It extends through the body from the mouth to the anus. Organs of the gastrointestinal tract include the mouth, esophagus, stomach, and small and large intestines. The accessory organs include salivary glands, liver and pancreas.
There are four main layers of the gastrointestinal tract:
*the mucosa
*the submucosa
*the muscularis externa
*the serosa, or adventitia
The gastrointestinal tract (GI) is bordered by a layer of epithelial cells (with glands) sitting on a lamina propria (or basement membrane), comprising the mucosa and adjacent to the submucosa. Submucosa contains blood vessels, lymphatic vessels and nerves.
Beneath the mucosa and submucosa are two layers of smooth muscle, lying in longitudinal and transverse directions, to allow contractions and peristalsis.
Within the stomach , but particularly in the small intestine, the surface area of the mucosa is greatly increased. The mucosa epithelium, which lines the lumen of the gastrointestinal tract is the surface that is contact with nutrients in the food eaten. The mucosal and submucosal layer is folded into microscopic villi on the surface of larger folds or ridges.
At the bases of the villi are the “crypts” where new epithelial cells are formed that migrate upward to the villi. These cells are sloughed off at a fairly rapid rate; the lifespan of villus cells in the small intestine is as little as 2 - 3 days (in man), that of colonic cells 3 – 8 days).
Exocrine and endocrine cells are found among the epithelial cells of the mucosa. The exocrine cells secrete a variety of substance, such as enzymes and juices, into the lumen of the gastrointestinal tract and the endocrine cells secrete various hormones into the blood.
Glandular cells are important in signaling the initiation and coordination of digestive processes, involving a large number of hormones neurotransmitters and paracrine factors. Mucous provided by “goblet” cells promotes lubrication within the lumen of the GI tract. In the small intestine, crypt cells are also the source of some digestive juices.
The epithelia cells of the mucosa have an apical (lumen –oriented) surface that is often additionally invaginated to form microvilli (or a brush border).
In the small intestine the brush border contains transporter and some digestive enzymes. Surface cells are held together by tight junctions near the apical (top) parts of the cells.
The outer layer do the gastrointestinal tract is called the serosa layer, or serosa. This layer is composed of visceral peritoneum, The visceral peritoneum is continuous with the parietal peritoneum lining the abdominal cavity.
Nutrients entering the blood or lymph for distribution to body tissues must first cross the brush border and ultimately the serosal surface of these cells to enter the intestinal fluid.
Transport across either or both of these surfaces may be independently and/or differentially controlled, depending upon the nutrient.
For there, capillaries and lymphatics take nutrients to the rest of the body. Nutrients not making it across the serosal membranes will remain with the mucosal cells until they are sloughed off, from whence they may be released by digestion and reabsorbed or lost with cell debris and bacteria in feces.
Once nutrients from the food have entered the blood stream, the circulating blood carried them throughout the body.
The feces eliminated by the intestinal tract are composed mainly of bacteria which have proliferated in the tract undigested of plant cell membranes which cannot be absorbed.
Although food residues are eliminated by the digestive system itself, the urinary system disposes of many other wastes or unneeded substance that enter the blood from gastrointestinal tract.
Nutritional Processes: Gastrointestinal Tract
