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How is carbon dioxide removed from blood before it is exhaled?

In humans, carbon dioxide is generated in the tissues during oxidation of glucose, in order to liberate energy, which is resumed to perform body functions. Before exhalation through lungs the gas is transported from the tissues to the lungs. Under normal resting conditions an average of 4 millilitres of CO{-2} is transported from the tissues to the lungs in each decilitre of blood.

This transport is performed by venous system of the body and pulmonary artery of the heart. The venous system first transports deoxygenated blood carrying CO{-2} to the heart. From there, blood is transported to lungs for purification by the pulmonary artery.

To begin with CO{-2} diffuses out of the tissue cells in the dissolved molecular CO{-2} form. On entering the capillary, it initiates a host of almost instantaneous physical and chemical reactions. A small portion of the CO{-2} is transported in a dissolved state to the lungs. This is about 7 per cent of all the CO{-2} transported.

Nearly 70 per cent of CO{-2} is transported as bicarbonate ions. The dissolved CO{-2} in the blood first reacts with water to form carbonic acid. This can be effected by the enzyme carbonic anhydrase, which catalyse the reaction between CO{-2} and water, accelerating 5000-fold.

Since the enzyme is found only in the red blood cells (RBC) carbonic acid formation is so rapid in them than in the plasma. In the red blood cells it occurs in a small fraction of a second. In no time the carbonic acid formed in the red blood cells dissociates into hydrogen ions (H{++}) and bicarbonate ions (HCO{-3}) ions. Most of the H{++} ions then combine with the haemoglobin (Hb) in the red blood cells because Hb is a powerful acid-base buffer. In turn, many of the bicarbonate ions diffuse into the plasma while chloride ions diffuse into the red cells to take their place.

This is possible by the presence of a special bicarbonate carrier protein in the red cell membrane that shuttles the two ions in opposite directions at rapid velocities. Thus, the chloride content of venous red blood cells is greater than that of arterial cells, a phenomenon called the chloride shift. In the alveolar capillaries the carbonate ions under the influence of carbonic anhydrase by reversible reaction again produce the CO{-2} and water molecules. These CO{-2} molecules are then easily released into the alveoli for exhalation.

Transport of CO{-2} in combination with haemoglobin is also possible. In addition to reacting with water, CO{-2} also reacts directly with haemoglobin to form the compound of carbaminohaemoglobin (CO{-2}HHb).

This combination of carbon dioxide with the haemoglobin is a reversible reaction that occurs with a very loose bond, so that the carbon dioxide is easily released into the alveoli where the partial pressure of CO{-2} is lower than the tissue capillaries. Nearly 23 per cent of CO{-2} is transported this way.

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How is carbon dioxide removed from blood before it is exhaled?