1. Name and describe the function of each structure included on the human body.

At the end of the esophagus there is a muscular valve, or sphincter, through which food enters the stomach. This sphincter muscle keeps food in the stomach from being forced back into the esophagus. Peristalsis in the stomach churns the food and mixes it with mucus and with gastric juices, which contain enzymes and hydrochloric acid. These gastric juices are secreted from millions of small glands in the lining of the upper stomach walls. These glands pour about three quarts of fluid into the stomach daily. Similar glands in the small intestine also secrete enzymes and fluid. Hydrochloric acid secreted by the stomach sets up the sour or acid condition necessary for digestion. Certain remedies for indigestion are advertised as correcting this acid condition. If these remedies actually do get rid of the stomach acids it is not wise to take them. Acid is required for digestion to be properly undertaken in the stomach.

The stomach churns the food into a thick liquid, called chyme, before it is passed on by peristalsis into the small intestine. Another strong sphincter muscle further mashes the chyme and has some control over the rate at which it is passed out of the stomach into the duodenum, or upper small intestine. The sphincter also prevents the chyme from passing back into the stomach.

Little by little, as the digestive process in the stomach is completed, all the chyme is passed through the sphincter into the duodenum. This peristalsis is regulated by the autonomic nervous system. This process does not take place all at once. It continues over a period of time.

From the time a meal is eaten, it takes from 30 to 40 hours for food to travel the length of the alimentary canal. Different kinds of food, depending on their components, are held in the stomach for varying lengths of time. Starch and sugar are held in the stomach for a short time only, usually no more than one to two hours. Protein foods are there from three to five hours. Fat foods may remain in the stomach even longer than proteins. This is why eating a heavy dinner of meat, potatoes, and gravy satisfies our hunger longer than one made up entirely of sweets or greens. Furthermore, food made up of easily digested carbohydrates passes quickly from the stomach and into the small intestine.

The stomach, though important, is not considered by physicians to be essential to life. People who have had their stomachs completely or partially removed are frequently able to live by taking special foods in small quantities many times a day. The small intestine is then able to perform all necessary digestion.

Work of the Small Intestine

The small intestine, which is from 22 to 25 feet (6.7 to 7.6 meters) long, is the longest part of the digestive tract of humans. The main parts of the small intestine are the duodenum, jejunum, and ileum. Food remains in the small intestine for several hours. Two large glands, the liver and the pancreas, connect with the small intestine by ducts, or tubes. Through these ducts the liver and pancreas pour secretions which further aid digestion.

Fluid from the pancreas is called pancreatic juice. Fluid from the liver is called bile. The pancreas is one of the most important glands in the body. It secretes up to a pint of pancreatic juice a day. This digestive fluid contains enzymes which help digest carbohydrates, proteins, and fats.

One of these enzymes is trypsin, which helps digest protein foods. Other enzymes are amylase and maltase, which help digest carbohydrates. The pancreatic enzyme lipase, along with bile from the liver, helps digest fat. Bile, however, does not contain important enzymes.

Bile is stored in the gall bladder, a small hollow organ located just under the liver. We could not live without the liver but the gall bladder can be removed by surgery without serious effect.

The liver stores glycogen for later use by the body and furnishes clotting material for the blood. When fully digested, proteins are changed into amino acids; fat foods are changed into fatty acids; and carbohydrates are changed into sugars. These soluble food products are dissolved and then absorbed into the bloodstream through the walls of the small intestine.

While food is in the small intestine it is further diluted by fluid secreted by the intestinal glands. In an adult the small intestine is about 21 feet (6.4 meters) long. By the time the diluted food products have traveled its length, most of their nutrients have been absorbed into the bloodstream.

2. Discuss how each system works. Follow material through a system, or show how signals, impulses, or substances are controlled, secreted or used.

Before your body can use the nutrients in the food you consume, the cells must break down the nutrients both physically and chemically. This process of breaking down food into molecules the body can use is called digestion. Digestion occurs in the alimentary canal, or digestive tract, which begins at the mouth and winds through the body to the anus. Located along the alimentary canal are the stomach and the other organs that aid in digestion. Other digestive organs, such as the liver and pancreas, are not part of the alimentary canal but deliver secretions into the canal through the ducts.

