The stomach has a pH between 1.5 and 3.5 generally and this is due to the cells in the stomach releasing hydrochloric acid. The low pH is useful for “unraveling” proteins making them easier to digest and killing bacteria and other pathogens. The intestine on the other hand is around pH 6 to 7 which is important because the low pH of the stomach is potentially dangerous to the body. Therefore, it makes sense to only have the stomach being really acidic rather than the whole digestive tract. To facilitate the change from low pH in the stomach to mid pH in the intestine, there are cells that release sodium bicarbonate. Sodium bicarbonate is more commonly known as baking soda and is useful for neutralizing acids. So as cells in the intestines gradually release more and more sodium bicarbonate, the pH raises from around 1 to around 7.

The stomach usually has a pH of 2 or 3. That’s still really acidic. The stomach is protected by having a layer of mucus between the inside the stomach and the actual stomach tissue. Some of the cells that line the stomach also make a buffer (a high-pH liquid) that helps to keep the pH right by the cells closer to neutral.

When there is nothing in the stomach, less acid is produced. This may seem like a waste of energy and materials, creating a low pH, then trying to protect itself. The acidic environment of the stomach is important, though. Not only does it help to break down certain foods, it kills germs before they enter the small intestine. The lining of the small intestine is more delicate so that nutrients can be absorbed there.

The small intestine stays close to neutral because the pancreas dumps a lot of buffers in right where the stomach connects to the small intestine. This pH is much closer to water and does not damage the tissue.

Why do you think the digestive system has zones that are so different from each other?

You may want to explore a career in physiology or medicine.