Material moves through the rest of the tract by mass movements, which transport tract contents over long distances. The motility of the GI tract is accomplished via a diffuse neuroendocrine system that involves the autonomic nervous system and hormones. The enteric nervous system is composed of several groups of nerve cells:. With this understanding of the basic structural features of the GI tract, we will now take a more focused look at different portions of the tract.
Be sure you understand the function of each portion of the tract before you study the structure in detail in the Laboratory Slides. Food enters the digestive tract in the oral cavity, where it is masticated into particles on which digestive enzymes can act more efficiently.
In the mouth, food particles are mixed with saliva, which lubricates them and initiates their digestion. The salivary glands will be discussed in detail in the next laboratory. The tongue is a muscular organ covered by oral mucosa that manipulates the food and contains the sensory organs for taste.
The taste buds will be discussed in detail in the Laboratory on Sensory Systems. The esophagus is a muscular tube that transports food from the pharynx to the stomach. It is lined by a stratified squamous epithelium and has a prominent muscularis mucosa and thick muscularis externa.
The muscularis externa of the esophagus is unique in that it transitions from striated to smooth muscle over the length of the tube. The esophagus ends in the gastro-esophageal junction. The stomach is the site where food is mixed with gastric juice and reduced to a fluid mass called chyme. The layers of the stomach wall follow the basic plan described above. The gastric glands are the basic structure of the stomach wall and can be thought of as tiny pits, or indentations, lined by epithelial cells.
Gastric glands are structured as a gastric pit that opens into the lumen, followed by an isthmus, neck, and base. There are several types of cells that are important in producing stomach secretions:. An increased absorptive area is useful because digested nutrients including sugars and amino acids pass into the villi, which is semi-permeable, through diffusion, which is effective only at short distances. In other words, the increased surface area in contact with the fluid in the lumen decreases the average distance traveled by the nutrient molecules, so the effectiveness of diffusion increases.
The small intestine uses different enzymes and processes to digest proteins, lipids, and carbohydrates. The small intestine is where most chemical digestion takes place. Most of the digestive enzymes in the small intestine are secreted by the pancreas and enter the small intestine via the pancreatic duct. These enzymes enter the small intestine in response to the hormone cholecystokinin, which is produced in response to the presence of nutrients.
The hormone secretin also causes bicarbonate to be released into the small intestine from the pancreas to neutralize the potentially harmful acid coming from the stomach. The three major classes of nutrients that undergo digestion are proteins, lipids fats , and carbohydrates. Proteins are degraded into small peptides and amino acids before absorption. Their chemical breakdown begins in the stomach and continues through the large intestine. Proteolytic enzymes, including trypsin and chymotrypsin, are secreted by the pancreas and cleave proteins into smaller peptides.
Carboxypeptidase, a pancreatic brush border enzyme, splits one amino acid at a time. Aminopeptidase and dipeptidase free the end amino acid products. Lipids fats are degraded into fatty acids and glycerol. Pancreatic lipase breaks down triglycerides into free fatty acids and monoglycerides. Pancreatic lipase works with the help of the salts from bile secreted by the liver and the gallbladder.
Bile salts attach to triglycerides and help to emulsify them; this aids access by pancreatic lipase because the lipase is water-soluble, but the fatty triglycerides are hydrophobic and tend to orient toward each other and away from the watery intestinal surroundings. The bile salts act to hold the triglycerides in their watery surroundings until the lipase can break them into the smaller components that are able to enter the villi for absorption.
Some carbohydrates are degraded into simple sugars, or monosaccharides e. Tunica submucosa lies immediately beneath the mucosa, and is a layer of loose to dense connective tissue containing blood and lymphatic vessels.
The submucosa also contains the submucous plexus, a critical component of the digestive tract's nervous system which provides nervous control to the mucosa. Tunica mucosa is the innermost layer of the digestive tube and lines the lumen. Among the four tunics, the mucosa is most variable in structure and function, endowing the tube with an ability to perform diverse and specialized digestive tasks along its length. Of critical importance in this regard are the epithelial cells that cover the mucosa and are thus in direct contact with the lumen.
This epithelial cell sheet lamina epithelialis is distinctly different in different regions of the tract. Oka, S. Ishihara, Y. Mishima et al. Yanaba, A. Yoshizaki, Y. Asano, T. Kadono, T. Tedder, and S. De Jonge, K. Kwikkers, A. Te Velde et al. Nakaya, Y. Xiao, X. Zhou et al. Cobbold, E. Adams, C. Farquhar et al. Carr, A. Kelman, G. Ikeda, M. Kinoshita, H. Kayama et al. Qiao, T. Sun, and F. Houdijk, E. Rijnsburger, J. Jansen et al. Chen, J. Shi, M. Qi, H. Yin, and C.
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Mannoor et al. Mellor and D. Ravindran, J. Loebbermann, H. Nakaya et al. Revelo, S. Winer, and D. Sundrud, S. Koralov, M. Feuerer et al. View at: Publisher Site Google Scholar. More related articles. Download other formats More. Related articles. L-arginine supplementation improved responses to DSS-induced injury and inflammation e. Dietary arginine supplementation had beneficial effects on clinical and biochemical parameters of colitis e. L-arginine supplementation improved intestinal mucosal immune function and maintained barrier integrity in response to LPS challenge e.
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