HOW IS CARBON DIOXIDE REMOVED FROM BLOOD BEFORE IT IS EXHALED ?

       In humans,carbon dioxide[ CO₂  ] 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 CO₂    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 decoxygenated blood carring to the CO₂    of the heart.From there, blood is transported to lungs for purification by the pulmonary artery.

       To begin with CO₂   diffuses out of the tissue cells in the dissolved molecular CO₂   form.On entering the capillary, it intiates a host of almost instantaneous physical and chemical reactions.A small portion of the carbon dioxide is transported in a dissolved state to the lungs.This is about 7 per cent of all the CO₂ transported.Nearly 70 per cent of  CO₂  is transported as bicarbonate ions.The dissolved  CO₂   in the blood first reacts with water to form carbonic acid.This can be effected by the enzymes carbonic anhydrase, which catalyse the reaction between  CO₂  and water, accelerating 5000 fold.

       Since the enzymes 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[ HCO₃ ] 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 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 CO₂   and water molecules.These  CO₂  molecules are then easily released into the alveoli for exhalation.Transport of CO₂    in combination with haemoglobin is also possible.

       In addition to reacting with water, CO₂  also reacts directly with haemoglobin to form the compound of carbaminohaemoglobin [  CO₂   HHB].This combination of carbon dioxide with 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 CO₂    is lower than the tissue capillaries.Nearly 23 per cent of   CO₂  is transported this way.