Which substance neutralizes stomach acid as it enters the duodenum?

Bicarbonate is the substance that neutralizes stomach acid as it enters the duodenum. When partially digested food, known as chyme, moves from the stomach into the duodenum, it is highly acidic because of the gastric juices in the stomach. To protect the lining of the duodenum and to create a suitable pH for digestive enzymes to function, bicarbonate is secreted by the pancreas into the duodenum. This raises the pH of the chyme, making it less acidic and allowing the digestive process to continue properly.

In contrast, bile plays a role in the emulsification of fats rather than directly neutralizing stomach acid. Pepsin is an enzyme that digests proteins but functions optimally in the acidic environment of the stomach and does not act in the duodenum where the pH is higher. Gastric acid, which largely consists of hydrochloric acid, is responsible for the acidity of the stomach contents and does not neutralize acids; instead, it is the very substance that bicarbonate neutralizes as it exits the stomach. Therefore, bicarbonate's role in increasing the pH in the duodenum is vital for maintaining digestive health.