Question
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Carbon Dioxide combines with Hb to form
A. Carbaminohemoglobin
B. Carboxyhemoglobin
C. Oxyhaemoglobin
D. Methemoglobin

Answer
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Hint: Haemoglobin is made up of two words 'haem' and 'globin'. Globin is the protein part of the haemoglobin and haem is the prosthetic part of the non-protein part. Globin is present in a major amount in the haemoglobin as compared to haem part. Haemoglobin is essential in the transport of oxygen in the body. Haemoglobin is present in the red blood cells and it is the characteristic respiratory protein in humans.

Complete answer:
Carbon dioxide is transported by the haemoglobin in three main methods. The blood transports carbon dioxide comparatively easier than oxygen because of the higher solubility of carbon dioxide.
There are three ways of carbon dioxide transport. A very little amount of it is transported in the dissolved state in plasma. Some amount of carbon dioxide reacts with the amine group of haemoglobin and forms carbaminohemoglobin. A major amount of carbon dioxide is transported by plasma as bicarbonates.
Carbon dioxide produced by the tissues diffuses into the red blood cells, where it reacts with water to form carbonic acid. This carbonic acid then dissociates into hydrogen and bicarbonate ions. The majority of bicarbonate ions formed within the RBCs diffuses out into the plasma along a concentration gradient.
When the deoxygenated blood reaches the alveoli of the lung, then carbaminohemoglobin and sodium bicarbonate dissociate because the partial pressure of Carbon dioxide is low as compared to that of oxygen in the alveoli.

Hence, the correct answer is option (A).

Note: Oxygen is transported in the body by the two main methods. A very little amount of oxygen travels through plasma in the dissolved state. Rest of the quantity of it is transported through haemoglobin in the form of oxyhaemoglobin. Oxygen can reversibly bind with haemoglobin to form oxyhaemoglobin. At the tissue level, this oxygen is freed from haemoglobin and carbon dioxide binds with haemoglobin.