Question

Blood is the chief method of transport for vital and waste electrolytes. The blood receives an excess of carbon dioxide (CO2) from metabolic activities which slight lowers blood pH, yet the pH of the blood remains relatively constant. Explain how CO2 is buffered in the blood causing stabilisation of blood pH?

Answer 1

Blood is the chief method of transport for vital and waste electrolytes. The blood receives an excess of carbon dioxide (CO2) from metabolic activities which slight lowers blood pH, yet the pH of the blood remains relatively constant. Explain how CO2 is buffered in the blood causing stabilisation of blood pH?

There are 2 buffer systems in the blood where CO2 plays an important role.

1. CO₂ can bind to hemoglobin. As PCO₂ rises, hemoglobin releases O₂ more readily. PCO₂ and pH are related factors because low blood pH (acidity) results from high PCO₂. As CO₂ enters the blood, much of it is temporarily converted to carbonic acid (H₂CO₃), a reaction catalyzed by an enzyme in red blood cells called carbonic anhydrase (CA):

$$\mathrm{CO}_{2+} \mathrm{H}_{2}\mathrm{O} - \mathrm{CA} \rightarrow \mathrm{H}_{2}\mathrm{CO}_{3} \leftrightarrow \mathrm{H}_{4+} \mathrm{HCO}_{3}$$

The carbonic acid thus formed in red blood cells dissociates into hydrogen ions and bicarbonate ions. As the H⁺ concentration increases, pH decreases. Thus, an increased PCO₂ produces a more acidic environment, which helps release O₂ from hemoglobin.

2. The greatest percentage of CO₂—about 70%—is transported in blood plasma as bicarbonate ions (HCO₃⁻). As CO₂ diffuses into systemic capillaries and enters red blood cells, it reacts with water in the presence of the enzyme carbonic anhydrase (CA) to form carbonic acid, which dissociates into H⁻ and HCO₃⁻.

$$\mathrm{CO}_{2+} \mathrm{H}_{2}\mathrm{O} - \mathrm{CA} \rightarrow \mathrm{H}_{2}\mathrm{CO}_{3} \leftrightarrow \mathrm{H}_{4+} \mathrm{HCO}_{3}$$

Thus, as blood picks up CO₂, HCO₃⁻ accumulates inside RBCs. Some HCO₃⁻ moves out into the blood plasma, down its concentration gradient. In exchange, chloride ions (Cl⁻) move from plasma into the RBCs. This exchange of negative ions, which maintains the electrical balance between blood plasma and RBC cytosol, is known as the chloride shift. The net effect of these reactions is that CO₂ is removed from tissue cells and transported in blood plasma as HCO₃⁻. As blood passes through pulmonary capillaries in the lungs, all these reactions reverse and CO₂ is exhaled.