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GI Tract and Function
Coach Hale, Tue Oct 11 2005, 09:29AM

GI TRACT AND FUNCTION
BY Jamie Hale

In a normal adult male the GI tract is approximately 25 feet and consists of the following components.

Mouth- the externally visible part of the oral cavity on the face and the system of organs surrounding the opening
Pharynx- the passage to the stomach and lungs; in the front part of the neck below the chin and above the collarbone
Esophagus- the passage between the pharynx and the stomach
Cardia- the opening into the stomach and that part of the stomach connected to the esophagus
Stomach- . vertebrates' digestive organ: an organ resembling a sac in which food is mixed and partially digested. It forms part of the digestive tract of vertebrates and is situated between the esophagus and the small intestine
Bowel or intestine:
Small intestine- (which has three parts- duodenum, jejunum, ileum ) the longest part of the alimentary canal; where digestion is completed
Duodenum- the part of the small intestine between the stomach and the jejunum
Jejunum- the part of the small intestine between the duodenum and the ileum
Ileum- the part of the small intestine between the jejunum and the cecum
Large intestine- (which has three parts- cecum, colon, rectum ) beginning with the cecum and ending with the rectum; includes the cecum and the colon and the rectum; extracts moisture from food residues which are later excreted as feces
cecum- the cavity in which the large intestine begins and into which the ileum opens
colon- the part of the large intestine between the cecum and the rectum; it extracts moisture from food residues before they are excreted
rectum- the lower part of the large intestine, between the colon and the anal canal (anus- excretory opening at the end of the alimentary canal )

Food after being mostly mechanically broken down in the mouth by teeth, tongue and saliva passes through the esophagus to the stomach where the process continues. It then passes to the small intestine where further breakdown continues, and the useful particles are absorbed into the bloodstream. The remaining particles pass through the large intestine and are expelled as feces.

The digestive system has a lining called mucosa ( mucus-secreting membrane lining all body cavities or passages that communicate with the exterior ). In the mouth, stomach and small intestine, the mucosa contains tiny glands that produce juices to help digest food. The liver and pancreas produce digestive juices that reach the intestine through small tubes. Other parts of organ systems play a key role in the digestive system as well.

Digestion is the process by which food and drink are broken down into their smallest parts so that the body can use them to build and nourish cells and to provide energy. The chemical process varies somewhat for different types of foods. Bacteria which naturally live in the GI tract do a lot of the actual chemical work of digesting for us.

The movement of organ walls cause movement of food through the system. Typical movement of the esophagus, stomach and intestine is call peristalsis. This action looks like an ocean wave moving through the muscle. The muscle of the organ causes a narrowing which then sends the narrowed portion slowly down the length of the organ. These waves of narrowing push the food and fluid in front of them through each hollow organ.

The first major movement occurs when food is swallowed. Once the swallow begins, movement becomes involuntary and proceeds under nerve control. The food is pushed into the esophagus. At the junction of the esophagus and stomach, there is a ring like valve closing the passage between the two. When the food approaches the closed ring, the surrounding muscles relax and allow food to pass. Now, the food enters the stomach which has three mechanical tasks. First, the stomach stores the swallowed food and liquid. The second job is to mix up the food, liquid, and digestive juice produced by the stomach . The third job of the stomach is to empty its contents slowly into the small intestine. As the food is digested in the small intestine and dissolved into the juices from the pancreas, liver, and intestine, the contents of the intestine are mixed and pushed forward to allow further digestion. Finally, all of the digested nutrients are absorbed through the intestinal walls. The waste products of this process include undigested parts of food, known as fiber, and older cells that have been shed from mucosa. These materials are launched into the colon, where they stay, usually for 1-2 days, until the feces are expelled by a bowel movement.

The glands the act first in the mouth are the salivary glands. Saliva produced by these glands contain an enzyme that begins to digest starch from food into small molecules. The next set of digestive glands is in the stomach lining. They produce stomach acid and an enzyme that digests protein.

After the stomach empties the food and juice mixture into the small intestine, the juices of two other digestive organs mix with the food to continue the process. One of these organs is the pancreas. It produces a juice that contains a wide variety of enzymes to break down carbohydrate, fat and protein in food. Other enzymes that are active in the process come from the glands in the wall intestine or even part of that wall. The liver produces another digestive enzyme called bile ( a digestive juice secreted by the liver and stored in the gallbladder; aids in the digestion of fats ). The bile is stored in between meals in the gallbladder (a muscular sac attached to the liver that secretes bile and stores it until needed for digestion ). At mealtime, it is squeezed out of the gallbladder into the bile ducts to reach the intestine and mix with the fat in food. The bile acids dissolve the fat into the watery contents of the intestine, much like detergents that dissolve grease from a frying pan. After the fat is dissolved, it is digested by enzymes from the pancreas and the lining of the intestine.

The digestible carbohydrates are broken into simpler molecules by enzymes in the salvia, in juice produced by the pancreas, and in the lining of the small intestine. Starch is digested in two steps: First, and enzyme (amylase) in the saliva and pancreatic juice breaks the starch into molecules called (maltose); then an enzyme in the lining of the small intesitine (maltase ) split’s the maltose into glucose molecules that can be absorbed into the blood. Glucose is carried through the bloodstream to the liver, where it is stored or used to provide energy for the work of the body. Table sugar must also be digested to be useful. An enzyme in the lining of the small intestien digests table sugar into glucose and fructose, each of which can be absorbed from the intestinal cavity into the blood. Milk contains anouterh type of sugar, lactose, which is changed into absorblale molecules by an enzyme called lactase, also found in the intestinal lining.

Protein must be digested by enzymes before they can be used to build and repair body tissues. An enzyme in the juice of the stomach starts the digestion of swallowed protein. Further digestion of the protein is completed in the small intestine. Here, several enzymes from the pancreatic juice and lining of the intestine carry outh the breakdown of huge protein molecules into small molecules called amino acids ( organic compounds containing an amino group and a carboxylic acid group Example: "Proteins are composed of various proportions of about 20 common amino acids"). These small molecules can be absorbed from the hollow of the small intestine into the blood and then carried to all parts of the body to build the walls and other parts of cells.

The first step in the digestion of fat is to dissolve it into the watery content of the intestinal cavity. The bile acids produced by the liver act as natural detergents to dissolve fat in water and allow the enzymes to break the large fat molecules into smaller molecules, some of which are fatty acids and cholesterol. The bile acids combine with the fatty acids and cholesterol and help these molecules to move into the cells of the mucosa. In these cells the small molecules are formed back into large molecules, most of which pass into vessels (called lymphatic) near the intestine. These small vessels carry the reformed fat to the veins of the chest, and the blood carries the fat to storage depots in different parts of the body.

Vitamins are another part of our food that is absorbed from the small intestine. Vitmains are classified as two different types: water soluble (all the B vitamins and Vitamin C) and fat soluble (vitamins A, D and K)

Refernces
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