Carbon dioxide produced by respiring cells diffuses into the blood and is carried to the lungs in three ways:• about 5% is dissolved in the plasma
• about 10 to 20% combines with the amino groups of haemoglobin to form carbaminohaemoglobin
• about 75 to 85% is carried as hydrogencarbonate ions (HCO3−) in the plasmaFormation of hydrogencarbonate ions
Carbon dioxide diffuses into erythrocytes. The enzyme carbonic anhydrase catalyses its reaction with water to form carbonic acid, which dissociates:CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3− • The hydrogencarbonate ions diffuse out of the erythrocyte into the plasma, where they are carried in solution.
• To balance the loss of negative charge, chloride ions (Cl−) diffuse into the erythrocyte from the plasma. This is the chloride shift.
• The hydrogen ions are taken up by haemoglobin, forming haemoglobinic acid (HHb). This prevents the pH of the blood falling, so haemoglobin acts as a buffer, and it encourages oxyhaemoglobin to release its oxygen.
In the lungs the process reverses. pCO2 in the alveoli is low, so carbon dioxide diffuses out of the blood. Hydrogencarbonate ions re-enter the erythrocytes, combine with hydrogen ions released from haemoglobin to form carbonic acid, and carbonic anhydrase converts this back to carbon dioxide and water. The carbon dioxide diffuses into the alveoli and is exhaled.Because carbonic anhydrase speeds up this reaction enormously, most carbon dioxide can be loaded and unloaded in the short time blood spends in a capillary.
• about 10 to 20% combines with the amino groups of haemoglobin to form carbaminohaemoglobin
• about 75 to 85% is carried as hydrogencarbonate ions (HCO3−) in the plasmaFormation of hydrogencarbonate ions
Carbon dioxide diffuses into erythrocytes. The enzyme carbonic anhydrase catalyses its reaction with water to form carbonic acid, which dissociates:
• To balance the loss of negative charge, chloride ions (Cl−) diffuse into the erythrocyte from the plasma. This is the chloride shift.
• The hydrogen ions are taken up by haemoglobin, forming haemoglobinic acid (HHb). This prevents the pH of the blood falling, so haemoglobin acts as a buffer, and it encourages oxyhaemoglobin to release its oxygen.
In the lungs the process reverses. pCO2 in the alveoli is low, so carbon dioxide diffuses out of the blood. Hydrogencarbonate ions re-enter the erythrocytes, combine with hydrogen ions released from haemoglobin to form carbonic acid, and carbonic anhydrase converts this back to carbon dioxide and water. The carbon dioxide diffuses into the alveoli and is exhaled.Because carbonic anhydrase speeds up this reaction enormously, most carbon dioxide can be loaded and unloaded in the short time blood spends in a capillary.