When I take a bite of food, I begin the mechanical phase of digestion. In this phase the body physically breaks down chunks of food into small particles. Mechanical digestion increases the surface area of food on which digestive enzymes can act. Incisors - sharp, flat front teeth - cut the food. Then the broad, flat surfaces of molars, or back teeth, grind it up. The tongue helps keep this food between the chewing surfaces of the upper and lower teeth by manipulating it againts the hard plate, the bony, membrane-covered roof of the mouth. This is distinctly different structure from the soft plate, the area located just behind the hard plate, made of folded membranes and seperating the mouth cavity from the nasal cavity.

While the mechanical phase of digestion is occuring, the chemical phase of digestion is also taking place. Preparations for this phase begin even before the first bite of food is taken. The mouth starts to water - that is, the salivary glands increase their production of saliva, a mixture of water, mucus, and digestive enzyme called salivary amylase. In addition to the tiny salivary glands located in the lining of the mouth, there are three pairs of larger salivary glands.

The mucus in the saliva softens and lubricates food and helps hold it together. In addition, the salivary amylase begins the chemical digestion of carbohydrates by breaking down some starch into the disaccharide maltose.

Once food has been thoroughly chewed, moistened, and rolled into a bolus, or ball, it is forced by the swallowing action of the tongue into the pharynx. The pharynx is the open area that begins at the back of the mouth and serves as passageway for both air and food. During swalowing a flap of tissue called the epiglottis prevents food from entering the trachea, or windpipe. Instead the bolus passes into the esophagus, the approximatly 25-cm-long muscular tube that connects the pharynx with the stomach. The esophagus has two muscle layers - a circular layer that wraps around the esophagus and a longitudial layer that runs the lenght of the tube. Alternating contractions of these muscle layers push the bolus through the esophagus into the stomach. This series of rhytmic muscular contractions and relaxations is called peristalsis.

The bolus then enters the stomach. The stomach, an organ of both mechanical and chemical digestion, is located in the upper left side of the abdominal cavity, just below the diaphragm. It is an elastic bag that is J-shaped when full and that lies in folds when empty. In addition to circular and longitudinal muscles, the walls of the stomach have a third, diagonal layer of muscles. Together these muscles can twist the stomach and churn its contents. This churning helps the stomach perform mechanical digestion.

The inner lining of the stomach is a thick, wrinkled mucous membrane dotted with small openings called gastric pits. Gastric pits are the open ends of gastric glands that produce the cells that release secretions into the stomach. Some of these glands secrete mucus, others secrete digestive enzymes, and still others secrete hydrochloric acid. The mixture of these secretions forms the acidic gastric fluid.

Gastric fluid carries out chemical digestion in the stomach. The chemical breakdown of carbohydrates that started in the mouth continues in the stomach. The stomach also begins the chemical digestion of proteins. This process starts as the digestive enzyme pepsin splits complex protein molecules into shorter chains of amino acids called peptides. Pepsin is secreted as an inactive fluid called pepsinogen, which is converted into pepsin at a low pH. The presence of hydrochloric acid within the stomach ensures a low pH. In addition to helping to transform pepsinogen into pepsin, hydrochloric acid dissolves minerals and kills bacteria that enter the stomach along with food.

The mucus secreted in the stomach is vital to the survival of this organ. It forms a coating that protects the lining from hydrochloric acid and prevents pepsin from digesting the proteins that make up stomach tissue. In some people the mucus coating of the stomach tissue breaks down, allowing digestive enzymes to eat through part of the stomach lining. The sore that results is called an ulcer.

The food enters the stomach when the cardiac sphincter opens. The cardiac sphincer is a circular muscle located between the esophagus and the stomach. Once the food is in the stomach, the cardiac sphincter closes to prevent the food from regurgitating back into the esophagus. Food ussually remains in the stomach for two to three hours. During this time muscle contractions in the stomach walls churn its contents, breaking up food particles and mixing them with gastric fluid. This process forms a mixture called chyme, a name that comes from the greek word meaning "juice". Chyme ussually contains fats, sugars, maltose, starches, vitamins, minerals, coagulated milk protein, peptides, and proteins that were not changed by pepsin.

