When we talk about the digestive system, we should start with the brain because even before the food comes into the mouth, we're thinking about it — we're planning what we want to eat, smelling its aroma as it simmers on the stove, looking at it on the plate. We do, in a very real sense, eat with our eyes, or more specifically, with our heads. When we see or smell food or even if we think about a food we love, the brain sends signals to the nerves that control the gastrointestinal tract.
These signals put the digestive system on alert, as it were — our mouth begins to water, the stomach starts to contract to be ready to receive the food, and the pancreas, a glandular organ that releases enzymes essential to digestion, starts to secrete chemicals that will break down the food.
Inside the mouth the food is ground and broken down by the teeth while the saliva excreted there lubricates the food. Although we tend to secrete saliva more during meals or when thinking about food, we secrete small amounts of saliva to moisturize the mouth throughout the day. But saliva does more than simply moisten the food. It contains enzymes that start the chemical breakdown of the food, a process that will continue in the stomach and the intestines.
Once the food is chewed and moisturized by the saliva, it is pushed back by the tongue into the throat, where muscles propel the food into the food pipe, or the esophagus. The esophagus pushes the food downward by an action that we call peristalsis, which is basically an orderly sequence of contractions like the wave motion moving across stadium bleachers.
These contractions, which push the food down into the stomach, are powerful enough to allow us to swallow even if lying down — or upside down. Astronauts, for example, have no trouble swallowing in space, where no gravity forces food from the mouth to the stomach. Between the esophagus and the stomach a sphincter ensures that the passage normally opens only one way — from the esophagus into the stomach.
The Stomach The stomach breaks down the food not only physically with its powerful contractions but also chemically through the action of enzymes originally mixed into the food in the mouth and the stomach's own powerful acids and enzymes.
Although most of the enzymes, which chemically break down the food, are secreted in the small intestine, the small amounts secreted with the saliva and in the stomach juices jump-start the process. By the time the food leaves the stomach it has the consistency of porridge.