Question

Maximum amount (about $70\% - 75\%$) of carbon dioxide transport occurs as
(a) Dissolved in plasma
(b) Carbaminohemoglobin complex
(c) Bicarbonates
(d) None of the above

Answer 1

Hint: Carbon dioxide is a waste product that requires urgent removal. The maximum amount of carbon dioxide transport occurs as a molecule formed in a reaction catalyzed by an enzyme exclusively located in the erythrocytes.

Complete step by step answer:
The transport of carbon dioxide by blood is much easier than that of oxygen due to the high solubility of carbon dioxide in water. About $7\%$ of carbon dioxide is transported as dissolved in plasma, $20\% - 23\%$ as carbaminohemoglobin i.e haemoglobin that is bound to carbon dioxide, and $70\% - 75\%$ as bicarbonates. Most of the carbon dioxide is transported by the blood in the form of sodium bicarbonate in plasma.
- The carbon dioxide diffuses into the blood and enters the red blood cells, where it reacts with water to form carbonic acids in the presence of carbonic anhydrases located inside the cytosol of RBCs.
- The carbonic acid formed later dissociates into hydrogen and bicarbonate ions. This leads to the accumulation of bicarbonate ions in the RBCs as more and more carbon dioxide is picked up by the blood.
- To maintain the balance between ions, bicarbonate ions move out into the blood plasma down its concentration gradient.
- The electrical instability created by the influx of bicarbonate ions into the blood plasma is neutralized by the entry of chloride ions into the cytosol of RBCs. This phenomenon is known as the chloride shift.
So, the correct answer is ‘(c) Bicarbonates.’

Note:
- The amount of carbon dioxide that can be transported in the blood is influenced by the percent saturation of oxyhemoglobin i.e oxygen bonded to haemoglobin.
- The lower the amount of oxyhemoglobin in the blood, the higher the carbon dioxide carrying capacity of haemoglobin. This relationship is known as Haldane’s effect.