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The mouth (oral cavity or buccal cavity) is formed by cheeks, lips, teeth, tongue, hard and soft palates. There are three pairs of salivary glands that secrete saliva. Saliva contains important digestive enzymes that help in chemical breakdown of the food. Food is broken down mechanically by the teeth. The tongue manipulates the bolus and moves it back and forth in the mouth while chewing. The rough parts of the tongue are called papillae and they are taste buds. On the top of the mouth there are hard and soft palates. The soft palate is the part that forms a block between the mouth and the nasopharynx which is connected to the hard palate which makes the roof of the mouth. When the mouth is done doing its job, the food is then referred to as a bolus and is swallowed. From the mouth the bolus moves to the pharynx (throat) and down the pharynx where it splits into two paths, one going to the lungs (trachea) and one going to the stomach. the one going to the stomach is called the esophagus. When the food is swallowed, a piece of cartilage called the epiglottis covers the trachea so no food gets into the lungs. In the left upper quadrant of the abdominal cavity, the stomach does both chemical and mechanical digestion. In the lower esophageal (cardiac) sphincter it is composed of muscle fibers that constrict when food is passed through the stomach, this helps the food from coming back into the esophagus. The upper part of the stomach are storage areas. the lining of the stomach consists of mucous membranes and contains lots of macroscopic folds (rugae). The rugae unfold when the stomach fills to make more room. There are also digestive glands that secrete hydrochloric acid and enzymes (mechanical). This turns the bolus into a liquid which moves out of the stomach to the pyloric sphincter which controls the speed the bolus (now liquid) enters the small intestine. The small intestine is a 20 foot long coiled up tube that connects with the large intestine. The small intestine has three parts: first, duodenum (upper segment) and is 10 inches long. Second, jejunum and is 8 feet long. Third is the ileum and is 12 feet long. Digestion is finished here. Nutrients from the liquid is absorbed by finger like things that are called villi then it goes to the bloodstream and lymphatic system. The left over liquid is sent into the large intestine. The large intestine ends at the anus. No digestion takes place in the large intestine. The only secretion that happens is in the colon where mucous coats the extra stuff so it can move smoothly through the body. The colon's function is to absorb water and minerals and get rid of undigested material. At the end of the large intestine it meets up with the rectum and anus that is in charge of defecation. The liver, pancreas, and gallbladder all aid in digestion as well. The liver weighs about three to four pounds. The liver produces bile which helps absorb fat from the bolus in the small intestine. The liver also removes sugar from the bloodstream to make starch and store it for later. It also stores vitamins like B12, A, D, E, K. It destroys or lessens any thing that may be toxic so it is not as harmful if at all. And it produces various blood proteins like prothrombin and fibrinogen which aid in blood clotting. The pancreas secretes insulin straight into the blood to maintain normal blood glucose (sugar) levels. It also produces digestive enzymes that help the small intestine in digestion. The digestive enzymes that the pancreas produces contain trypsin which help break down protein; amylase which helps break down carbohydrates; and lipase which breaks down fat. And finally the gallbladder is the storage area for bile that is produced by the liver. Whenever bile is needed for digestion the gallbladder releases it. Without bile, fat digestion is not possible.
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