The Digestive System The Digestive System By: Aubrey

Bibliography http://celiacdisease.about.com http://www.biology-online.org http://mechanicaldigestion.org http://www.nlm.nih.gov http://www.livestrong.com http://www.britannica.com http://www.enchantedlearning.com http://en.wikipedia.org http://www.webmd.com/ http://www.buzzle.com

Mucus Mucus is something everyone has, and some people wish they had a lot less of the stringy, gooey stuff.  Sure, it can be gross to blow globs of snot into tissue after tissue when you have a cold or sinus infection, but mucus actually serves a very important purpose.

Chemical Digestion Chemical digestion doesn't begin in your stomach, but in your mouth. The moment you see, smell or even think about food, your mouth begins to produce extra saliva. Saliva contains an amylase enzyme called ptyalin, which breaks starches down into dextrose and maltose by adding a water molecule into the starch compound. Once food is swallowed, powerful muscles in the esophagus push it downward into the stomach in a continuous wave motion.

Mouth The digestive process begins in the mouth. Food is partly broken down by the process of chewing and by the chemical action of salivary enzymes (these enzymes are produced by the salivary glands and break down starches into smaller molecules).

Enzymes Enzymes have extremely interesting properties that make them little chemical-reaction machines. The purpose of an enzyme in a cell is to allow the cell to carry out chemical reactions very quickly. These reactions allow the cell to build things or take things apart as needed. This is how a cell grows and reproduces. At the most basic level, a cell is really a little bag full of chemical reactions that are made possible by enzymes! Enzymes are made from amino acids, and they are proteins. When an enzyme is formed, it is made by stringing together between 100 and 1,000 amino acids in a very specific and unique order. The chain of amino acids then folds into a unique shape. That shape allows the enzyme to carry out specific chemical reactions -- an enzyme acts as a very efficient catalyst for a specific chemical reaction. The enzyme speeds that reaction up tremendously.

Mechanical Digestion Mechanical digestion happens in the mouth with the help of the saliva, teeth and tongue. It is the actual break down of food into smaller bits. This makes food easier to digest. The chemical content of the food is extracted during the process.  This is largely carried out by by digestive enzymes which are enzymes that break down polymeric macromolecules into their smaller parts, so that they can be absorbed by the body.  

Salivary Amylase Amylase is an enzyme found in human saliva which is responsible for the breaking down of starch into sugar. The process of digestion begins with the chewing of food, in the presence of salivary amylase in the mouth, to convert the starch in food to sugar. Foods that are high in starch, like potatoes or bread, will taste slightly sweet when chewed - this is because of the action of amylase. The enzyme amylase is also secreted by the pancreas, where it is called pancreatic amylase; its presence in the gastrointestinal tract aids in the breakdown of food molecules into energy for the body to store and use. The diagram of the digestive system will clarify the sequence of digestion and also help in the identification of the organs of the digestive system.

The Esophagus The esophagus is a muscular tube connecting the throat (pharynx) with the stomach. The esophagus is about 8 inches long, and is lined by moist pink tissue called mucosa. The esophagus runs behind the windpipe (trachea) and heart, and in front of the spine. Just before entering the stomach, the esophagus passes through the diaphragm.

The Stomach The stomach is a muscular organ located on the left side of the upper abdomen. The stomach receives food from the esophagus. As food reaches the end of the esophagus, it enters the stomach through a muscular valve called the lower esophageal sphincter. The stomach secretes acid and enzymes that digest food. Ridges of muscle tissue called rugae line the stomach. The stomach muscles contract periodically, churning food to enhance digestion. The pyloric sphincter is a muscular valve that opens to allow food to pass from the stomach to the small intestine.

Gastric Juices The gastric juice is made up of hydrochloric acid, pepsinogen and other digestive enzymes, intrinsic factor, gastrin, mucus, and bicarbonates. It has a pH ranging from 1 to 2. Its low pH is essential in activating many digestive enzymes and in destroying various pathogens. The gastric juice also contains bicarbonates to neutralize the acid and thereby regulate the pH level in the stomach.

