The purpose of the digestive system is to convert food that has been eaten into nutrients capable of being absorbed and used for bodily functions and growth. Both physical and chemical processes are involved; for example, the teeth chew the food and are assisted by the action of an enzyme called ptyalin, which is contained in the saliva. The digestive (or alimentary) tract begins at the mouth and ends at the anus, being about 8-10 meters in length in mature humans. Different animals show specific adaptations to their diet. The dog, for instance, does little chewing, most of the initial breaking-down process occurring in the stomach; the cow, too, is different, being capable of regurgitating and 'chewing its cud'. But, despite these adaptations, all vertebrates have the same fundamental pattern of digestion. The following description of the digestive system refers to that of the human being.
Stages in the Digestive Process
The digestive system can be divided into three sections, concerned with preparation, absorption and elimination. The lips, teeth, tongue, mouth, pharynx and esophagus, or gullet, are concerned with the preparation of food for digestion. The absorption or real digestion takes place in the stomach and small intestine with the aid of such glands as the pancreas and liver.
The storage and elimination of waste is the concern of the large intestine, consisting of the caecum, colon and rectum, and the anal canal and anus.
After the food has entered the mouth with the aid of the lips, it is reduced to a softer, finer state by the action of the teeth, tongue and salivary juices. It is then swallowed and, in a series of involuntary movements, passes into the pharynx and down the esophagus, which is a muscular tube ending at the diaphragm. The bolus (soft, masticated mass of food) then passes into the stomach.
This whole process has taken 15-20 seconds.
In the stomach, gastric glands secrete acid and digestive enzymes to break down further the food, which then passes into the small intestine, its passage being regulated by the activity of a muscle known as the sphincter. The small intestine, where the major part of the digestive process occurs, consists of the duodenum, jejunum and ileum. In the duodenum, bile from the liver and pancreatic juices are mixed with the food, which then proceeds to the jejunum and ileum. Both of these organs are convoluted in order to increase the surface area for absorption and secretion of intestinal juices, which are rich in enzymes and mucus to assist the passage of food.
From the ileum, the food enters the caecum, the first section of the large intestine, through the ileo-colic sphincter. From the caecum, it moves up through the colon, which is lined with a mucous membrane, into the transverse colon and then the descending colon. At this stage, the food residue, or feces, begins to harden, water being drawn from it as it passes along the colon to enter the rectum, which is normally empty, filling only just before defecation.
The rectum is lined by a mucous membrane and separated from the anus by two muscles, the levatores ani, which are important in controlling defecation and maintaining the stability of the abdominal and pelvic organs. The mucus-lined anal canal is surrounded by sphincters, only the external one being voluntary. The mucous membrane of the anal canal merges with the skin at the anus, where waste products emerge, so completing the process of digestion.