Swallowed food is passed through the esophagus to the stomach by wavelike contractions known as peristalsis. The mucosa of the stomach secretes hydrochloric acid and pepsinogen. Upon entering the lumen of the stomach, pepsinogen is converted into the active protein-digesting enzyme known as pepsin. The stomach partially digests proteins and functions to store its contents, called chyme, for later processing by the small intestine.
Mastication (chewing) of food mixes it with saliva, secreted by the salivary glands. In addition to mucus and various antimicrobial agents, saliva contains salivary amylase, an enzyme that can catalyze the partial digestion of starch. Deglutition (swallowing) begins as a voluntary activity in which the larynx is raised so that the epiglottis covers the entrance to the respiratory tract (chapter 16), preventing ingested material from entering. Swallowing involves three phases—oral, pharyngeal, and esophageal—that require the coordinated contraction of 25 pairs of muscles in the mouth, pharynx, larynx, and esophagus. The formation of a bolus (a mass to be swallowed) of food in the mouth is under voluntary control, while the pharyngeal and esophageal phases are involuntary and cannot be stopped once they have begun. The pharyngeal phase involves striated muscles of the larynx, pharynx, and mouth (the tongue and suprahyoid muscles), which are innervated by somatic motor neurons. The lower esophagus contains smooth muscles, innervated by autonomic neurons. The pattern of contractions required for swallowing is coordinated by interacting neurons in the medulla oblongata, which function as a swallowing center.
Once in the stomach, the ingested material is churned and mixed with hydrochloric acid and the protein-digesting enzyme pepsin. The mixture thus produced is pushed by muscular contractions of the stomach past the pyloric sphincter (pylorus = gatekeeper), which guards the junction of the stomach and the duodenum of the small intestine.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.