Peristalsis forces chyme out of the stomach and into the small intestine. The pyloric sphincter, a circular muscle between the stomach and the small intestine, regulates the flow of chyme. Each time the pyloric sphincter opens, about 5 to 15 mL of chyme moves into the small intestine until finally the stomach is empty.

When chyme enters the small intestine, it mixes with secretions from the liver and pancreas. The liver is a large organ located to the right of the stomach in the upper right area of the abdominal cavity just below the diaphragm. The liver plays a vital role in the digestion of fats by secreting bile. Bile is not a digestive enzyme but an emulsifying agent that breaks fat globules into small droplets, forming a milky emulsion. This process exposes a greater surface area of fats to the action of digestive enzymes and prevents small fat droplets from rejoining into large globules.

The bile secreted by the liver passes from the liver through a Y-shaped duct. The bile travels down one branch of the Y, the hepatic duct, and then up the other branch, the cystic duct, to the gallbladder, a saclike organ that stores and concentrates bile. When chyme is present in the small intestine, the gallbladder releases bile through the common bile duct into the small intestine

The pancreas is an organ that lies behind the stomach, againts the back wall of the abdominal cavity. It performs two highly different functions. As part of the digestive system the pancreas secretes pancreatic fluid, which contains digestive enzymes that help complete the breakdown of nutrients in the chyme. This pancreatic fluid enters the small intestine through the pancreatic duct, which joins the common bile duct just before it enters the intestine.

Pancreatic fluid contains sodium bicarbonate, which changes the pH of the chyme from acid to base. Once the pH has changed, the many enzymes in the pancreatic fluid are activated. Pancreatic amylase splits into disaccharides the molecules of starch or glycogen that were not acted upon by salivary amylase. Pancreatic lipase breaks fat into fatty acids and glycerol. Trypsin, chymotrypsin, and carboxypeptidase split proteins into peptides.

After the bile, pancreatic fluid and chyme are mixed the food continues through the small intestine. If you could stretch the small intestine to its full lenght, you would find it is nearly 7m long. The duodenum, the first section of this coiled tube makes up the first 25 cm of that lenght. The jejunum, the middle section, is about 2.5 m long. The ileum, which makes up the remaining portion of the small intestine, is approximately 4 m in lenght.

The secretions from the liver and pancreas enter the duodenum, where they continue the chemical digestion of the chyme. When the secretions from the liver and pancreas along with the chyme enter the duodenum, they trigger intestinal mucous glands to release large quantities of mucus. This mucus protects the intestinal wall from protein-digesting enzymes and the acidic chyme. Glands in the mucous lining of the small intestine release the following enzymes:

• Peptidase completes protein digestion by breaking down peptides into amino acids

• Maltase, lactase, and sucrase split the disaccharides maltose, lactose, and sucrose into monosaccharides

• Intestinal lipase splits fats into glycerol and fatty acids

The end products of digestion - amino acids, monosaccharides, glycerol and fatty acids - are then absorbed into the circulatory system through blood and lymph vessels in the lining of the small intestine. The structure of this lining provides a huge surface area throug hwhich absorbtion takes place. Absorbtion is the process by which the end products of digestion are transfered into the circulatory system. The highly folded lining is covered with millions of fingerlike projections called villi. The cells covering the villi, in turn, have extensions on their cell membranes called microvilli. The folds, villi, and microvilli give the small intestine a surface area of about 250 cm2, roughly the size of a tennis court. Nutrients are absorbed through this surface by means of diffusion and active transport.

Inside each villus are capillaries and tiny lymph vessels called lacteals. Glycerol and fatty acids enter the lacteals, are carried through the lymph vessels, and eventually reach the bloodstream through lymphatic ducts near the heart. Amino acids and monosaccharides enter the capillaries and are carried to the liver. The liver detoxifies the blood and removes any excess glucose, converting it to glycogen for storage. The filtered blood then carries the nutrients to all parts of the body.

Once absorbtion in the small intestine is complete, peristalsis moves the remaining material on to the large intestine. The large intestine or colon is the final organ of digestion. The four major parts of the colon are ascending colon, transverse colon, descending colon, and sigmoid colon. The rectum and the anal canal that it leads into are the names given to the very short final portions of the large intestine.

