The Mammal's Digestive System

There are many pieces that make up a mammal's digestive system. These are the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.

The mammalian digestive system is made up of the alimentary canal and the various accessory glands that secrete digestive fluids into the canal and through the ducts. The mammalian digestive track is a four-layered wall. First the lumen, the cavity, is lined by a mucous membrane, the mucosa. Then comes the layer of connective tissue, which is followed by the smooth muscle. Last comes the outer most layer is a sheath of connective tissue that is attached to the membrane of the body cavity. After the food enters the mouth the peristalsis pushes the food down the tract. There are sphincters that regulate the passage of food between the cambers of the canal.

To being the trip of the digestive system we start with the oral cavity. The sensation of food in the oral cavity sends a reflex that activates the salivary glands that deliver saliva through ducts to the oral cavity. Saliva can even appear in the oral cavity before food is there, simply in anticipation.

Next the pharynx is in the throat which intersects the windpipe and the esophagus. When people swallow, the top of the trachea (windpipe) is closed off by the cartilaginous flap, called the epiglottis. This action prevents food or liquid from entering the windpipe.

The esophagus moves food from the pharynx down to the stomach. The muscles at the top of the esophagus are striated (voluntary). So the start of swallowing is voluntary, yet involuntary waves of contractions are taken over by the smooth muscles. The saliva continues to hydrolyze starch and glycogen as the food passes through the esophagus.

The stomach is a very elastic organ and is located just below the diaphragm and to the left side of the abdominal cavity. There is an epithelium that lines the lumen of the stomach which secretes gastric juice. Gastric juice is very acidic with a pH level of 2 so that all of the bacteria are killed. Also located in the gastric juice is pepsin, which is an enzyme that hydrolyzes proteins. The pepsin only breaks the peptide bonds adjacent to specific amino acids.

Even though some digestion takes place in the oral cavity and stomach, a lot of the digestion takes place in the small intestine. The pancreas, liver, and gallbladder are all also part of digestion. The pancreas makes several hydrolytic enzymes and an alkaline solution. The liver has many different functions one of which is to produce bile. The small intestine contains the duodenum in the first twenty-five centimeters. This is were the digestive fluid from the pancreas, liver, gallbladder and gland cell from the intestinal wall mixes with the acid chyme seeping from the stomach. There is limited absorption in the stomach and large intestine: mainly it occurs in the small intestine. There is a brush border that has a huge microvillar surface that is used for absorbing the nutrients.

The large intestine or colon is attached to the small intestine. The major function of the colon is to reabsorb water that has entered the alimentary canal. The colon finishes up the work by reclaiming the water that has remained in the lumen. The final phase of the digestive system is when the feces reach the terminal part of the colon called the rectum. This is where the feces are stored until they are eliminated.

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