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| Transport of Gases |
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| Oxygen is transported in a combined state. The haemoglobin pigment (Hb) has an affinity for oxygen. In the lungs, it combines with oxygen and forms HbO2, oxyhaemoglobin. |
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| It is found that haemoglobin has more affinity for oxygen under high partial pressure of oxygen, low temperatures and low acidity conditions. |
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| However, in the tissues the partial pressure of oxygen is low as it is constantly being used up. Temperature is higher because of metabolic reactions and the acidity is also high. Under these conditions, the oxyhaemoglobin gives up its oxygen and forms haemoglobin again. |
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| Some of the carbon dioxide is transported in haemoglobin. However, most of the carbon dioxide is transported as bicarbonate ions. Carbon dioxide first combines with water to form carboxylic acid. This then splits into hydrogen ions and bicarbonate ions. The hydrogen ions are absorbed by the haemoglobin. |
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| In the lungs, the reverse reaction takes place and the haemoglobin gives up the hydrogen ions, which combine with the bicarbonate to form the carboxylic acid. This then forms carbon dioxide and water. |
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| Both the reactions are catalysed by an enzyme called the carbonic anhydrase. |
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