Digestive System.
Publié le 11/05/2013
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The stomach, located in the upper abdomen just below the diaphragm, is a saclike structure with strong, muscular walls.
The stomach can expand significantly to storeall the food from a meal for both mechanical and chemical processing.
The stomach contracts about three times per minute, churning the food and mixing it with gastricjuice.
This fluid, secreted by thousands of gastric glands in the lining of the stomach, consists of water, hydrochloric acid, an enzyme called pepsin, and mucin (the main component of mucus).
Hydrochloric acid creates the acidic environment that pepsin needs to begin breaking down proteins.
It also kills microorganisms that may havebeen ingested in the food.
Mucin coats the stomach, protecting it from the effects of the acid and pepsin.
About four hours or less after a meal, food processed by thestomach, called chyme, begins passing a little at a time through the pyloric sphincter into the duodenum, the first portion of the small intestine.
C The Small Intestine
Most digestion, as well as absorption of digested food, occurs in the small intestine.
This narrow, twisting tube, about 2.5 cm (1 in) in diameter, fills most of the lowerabdomen, extending about 6 m (20 ft) in length.
Over a period of three to six hours, peristalsis moves chyme through the duodenum into the next portion of the smallintestine, the jejunum, and finally into the ileum, the last section of the small intestine.
During this time, the liver secretes bile into the small intestine through the bileduct.
Bile breaks large fat globules into small droplets, which enzymes in the small intestine can act upon.
Pancreatic juice, secreted by the pancreas, enters the smallintestine through the pancreatic duct.
Pancreatic juice contains enzymes that break down sugars and starches into simple sugars, fats into fatty acids and glycerol, andproteins into amino acids.
Glands in the intestinal walls secrete additional enzymes that break down starches and complex sugars into nutrients that the intestineabsorbs.
Structures called Brunner’s glands secrete mucus to protect the intestinal walls from the acid effects of digestive juices.
The small intestine’s capacity for absorption is increased by millions of fingerlike projections called villi, which line the inner walls of the small intestine.
Each villus isabout 0.5 to 1.5 mm (0.02 to 0.06 in) long and covered with a single layer of cells.
Even tinier fingerlike projections called microvilli cover the cell surfaces.
Thiscombination of villi and microvilli increases the surface area of the small intestine’s lining by about 150 times, multiplying its capacity for absorption.
Beneath the villi’ssingle layer of cells are capillaries (tiny vessels) of the bloodstream and the lymphatic system.
These capillaries allow nutrients produced by digestion to travel to the cells of the body.
Simple sugars and amino acids pass through the capillaries to enter the bloodstream.
Fatty acids and glycerol pass through to the lymphatic system.
D The Large Intestine
A watery residue of indigestible food and digestive juices remains unabsorbed.
This residue leaves the ileum of the small intestine and moves by peristalsis into the largeintestine, where it spends 12 to 24 hours.
The large intestine forms an inverted U over the coils of the small intestine.
It starts on the lower right-hand side of the bodyand ends on the lower left-hand side.
The large intestine is 1.5 to 1.8 m (5 to 6 ft) long and about 6 cm (2.5 in) in diameter.
The large intestine serves several important functions.
It absorbs water—about 6 liters (1.6 gallons) daily—as well as dissolved salts from the residue passed on by thesmall intestine.
In addition, bacteria in the large intestine promote the breakdown of undigested materials and make several vitamins, notably vitamin K, which the bodyneeds for blood clotting.
The large intestine moves its remaining contents toward the rectum, which makes up the final 15 to 20 cm (6 to 8 in) of the alimentary canal.The rectum stores the feces—waste material that consists largely of undigested food, digestive juices, bacteria, and mucus—until elimination.
Then, muscle contractionsin the walls of the rectum push the feces toward the anus.
When sphincters between the rectum and anus relax, the feces pass out of the body.
IV REGULATION OF THE DIGESTIVE PROCESS
The body coordinates the various steps of digestion so that the process proceeds smoothly and cells obtain a steady supply of nutrients and energy.
The centralnervous system and various glands control activities that regulate the digestive process, such as the secretion of enzymes and fluids.
For example, the presence of foodin the esophagus, stomach, or intestines triggers peristalsis.
Food entering the stomach also stimulates the central nervous system to initiate the release of gastric juice.And as hydrochloric acid passes from the stomach, the small intestine produces secretin, a substance that simulates secretion of pancreatic juice.
V AILMENTS OF THE DIGESTIVE SYSTEM
Infection of or damage to any part of the digestive system may affect digestion as well as other bodily functions.
Common infectious agents that attack digestive organsinclude the mumps virus, which often infects the salivary glands; the bacterium Helicobacter pylori, which causes most stomach and duodenal ulcers; and viruses and bacteria that cause various forms of gastroenteritis, often called stomach flu or traveler’s diarrhea.
Appendicitis is an inflammation of the appendix, a tube-like pouchabout 9 cm (3.5 in) long that branches off the large intestine.
It occurs most commonly among children and young adults.
Diarrhea—frequent elimination of loose,watery feces—is a symptom of many disorders that occurs when the large intestine is irritated or inflamed.
As a result, food residues move through it too quickly for itto absorb the excess water.
The opposite condition, constipation, occurs when the large intestine absorbs too much water because food residues are moving slowly.
As aresult, the feces become hard and dry, which may make elimination difficult.
Cancerous tumors may develop in any part of the digestive system, though they most commonly occur in the large intestine, rectum, and anus ( see Colorectal Cancer). Colitis, which has various causes, is a potentially life-threatening inflammation of the large intestine ( see Ulcerative Colitis).
Chronic conditions that cause at least intermittent distress include irritable bowel syndrome, caused by spasms of muscles in the lower intestine, and Crohn’s disease, an inflammation of the intestines.Abnormal sensitivity to proteins called glutens can damage the lining of the small intestine and hinder absorption of nutrients, leading to malnutrition and otherproblems.
The eating disorders anorexia nervosa and bulimia disrupt the normal functioning of the digestive system and are potentially fatal.
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