Figure 1: Acid secretion and ion transport mechanism. Luminal acidity and food contents in the stomach increase intragastric pH, which promotes gastrin secretion that increases the rate of acid secretion (HCl) by parietal cells. The concentration of bicarbonate (HCO3 -) in the blood is increased as a response to the acid secreted in the stomach, and the extra bicarbonate is generated by the same parietal cells that produce the HCl in the lumen in order to lower the pH to digest a meal. Once the food reaches the duodenum, the cells in its lumen and pancreas secrete bicarbonate to increase the pHe of the lumen. As a consequence, while food moves from the esophagus to the stomach, and finally to the duodenum, the concentration of bicarbonate in blood increases and then decreases back to its normal levels.