and skeletal muscle, and the lower third has only smooth muscle. There is no serosal layer but there is an adventitial layer. It is not covered by visceral peritoneum. The act of swallowing is called deglutition. It is the movement of food as a bolus from the mouth to the stomach. It takes about a second for liquids to pass through the esophagus and 4-8 seconds for solid or semisolid food to pass through. The process is not passive and involves a complex series of muscular activities that are both conscious and unconscious phases. The voluntary phase of swallowing is controllable. Chewing is over and the tongue moves upward and backward against the palate so that food can get into the oropharynx. There are muscles that kick in to prevent food from getting into the trachea or nasopharynx. The pharyngeal phase involves receptors in the oropharynx that sense food, sending it to the “deglutition center” in the medulla oblongata. Breathing stops briefly by the closure of the epiglottis and pharyngeal muscles constrict to move the bolus through the pharynx. The upper esophageal sphincter allows food to enter the esophagus. The esophageal phase involves peristalsis, controlled by the medulla oblongata. There are circular and longitudinal muscles that contract to push the bolus through. It is a short reflex that relaxes the lower esophageal sphincter to allow food to pass into the stomach.
STOMACH ANATOMY AND PHYSIOLOGY Chemical digestion starts minimally in the mouth but really advances in the stomach. The stomach links the esophagus to the small intestine. It contracts readily to cause mechanical digestion. It can stretch to more than 75 times its empty size in order to take in as much as four liters of food or fluid. It is also a receptacle for food, letting in only a little bit of food at a time into the small intestine. The food that is mixed with digestive juices is called chyme, which is made in the stomach. Little nutrient absorption occurs. The four main regions of the stomach are the cardia, the fundus, the body, and the pylorus. The cardia is the first part, located just after the esophagus. The next part is the fundus, which is dome-shaped. Below this is the main portion—the body. The pylorus is the last part, connecting the stomach to the duodenum. It is funnel-shaped, with the largest part being called the pyloric
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