Hemoglobin is an important intracellular protein buffer present inside the red blood cells (RBC). When the partial pressure of carbon dioxide (pCO2) is increased, it freely diffuses into the RBC where it reacts with water molecules to form carbonic acid which dissociates to form bicarbonate and hydrogen ions by the enzyme carbonic anhydrase. Hydrogen ions liberated in this reaction are buffered by hemoglobin. Oxyhemoglobin is a stronger acid than deoxyhemoglobin. Oxygenation of hemoglobin causes an increase in net titratable hydrogen ion due to the Haldane effect. As the oxygen saturation of hemoglobin (sO2) increases, the base excess is changed in the acidic direction, or as the sO2 decreases, the base excess is changed in alkaline direction. The changes in the level of the enzyme carbonic anhydrase in RBC are related to the changes in pH, pCO2, and bicarbonate levels in the blood. The understanding of the acid-base balance is a challenging task, but at the same time, it has immense clinical value. The relationship of carbonic anhydrase enzyme present inside the RBC in maintaining the acid-base balance to the commonly employed arterial blood gas (ABG) parameters like pH, pCO2 bicarbonate, and base excess may help us for better understanding.
Part of the book: Erythrocyte