The Digestive SystemAddison Martin

General Information

Functionof the Digestive System

The gastrointestinal tract (abbreviated here as GI) is the system of organs that takes in and mechanically and chemically breaks down food so that it can be absorbed and used in the body, and then expels the unused material.

In humans, the GI has several organs, the oral cavity, pharynx, esophagus, stomach, small intestine, and the large intestine.http://meta.wikimedia.org/wiki/File:Digestive_system_diagram_en.svg

Phases of DigestionCephalic, Gastric, and Intestinal phase

Cephalic Phase – This is the phase before food enters the stomach. The sight, smell, taste, and sensation of food causes the brain to prepare the body for digestion

Gastric Phase – This phase can take 3-4 hours and takes place in the stomach. The presence of food in the stomach causes the it to secrete enzymes and acids to break down food.

Intestinal Phase – This phase begins with food entering the intestines. This is when nutrients are absorbed.

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Organs & PartsOf the Digestive System

The Oral Cavity

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The function of the oral cavity is to mechanically break down food and to begin breaking it down.

The oral cavity houses the tongue and teeth, which aid in the mechanical break down of food and helps to create balls of food called a bolus.

The salivary ducts also lead into the oral cavity.

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The Salivary Glands

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There are four major salivary glands with hundreds of minor salivary glands throughout the mouth cavity.

The salivary glands produce saliva, which keeps food moist, begin digestion, help create a food bolus, and keep the oral cavity moist and lubricated.

Saliva is composed of mostly water but also electrolytes, cheek cells, mucus, various enzymes, and other small biochemical compounds.

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Saliva begins the digestive process because of enzymes in the saliva which begin breaking materials down. Here are a list of some of the enzymes found in saliva and their function:

Amylase – breaks down starch and lipase fatLysozyme – an enzyme that causes bacteria to lyse

Saliva contains other enzymes. Some enzymes are designed so that they do not function unless in the acidic environment of the stomach.

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The Pharynxhttp://www.easttroy.k12.wi.us/hs/dept/science/bottum/Adv%20Biology/

The pharynx is the throat. The pharynx’s function is to separate the windpipe from the esophagus.

The glottis, or opening to the windpipe is blocked by the cartilaginous flag called the epiglottis.

The pharynx ensures that a food bolus will go down the esophagus and not the windpipe, protecting the respiratory system.

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The Esophagushttp://doctorscience.wordpress.com/

The esophagus is the tube leading food from the oral cavity and pharynx through the diaphragm and into the stomach.

Although the control of the pharynx and the act of swallowing is voluntary, the transportation of a bolus to the stomach is involuntary.

The esophagus moves a food bolus to the stomach in through rhythmic contractions of the smooth muscle called peristalsis.

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The Stomachhttp://shirt.woot.com/Forums

The elasticity and large capacity of the stomach allows the stomach to store food, and thus we do not nee to constantly eat.

An acidic mixture of several liquids, called gastric juice, is secreted by the stomach to disrupt the extracellular matrix between plant and animal cells and kill bacterial cells.

Gastric juice also contains pepsin. Pepsin is an enzyme that begins to hydrolyze polypeptides. This enzyme is unique because of its functionality in highly acidic environments.

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The inactive form of pepsin is secreted by specialized cells in the stomach and is called pepsinogen. Similar specialized cells secrete HCl, one of the main components of gastric juice, which activates the pepsinogen.

Mucus coating the stomach wall, rapid mitosis regenerating stomach wall cells, and the fact that pepsin is secreted in its inactive form keeps the stomach from digesting itself.

About every 20 seconds the stomach involuntarily churns its contents.

The enzymatic activity, and other chemical reactions within the stomach turns a bolus of food into a nutrient broth known as acid chyme.

The stomach is normally closed at both ends. The orifice from the esophagus opens when food travels down the esophagus. The pyloric sphincter regulates the passage of acid chyme into the intestine.

Ulcers can form in the GI tract (often in the stomach, esophagus, or most often in the duodenum). Ulcers found anywhere in the GI tract are called peptic ulcers.

An ulcer is defined as mucosal erosions equal or greater that .5 cm. This erosion of the mucosa, or mucus membrane, can sometime expose the submucosa which is the dense irregular connective tissue that connects the mucosa to the muscles of the GI tract.

The Small Intestine

Acid chyme enters the small intestine by the pyloric sphincter.

The small intestine is the site of the majority of the break down of macromolecules in food and absorption of nutrients. The hollow part of the intestine is the lumen.

The duodenum, the first section of the small intestine, is the small section of intestine where digestive juices from the pancreas, liver, gallbladder, various glands, and the small intestine itself enzymatically break down the acid chyme and neutralizes it.

