Food pipe and stomach

Chewing does two jobs at once. Saliva not only digests starch, it also wets the food, making it soft and easy to swallow. The tongue mixes the chewed food with saliva and pushes it into a long, bendy tube called the food pipe, or oesophagus.

How does food move down? The walls of the food pipe squeeze and relax one after another, in a wave, pushing the food along towards the stomach — a bit like squeezing toothpaste along a tube. (Scientists call this wave peristalsis.) The same wave-like movement happens all along the alimentary canal, moving food forward the whole way.

In the stomach, the walls squeeze and relax to churn (mix and mash) the food. The churned food is mixed with a liquid from the stomach's inner lining, which contains three things: digestive juice, acid and mucus.

Each has its own job:

• Digestive juice breaks down the proteins in food into simpler parts.
• Acid helps break down proteins too, and also kills many harmful bacteria that came in with the food.
• Mucus coats and protects the stomach lining from the acid, so the stomach does not damage itself.

By the time food leaves the stomach it is partly digested and has become a semi-liquid mass, ready for the next stage.

How do we know all this? In 1822, a man named Alexis St. Martin was accidentally shot in the stomach. His doctor, William Beaumont, treated him, but the wound never fully closed and left a small hole. Through that opening Beaumont could watch digestion happening, and he studied how different foods were broken down and even how feelings affect digestion.