The material that enters the large intestine contains an abundance of water and minerals. Cellulose present in this material draws even more water into the intestine. This influx of water increases the mass of the material in the small intestine and expands the large intestine walls, triggering muscle contractions.

These contractions form the rhytmic peristaltic wave that moves the watery mass through the lenght of the intestine and eventually out of the body. As this matter moves through the intestine, the body then absorbs water and minerals, solidifying the mass in the process. The solidified material is called feces.

As the fecal matter solidifies, mucous secretions of the intestinal wall lubricate the wall to make the passing of the feces less abrasive. Mucus also binds the fecal matter together and neutralizes any acids that are released when bacteria act upon the feces. Such bacteria also aid in the manufacture of vitamin K. The feces are eliminated through the anus.

3. Discuss diseases and dysfunctions related to each system.

An ulcer is a pitting of a mucous or skin surface, that results from an erosion or disintegration of the tissues. Ulcers of the gastrointestinal tract, called peptic ulcers, are relatively common and are thought to occur in 1 to 20 percent of the population in developed countries.

Peptic ulcers occur most commonly in the duodenum near the junction with the stomach and in the stomach wall. They usually occur singly as round or oval lesions. The erosions are usually shallow but can penetrate the entire wall, leading to hemorrhage and possibly death. Pain, the predominant symptom, occurs one to three hours after a meal and is usually relieved with alkalis. Although the cause of such ulcers is not established, researchers have found a link between infection of the stomach lining by Helicobacter pylori bacteria and ulcers. Some ulcers have also been linked with the use of the antiarthritis drugs called Nsaids.

At any rate, when gastric juices (consisting of hydrochloric acid, mucus, and a digestive enzyme called pepsin) act upon the walls of the digestive tract, a peptic ulcer results. The fact that ulcers of the duodenum are frequently associated with excess secretion of gastric acid and that ulcers of the stomach are not suggests that the two lesions may be separate disease entities. Entry of acid-peptic contents from the stomach into the lower esophagus can also cause ulcers in this area. Peptic ulcers tend to become chronic.

The "chronic peptic ulcer" seems to be more related to psychological factors than are "stress ulcers," discussed below, but no identifiable psychological injuries have been reported, and they are not more common in "executive types." The chronic peptic ulcer develops when there is imbalance between the normal "aggressive" factors, the acid-peptic secretions, and the normal "resistance" factors such as mucous secretions and rapid cellular replacement. Psychological influences may alter these factors through cerebral reactions altering lower brainstem function, with the resultant vagal nervous stimulation directly affecting the stomach and duodenum.

The stress ulcer differs from peptic ulcers in their cause and characteristics. They usually occur in the stomach and are seen as multiple, shallow, bleeding erosions. Although stress ulcers tend to heal rapidly because of their shallowness, they can perforate and cause severe bleeding. They tend to cause less pain than the peptic ulcer. The term stress has led to misconceptions about the role of psychological factors in the development of ulcers. These stress ulcers occur most often in patients who have been subjected to marked physical injury such as severe trauma, burns (resulting in Curling's ulcer), or major surgery, and are more common in elderly or debilitated patients. Stress ulcers that occur because of central nervous system disease are called Cushing's ulcers.

Ulcers, whether chronic or stress, are usually treated by medical therapy alone. The drugs most commonly used are alkaline buffering agents and a special class of antihistamine called cimetidine, which blocks the histamine-2 receptors in the stomach that regulate gastric secretion. Newer drugs and drugs under development include sucrasulfate, prostaglandin compounds, and anticholinergic agents that inhibit acid secretion. The latter include omeprazol, ranitidine, and pirenzepine, which exerts antipepsin and antigastrin activity as well. Carbenoxolone, which increases mucus secretion, and colloidal bismuth, an antipeptic, have also been used. To eradicate H. pylori, a three-drug regime is used, including colloidal bismuth, antibiotics, and metronidazole.

When drug therapy alone fails, surgical therapy is attempted. This approach may include removal of the distal part of the stomach, which produces the gastric stimulant hormone called gastrin, combined with a vagotomy (severing of the vagus nerves) to remove stimulation from the central nervous system. The need for surgery can sometimes be averted by using endoscopes to bring laser heat or electric currents to the ulcer site. Such treatments help to stop ulcer bleeding in high-risk patients.