Final answer:
The pancreas secretes bicarbonate to neutralize the acidity of the gastric juice from the stomach, creating an optimal alkaline environment for digestive enzymes in the small intestine.
Step-by-step explanation:
The alkaline substance that the pancreas secretes to counter the acidity in the gastric juice is bicarbonate. The pancreatic juice is produced in large quantities, with more than a liter each day being secreted. This juice contains the bicarbonate ion, which is crucial for neutralizing the highly acidic chyme that comes from the stomach.
When the acidic chyme enters the small intestine, it needs to be neutralized for the digestive enzymes to work effectively. The pancreas provides bicarbonate, which neutralizes the acid, thus making the environment more alkaline. This helps the pancreatic enzymes, specifically for digesting proteins and carbohydrates, to function at their optimal pH. Additionally, bicarbonate helps to inactivate pepsin, an enzyme from the stomach that works best in acidic conditions and would otherwise damage the intestine.