The process of taking food into the mouth is called ingestion. The mouth receives the ingested food, breaks it up into smaller pieces, mixes it with saliva, and sends the food as a bolus to the pharynx, then into the esophagus. In addition to the physical digestion of breaking the food into smaller pieces, some chemical digestion begins in the mouth, especially for starches. Then the esophagus transports the bolus of food to the stomach. A detailed description of this part of the process can be found in Chapter 5. The stomach acts as a blender, mixing the food with digestive juices secreted by specialized cells in the stomach lining. One of the digestive chemicals produced in the stomach is hydrochloric acid at a concentration strong enough to eat away shoe leather. A large amount of mucus present in the stomach protects the lining cells from this acid.
The contents of the stomach are squirted into the small intestine at regular intervals. Locally produced hormones control this process. The material at this time is called chyme and consists of a combination of ingested food, saliva, and stomach juices. The material in the small intestine will go through the rest of the digestive process and be absorbed into the lining cells of that part of the tube. Additional digestive juices are brought into the small intestine from the pancreas and gallbladder. The pancreas contributes additional enzymes to break down what is left of starch, protein fragments, and triglycerides. The final breakdown of the ingested food, including disaccharides, occurs at the surface of the columnar cells lining the tube, and is then absorbed into the lining cells. Nearly all the absorption of nutrients occurs in the small intestine. When nutrients leave the digestive tract, they go either to the body's tissues or to the liver. The liver is an accessory organ to the digestive tract that regulates much of what goes out to the body through the bloodstream. A specific description of this part of the process can be found in Chapter 5.
Most of the water that enters the digestive tract with food or from the digestive juices of the stomach and pancreas is actively removed from the tube by the large intestine. The removal of most of the water from the digestive tube creates the material that will be eliminated from the body in the form of feces. There are a large number of goblet cells in this portion of the tube to produce the mucus necessary to move the feces through the rest of the tract. Whatever has not been broken down or absorbed in the digestive process will be eliminated through the rectum and the anus. This is discussed further in Chapter 6.
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