Digestion is the process of metabolism whereby a biological entity processes a substance in order to chemically and mechanically convert the substance for the body to use. Preparation for digestion begins with the cephalic phase producing saliva and enzyme production. Mechanical and chemical digestion occur in the mouth and stomach; food is chewed, and mixed with saliva in the mouth, and further broken down through mechanical and chemical processing in the stomach. Absorption occurs in the stomach and gastrointestinal tract, and the process finishes with excretion.
Phases of human digestion
Cephalic phase - This phase occurs before food enters the stomach and involves preparation of the body for eating and digestion. Sight and thought stimulate the cerebral cortex. Taste and smell stimulus is sent to the hypothalamus and medulla oblongata. After this it is routed through the vagus nerve.
Gastric phase - This phase takes 3 to 4 hours. It is stimulated by distention of the stomach and alkaline pH. Distention activates long and myentric reflexes. This activates the release of acetylcholine which stimulates the release of more gastric juices. As protein enters the stomach, it binds to hydrogen ions, which raises the pH of the stomach to an alkaline level. This triggers G cells to release gastrin, which in turn stimulates parietal cells to secrete HCl. HCl release is also triggered by acetylcholine and histamine.
Intestinal phase - This phase has 2 parts, the excitatory and the inhibitory. Partially-digested food fills the duodenum. This triggers intestinal gastrin to be released. Enterogastric reflex inhibits vagal nuclei, activating sympathetic fibers causing the pyloric sphincter to tighten to prevent more food from entering, and inhibits local reflexes.
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