carboxy hemoglobin. 

7. Ability of desaturated hemoglobin to buffer hydrogen ions. 

A. This has a particularly unfavourable effect on the ability of blood to carry adequate oxygen. 

B. The action of carbonic anhydrase is particularly important here. 

C. The relationship of this to partial pressure is quantitatively different than is the case with oxygen. 

D. This favours carriage of carbon dioxide by the law of mass action. 

E. This form of transport is influenced more by the saturation of the haemoglobin with oxygen than the partial pressure of carbon dioxide in the blood.

Answer 1

A. Option 6 Carboxy hemoglobin. This is formed when people breathe even low concentrations of carbon monoxide. The great affinity of carbon monoxide for hemoglobin displaces most of the oxygen so that most hemoglobin is rendered useless and the patient suffers from potentially fatal anaemic hypoxia. 

B. Option 3 Carbon dioxide in red cells. Carbonic anhydrase is located in red cells rather than plasma. It provides the necessary acceleration of the formation of carbonic acid and hence bicarbonate during the short time available for the uptake of carbon dioxide at tissue level. Without it the dissolved carbon dioxide would build up a pressure which would seriously slow diffusion from the tissues. 

C. Option 1 Dissolved carbon dioxide. At any given partial pressure carbon dioxide is many times more soluble than oxygen. 

D. Option 7 Ability of desaturated hemoglobin to buffer hydrogen ions. As mentioned in (B), generation of bicarbonate is necessary for adequate carriage of carbon dioxide. Desaturated hemoglobin has a greater ability to buffer hydrogen ions than has oxy hemoglobin. The law of mass action predicts that the removal of hydrogen ions by buffering favours further conversion of carbon dioxide to bicarbonate. In addition, diffusion of bicarbonate ions from red cells to plasma lowers the concentration of the other product of the reaction. 

E. Option 5 Carbamino hemoglobin. Oxy hemoglobin cannot carry as much CO₂ in the carbamino compound as can reduced hemoglobin.