Anatomy & Physiology

Anatomy & Physiology Study Guide bolus into the esophagus. During this period, the respiratory centers are inhibited, and breathing stops. • The esophageal phase of swallowing begins as the contraction of pharyngeal muscles forces the bolus through the entrance to the esophagus. Once in the esophagus, the bolus is pushed toward the stomach by a peristaltic wave. The approach of the bolus triggers the opening of the lower esophageal sphincter, and the bolus then continues into the stomach. Primary peristaltic waves are peristaltic movements coordinated by afferent and efferent fibers in the glossopharyngeal and vagus nerves. For a typical bolus, the entire trip takes about nine seconds. Liquids take even less time, flowing ahead of the peristaltic contractions with the assistance of gravity. A dry or poorly lubricated bolus travels much more slowly, and a series of secondary peristaltic waves may be required to push it all the way to the stomach. Secondary peristaltic waves are local reflexes triggered by the stimulation of sensory receptors in the esophageal walls. 23.8 Stomach The stomach performs four major functions: (1) storage of ingested food, (2) mechanical breakdown of ingested food, (3) disruption of chemical bonds in food material through the action of acid and enzymes, and (4) production of intrinsic factor, a glycoprotein whose presence in the digestive tract is required for the absorption of vitamin B12. We can divide the stomach into four regions: • The smallest part of the stomach, the cardia , consists of the superior, medial portion of the stomach within 3 cm (1.2 in.) of the junction between the stomach and the esophagus. The cardia contains abundant mucous glands whose secretions coat the connection with the esophagus and help protect that tube from the acid and enzymes of the stomach. • The fundus is the portion of the stomach that is superior to the junction between the stomach and the esophagus. • The area of the stomach between the fundus is the body , the largest region of the stomach. The body functions as a mixing tank for ingested food and secretions produced in the stomach. Gastric ( gaster, stomach ) glands in the fundus and body secrete most of the acid and enzymes involved in gastric digestion. • The pylorus forms the sharp curve of the J (shape of the stomach). The pylorus is divided into a pyloric antrum (antron, cavity), which is connected to the body, and a pyloric canal , which empties into the duodenum, the proximal segment of the small intestine. The stomach’s volume increases while you eat and then decreases as chyme enters the small intestine. When the stomach is relaxed (empty), the mucosa is thrown into prominent folds called rugae . Regulation of Gastric Activity The CNS controls the production of acid and enzymes by the gastric mucosa; its regulated by short reflexes of the enteric nervous system, coordinated in the wall of the stomach; and regulated by hormones of the digestive tract. Gastric control proceeds in three overlapping phases, named Achieve Page 320 of 368 ©2018