stomach where further chemical digestion takes place. Another mixing wave pushes
another small amount of the contents of the stomach into the duodenum. This
continuous forward and backward movement of the stomach contents results in a
thorough mixing of the food with the secretions of the gastric glands.
(2) Chemical digestion. The main chemical activity of the stomach is to
begin the digestion of proteins. Gastrin, a hormone produced by the stomach, stimulate
the secretion of gastric juices, primarily hydrochloric acid. Hydrochloric acid activates
the production of pepsinogen which in turn becomes pepsin. Pepsin starts a chemical
breakdown of proteins.
e. Small Intestine. Mechanical digestion and chemical digestion take place in
the small intestine. In fact, the major part of digestion and absorption occurs in the
small intestine.
(1) Mechanical digestion. The two main mechanical digestive processes
which take place in the small intestine are segmentation and peristalsis. Segmentation
is a forward and backward movement of food within a particular segment of the small
intestine. The food is not being moved forward but is being thoroughly mixed with
digestive juices and intestinal mucosa. (The intestinal mucosa makes absorption
easier.) Peristalsis, the second mechanical process, propels the chyme onward through
the intestinal tract. Peristaltic contractions in the small intestine are much weaker than
those in the esophagus or stomach. Chyme remains in the small intestine for from
three to five hours because chyme moves through this organ at a slow rate of one
centimeter per minute.
(2) Chemical digestion. When fats, carbohydrates, and acids pass through
the pyloric sphincter, the chemical digestive process in the small intestine begins. The
chyme which enters the small intestine is made up of partly digested carbohydrates,
partially digested proteins, and largely undigested lipids (a group of fat and fat-like
substances which are a source of the body's fuel). These carbohydrates, proteins, and
lipids finish the process of digestion in the small intestine. Fats, carbohydrates, and
acids passing through the pyloric sphincter trigger the release of hormones from the
intestinal mucosa. One of these hormones inhibits gastric secretions produced by the
liver and also inhibits motility. Two other hormones stimulate the flow of bile and
pancreatic juices that help in the emulsifying of fats and the chemical breakdown of
carbohydrates.
f. Large Intestine. The chief functions of the large intestine are to finish the job
of absorption, make certain vitamins, form feces, and expel feces from the body. Both
mechanical digestion and chemical digestion work to perform these functions.
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