Chyme chyme, a thick semifluid mass of partially digested food and digestive secretions that is formed in the stomach and intestine during digestion. In the stomach, digestive juices are formed by the gastric glands; these secretions include the enzyme pepsin, which breaks down proteins, and hydrochloric acid. Once food is in the small intestine, it stimulates the pancreas to release fluid containing a high concentration of bicarbonate. This fluid neutralizes the highly acidic gastric juice, which would otherwise damage the membrane lining of the intestine, resulting in a duodenal ulcer. Other secretions from the pancreas, gallbladder, liver, and glands in the intestinal wall add to the total volume of chyme.

Villi Microscopic finger-like projections that line the inner wall of the small intestine. (Singular: villus.) After food passes from the stomach into the small intestine, nutrients in the food are absorbed into the body through the villi. Every person has millions of villi in his intestines.

Small Intestine The small intestine consists of three sections. The first portion, called the duodenum, connects to the stomach. The middle portion is the jejunum. The final section, called the ileum, attaches to the first portion of the large intestine

Appendix The appendix sits at the junction of the small intestine and large intestine. It’s a thin tube about four inches long. Normally, the appendix sits in the lower right abdomen. The function of the appendix is unknown. One theory is that the appendix acts as a storehouse for good bacteria, “rebooting” the digestive system after diarrheal illnesses. Other experts believe the appendix is just a useless remnant from our evolutionary past. Surgical removal of the appendix causes no observable health problems.

Duodenum The duodenum is the first part of the small intestine. It is located between the stomach and the middle part of the small intestine, or jejunum. After foods mix with stomach acid, they move into the duodenum, where they mix with bile from the gallbladder and digestive juices from the pancreas. Absorption of vitamins, minerals, and other nutrients begins in the duodenum.

Pancreas The pancreas is about 6 inches long and sits across the back of the abdomen, behind the stomach. The head of the pancreas is on the right side of the abdomen and is connected to the duodenum (the first section of the small intestine) through a small tube called the pancreatic duct. The narrow end of the pancreas, called the tail, extends to the left side of the body

Liver The liver is a large, meaty organ that sits on the right side of the belly. Weighing about 3 pounds, the liver is reddish-brown in color and feels rubbery to the touch. Normally you can't feel the liver, because it's protected by the rib cage. The liver has two large sections, called the right and the left lobes. The gallbladder sits under the liver, along with parts of the pancreas and intestines. The liver and these organs work together to digest, absorb, and process food. The liver's main job is to filter the blood coming from the digestive tract, before passing it to the rest of the body. The liver also detoxifies chemicals and metabolizes drugs. As it does so, the liver secretes bile that ends up back in the intestines. The liver also makes proteins important for blood clotting and other functions.

Bile/Bile Duct A bile duct is any of a number of long tube-like structures that carry bile. Bile, required for the digestion of food, is secreted by the liver into passages that carry bile toward the hepatic duct, which joins with the cystic duct (carrying bile to and from the gallbladder) to form the common bile duct, which opens into the intestine. The biliary tree is the whole network of various sized ducts branching through the liver.

Gall Bladder The gallbladder is a small pouch that sits just under the liver. The gallbladder stores bile produced by the liver. After meals, the gallbladder is empty and flat, like a deflated balloon. Before a meal, the gallbladder may be full of bile and about the size of a small pear. In response to signals, the gallbladder squeezes stored bile into the small intestine through a series of tubes called ducts. Bile helps digest fats, but the gallbladder itself is not essential. Removing the gallbladder in an otherwise healthy individual typically causes no observable problems with health or digestion yet there may be a small risk of diarrhea and fat malabsorption

Absorption Absorption is the movement of molecules across the gastrointestinal (GI) tract into the circulatory system. Most of the end-products of digestion, along with vitamins, minerals, and water, are absorbed in the small intestinal lumen by four mechanisms for absorption: (1) active transport, (2) passive diffusion, (3) endocytosis, and (4) facilitative diffusion. Active transport requires energy.

Large Intestine The large intestine (colon) extends from the cecum to the anus and includes the ascending colon, the transverse colon, the descending colon, the sigmoid colon, and the rectum.