The alimentary tract of the digestive system maintains the following four main activities: ingestion, digestion, absorption, and elimination. The alimentary canal begins in the mouth; leads through the thorax, abdomen, and pelvic region; and ends at the anus. 

When you eat a meal, a series of digestive processes begin to take place. These can be divided into the mechanical breakdown of food through mastication (chewing) and the chemical breakdown of food through enzymes. These enzymes are present in the secretions produced by various glands of the digestive system.

Enzymes are minute chemical substances composed of proteins that cause or speed up chemical changes in other substances without themselves being changed. Digestive enzymes are contained in the saliva of the salivary glands of the mouth, the gastric juice in the stomach, the intestinal juice in the small intestine, the pancreatic juice in the pancreas, and the bile in the liver.

Absorption is the process by which tiny nutrient particles of digested food pass through the intestinal walls into the blood and lymph vessels, which help distribute them to the cells of the body.

The bowels eliminate as feces whatever food substances they cannot digest or absorb, such as the plant fiber cellulose. Fecal matter also contains bile, which carries the waste products resulting from the breakdown (catabolism) of red blood cells. Nearly one-third of the excreted fecal matter is made up of dead intestinal bacteria. The body can function smoothly and efficiently only if the bowel removes these daily-generated waste products every day.

Good health results when each of these major activities in the digestive system is balanced and well coordinated with the rest of the body. By contrast, abnormalities begin to arise in the digestive system as well as in other parts of the body when one or more of these functions becomes impaired.