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• Mouth: Foodstuffs are broken down mechanically by chewing and saliva is added as a lubricant. In some species, saliva contains amylase, an enzyme that digests starch. • Esophagus: A simple conduit between the mouth and stomach - clearly important but only marginally interesting compared to other regions of the tube. • Stomach: Where the real action begins - enzymatic digestion of proteins initiated and foodstuffs reduced to liquid form. • Liver: The center of metabolic activity in the body - its major role in the digestive process is to provide bile salts to the small intestine, which are critical for digestion and absorption of fats. • Pancreas: Important roles as both an endocrine and exocrine organ - provides a potent mixture of digestive enzymes to the small intestine which are critical for digestion of fats, carbohydrates and protein. • The Gall Bladder: • The gall bladder is a pouch-shaped organ which lies near the liver. It accepts bile from the liver, and stores it. When food is digested, the gallbladder releases bile into the small intestine where it is able to help dissolve fats. • Small Intestine: The most exciting place to be in the entire digestive system - this is where the final stages of chemical enzymatic digestion occur and where almost almost all nutrients are absorbed. o Jejunum The jejunum is the next portion of the small intestine, and it has a lining which is specialized in the absorption of carbohydrates and proteins. The proteins have been broken down in the stomach by enzymes called pepsin and acid into amino acids. The carbohydrates are broken down in the duodenum by enzymes from the pancreas and liver into sugars. Fats are broken down in the duodenum by "lipase" from the pancreas into fatty acids. Amino acid, sugar, fatty acid particles, vitamins, minerals, electrolytes and water are small enough to soak into the villi of the jejunum and drop into the blood stream. The blood takes all these nutrients to all the other parts of the body to provide fuel to do their jobs. o Duodenum The duodenum begins just beyond the stomach and curves around the head of the pancreas and the entrance of the common bile duct, in a C-shaped formation. At the spot where the stomach and duodenum meet, is a muscle called the pyloric sphincter which prevents the regurgitation of material back into the stomach. The duodenum is responsible for further processing the material from the stomach (called chyme), by secreting enzymes which aid in digestion. Bile and pancreatic juice also enter the duodenum around its midpoint, and by moving the chyme in a shaking kind of motion, the duodenum mixes the chyme with these enzymes within its lumen, further aiding digestion. • Large Intestine: Major differences among species in extent and importance - in all animals water is absorbed, bacterial fermentation takes place and feces are formed. In carnivores, that's about the extent of it, but in herbivores like the horse, the large intestine is huge and of critical importance for utilization of cellulose. • Rectum The rectum (Latin for "straight") is an 8-inch chamber that connects the colon to the anus. It is the rectum's job to receive stool from the colon, to let the person know that there is stool to be evacuated, and to hold the stool until evacuation happens. When anything (gas or stool) comes into the rectum, sensors send a message to the brain. The brain then decides if the rectal contents can be released or not. If they can, the sphincters relax and the rectum contracts, disposing its contents. If the contents cannot be disposed, the sphincter contracts and the rectum accommodates so that the sensation temporarily goes away. • Anus The anus is the last part of the digestive tract. It is a 2-inch long canal consisting of the pelvic floor muscles and the two anal sphincters (internal and external). The lining of the upper anus is specialized to detect rectal contents. It lets you know whether the contents are liquid, gas, or solid. The anus is surrounded by sphincter muscles that are important in allowing control of stool. The pelvic floor muscle creates an angle between the rectum and the anus that stops stool from coming out when it is not supposed to. The internal sphincter is always tight, except when stool enters the rectum. It keeps us continent when we are asleep or otherwise unaware of the presence of stool. When we get an urge to go to the bathroom, we rely on our external sphincter to hold the stool until reaching a toilet, where it then relaxes to release the contents.