The energy required for maintenance and proper functioning of the human body is supplied by food. But for our cells to use the food, it must first be converted into simple chemicals, be absorbed and then enter the bloodstream and travel to where it is needed. This is the job of our digestive system.

The Senses

The sight, smell, or thought of food can initiate the digestive process by the secretion of saliva. Gastric juice also begins to flow (in the stomach) when stimulated by the sight or smell of food. 

The Mouth

Food enters the body firstly through the mouth. Here it is chewed and moistened to facilitate swallowing, and an enzyme is added to it to begin its breakdown into smaller particles. Particle size is essential because food 'as is', even when thoroughly chewed, cannot be absorbed from the intestine into the body unless altered chemically and physically beforehand.

The Stomach

Food is quickly swallowed (often without thorough chewing) and passes down the esophagus to the stomach. The stomach serves as a bag for the temporary storage and partial digestion of food, it churns and breaks up food and mixes the pieces with gastric juice (more enzymes and hydrochloric acid) and it produces enzymes and hormones. Alcohol, certain drugs, and moderate amounts of sugar and water are absorbed by the stomach wall.

The Small Intestine

After a variable length of time, the partially digested food leaves the stomach and enters the first 25cm of the small intestine, the duodenum. Here, secretions of the liver (bile to emulsify fat) and secretions of the pancreas (enzymes to break down carbohydrates, fat and protein) are added, along with a secretion of the duodenum itself.

The duodenum continues to the next portion of the small intestine, the jejunum, which in turn continues to the ileum. Intestinal juice is added as food moves through all portions of the small intestine, and most digestion and absorption should take place within this portion of the digestive tract.

Each day your digestive system will handle about 10L (18 pints), of food and drink from your stomach and intestines. In a healthy digestive system only about 125mL (one pint) of this leaves the body as faeces. The rest is absorbed into capillaries and lymph vessels in the walls of the intestines (see photo and illustration).

Absorption

The walls of a healthy small intestine are covered with finger-like protrusions (villi) and so highly folded that they have a surface area that would be larger than a tennis court if they were to be spread out flat.

The Large Intestine

After the small intestine has coiled for over 6m in the abdomen, the far end joins the large intestine or colon. All unabsorbed food passes into the large intestine as waste.

The key functions of the large intestine are the absorption of water and the elimination of solid wastes; however, one other significant contribution to health is made by the large intestine, it is known that quantities of vitamin K (which is necessary to the normal clotting of blood) and B-complex vitamins are manufactured by some of the bacteria in the large intestine. The B-complex vitamins are essential for the maintenance and normal function of the digestive tract, metabolism (use) of nutrients and a healthy nervous system.

During the passage of unabsorbed food through the small and large intestine, bacterial action converts the bile pigments into stercobilin, which produces the brown colour of faeces, the solid waste that is periodically eliminated from the intestine.

References:
1. "Anatomy", Microsoft(R) Encarta(R) 98 Encyclopedia. (c) 1993-1997 Microsoft Corporation.
2. "Anatomy and Physiology", William F. Evans, Second Edition, 1976 by PRENTICE-HALL, INC., Englewood Cliffs, New Jersey
3. "Science through the Microscope", Aladdin Books Ltd 1995, Shooting Star Press Inc., New York