Question

Blood carries carbon dioxide mainly in which form?
A. HbCO2
B. Carbonic acid
C. NaCO2
D. HbCO2 and carbon monoxide.

Answer 1

Hint: When carbon dioxide binds to the haemoglobin the resulting molecule is called the carbaminohaemoglobin. The binding of carbon dioxide to the haemoglobin is reversible. Thus, when it reaches the lungs the carbon dioxide can freely dissociate from the haemoglobin and be expelled from the body.

Complete answer:
When the carbon dioxide binds to the haemoglobin the carbaminohaemoglobin is formed, lowering haemoglobin’s affinity for the oxygen through the Bohr Effect. In the absence of oxygen, the unbound haemoglobin molecules have a greater chance of becoming carbaminohaemoglobin. The Haldane effect relates to the increased affinity of the deoxygenated haemoglobin for hydrogen ions offloading of the oxygen to the tissues hence results in the increased affinity of the haemoglobin for the carbon dioxide which can then be transported to the lung for removal. The carbaminohaemoglobin has a distinctive blue colour that may contribute to the dark red colour of the deoxygenated venous blood also for the blue colour of the veins and the purplish or the bluish colour of tissues present in the hypoxia. The haemoglobin can bind to the four molecules of carbon dioxide. The carbon dioxide molecules form a carbamate with the four terminal-amine groups of the four protein chains in the deoxy form of the molecule. Therefore, one haemoglobin molecule can transport four carbon dioxide molecules back to the lungs, where they are released when the molecule changes back to the oxyhemoglobin form.
Therefore, the correct answer is Option (A).

Note: The carbon dioxide can be transported to the blood through three methods. First, it is dissolved directly in the blood, bound to the plasma proteins or the haemoglobin, or converted directly into bicarbonate. The majority of the carbon dioxide is transported as part of the bicarbonate system.
The carbon dioxide diffuses into the red blood cells.