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The less acidic, and broken down material is easier for the rest of the small intestine to absorb and less harmful. (The duodenum is protected by mucus secreted by glands along its wall).

Enzymes within the duodenum, such as tryspin and chymotryspin, break down specific bonds within polypeptides.

The jejunum and ileum are the next sections of the small intestine, and are very similar.

The jejunum marks the beginning of mats of villi, finger-like projections to increase surface area, and microvilli, microscopic finger-like projections on the surface of villi, which absorb nutrients.

Within each villi are many capillaries and lacteal, small lymphatic vessels.

Two layers of cells separate the capillaries and lacteal from the lumen, allowing for easy absorption through passive or active transportation.

The Liver

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The major functions of the liver include protein metabolism and synthesis as well as bile production. But it also plays a role in detoxification, production of digestive juices, storing glycogen, hormone production, decomposition of red blood cells, and plasma protein synthesis.

The liver is the only organ that can regenerate itself, mostly due to hepatocytes, or liver cells, re-entering the cell cycle.

(Hepato-/hepatic means liver)

The blood vessels with newly absorbed nutrients from the villi and the rest of the small intestine converge at the hepatic portal vessel which leads directly to the liver. This ensures that the liver gets first access to the sugars and amino acids absorbed from a meal, due to the liver’s metabolic versatility and so the liver can regulate the level of sugars and other nutrients and remove the toxins in newly nutrient-enriched blood.

The liver is a vital organ present in all vertebrates.

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The Gallbladder

The gallbladder’s (or cholecyst) main function is to store and concentrate the bile, a green fluid produced by the liver containing bile salts, in preparation for digestion of fat.

Bile salts/acids created by the liver from cholesterol and stored in the gallbladder serve to coat tiny fat droplets and keep them from coalescing, a process called emulsification. The emulsification of fat allows for a greater surface area of fat to be exposed to lipase.

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The Pancreaswww.answers.com

The pancreas is both an exocrine gland and endocrine gland. This means that it creates important hormones (endocrine) as well as digestive enzymes (exocrine).

The exocrine pancreas secretes lipase, amylase, and alkaline fluids (to help neutralize the acid chyme) as well as carboxypeptidase, trypsin, and chymotrypsin in inactive forms into the duodenum by the pancreatic duct.

The endocrine pancreas produces glucagon, insulin, somatostatin, and other polypeptides.

The Biliary Tract

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The biliary tract, (or biliary tree) is the name of the various bile ducts that converge and lead from the liver and gallbladder to the common bile duct, which is the duct that secretes bile into the duodenum.

The biliary tract is not an organ, but rather the series of ducts.

The Large Intestine

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The large intestine, or colon, is where food enters after exiting the small intestine. At this T-shaped intersection of the small and large intestine, is a small pouch called a cecum. A finger-like projection from the cecum is the appendix.

Peristalsis moves the waste from the small intestine through the large intestine as most of the water that was used as a solvent of various digestive juices.

Many (mostly harmless) bacteria live in the colon as well and feed on byproducts that would otherwise be discarded.

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Some of these bacteria produce gasses (i.e. methane and hydrogen sulfide) and others even produce vitamins (i.e. biotin, folic acid, vitamin K and some kinds of vitamin B) which is then absorbed.

The colon can also secrete various material that is not needed to the body such as salts, iron, or calcium that is later expelled.

The rectum is the terminal portion of the colon where fecal matter is stored until it can be eliminated.

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Control & Regulation

Hormoneswww.skeptic.com

The digestive system is controlled by various hormones. The brain, stomach, duodenum, and pancreas all secrete hormones into the circulatory system that initiate different reactions.

Gastrin, released from the stomach, causes the stomach wall to secrete HCl.

Secretin, released from the duodenum when acid chyme enters the small intestine, signals the pancreas to produce bicarbonate to neutralize the acidic chyme.

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Cholecystokinin (CCK), released from the duodenum if there are amino acids or lipids in the chyme, causes the gallbladder to contract which release bile into the small intestine.

CCK and secretin and other hormones secreted from the wall of the duodenum are collectively called enterogastrones.

Enterogastrones can also control the speed at which material moves through the digestive system depending on the contents of the chyme entering the duodenum.

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Nervous Systemwww.faculty.washington.edu

The section of the nervous system that regulates the GI tract is a very complex is considered like a second brain. In fact, it contains more neurons than the spinal chord. This section of the nervous system is called the enteric nervous system. It is an offshoot of the autonomic nervous system which acts to control autonomous actions such peristalsis or sphincter contractions.

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