Chapter 18: Digestive System

Chapter Summary

Chapter 18: Digestive System

Introduction to the Digestive System (pp. 615-616)

  1. The digestive system mechanically and chemically breaks down food to forms that can be absorbed through the intestinal wall and transported by the blood and lymph for use at the cellular level.

  2. The digestive system consists of a gastrointestinal (GI) tract and accessory digestive organs.


Serous Membranes and Tunics of the Gastrointestinal Tract (pp. 616-620)

  1. Peritoneal membranes line the abdominal wall and cover the visceral organs. The GI tract is supported by a double layer of peritoneum called the mesentery.

    1. The lesser omentum and greater omentum are folds of peritoneum that extend from the stomach.

    2. Retroperitoneal organs are positioned behind the parietal peritoneum.

  2. The layers (tunics) of the abdominal GI tract are, from the inside outward, the mucosa, submucosa, tunica muscularis, and serosa.

    1. The mucosa consists of a simple columnar epithelium, a thin layer of connective tissue called the lamina propria, and thin layers of smooth muscle called the muscularis mucosae.

    2. The submucosa is composed of connective tissue; the tunica muscularis consists of layers of smooth muscle; and the serosa is composed of connective tissue covered with the visceral peritoneum.

    3. The submucosa contains the submucosal plexus, and the tunica muscularis contains the myenteric plexus of autonomic nerves.


Mouth, Pharynx, and Associated Structures (pp. 620-627)

  1. The oral cavity is formed by the cheeks, lips, and hard palate and soft palate. The tongue and teeth are contained in the oral cavity.

    1. Lingual tonsils and papillae with taste buds are located on the tongue.

    2. Structures of the palate include palatal folds, a cone-shaped projection called the palatine uvula, and palatine tonsils.

  2. The incisors and canines have one root each; the bicuspids and molars have two or three roots.

    1. Humans are diphyodont; they have deciduous and permanent sets of teeth.

    2. The roots of teeth fit into sockets called dental alveoli that are lined with a periodontal membrane. Fibers in the periodontal membrane insert into the cementum covering the roots, firmly anchoring the teeth in the sockets.

    3. Enamel forms the outer layer of the tooth crown; beneath the enamel is dentin.

    4. The interior of a tooth contains a pulp cavity, which is continuous through the apical foramen of the root with the connective tissue around the tooth.

  3. The major salivary glands are the parotid glands, the submandibular glands, and the sublingual glands.

  4. The muscular pharynx is a passageway connecting the oral and nasal cavities to the esophagus and larynx.


Esophagus and Stomach (pp. 628-632)

  1. Swallowing (deglutition) occurs in three phases and involves structures of the oral cavity, pharynx, and esophagus.

  2. Peristaltic waves of contraction push food through the lower esophageal sphincter into the stomach.

  3. The stomach consists of a cardia, fundus, body, and pylorus. It displays greater and lesser curvatures, and contains a pyloric sphincter at its junction with the duodenum.

    1. The mucosa of the stomach is thrown into distensible gastric folds; gastric pits and gastric glands are present in the mucosa.

    2. The parietal cells of the gastric glands secrete HCl, and the principal cells secrete pepsinogen.

Small Intestine (pp. 632-635)

  1. Regions of the small intestine include the duodenum, jejunum, and ileum; the bile duct and pancreatic duct empty into the duodenum.

  2. Fingerlike extensions of mucosa, called villi, project into the lumen, and at the bases of the villi the mucosa forms intestinal glands.

    1. New epithelial cells are formed in the intestinal crypts.

    2. The membrane of intestinal epithelial cells is folded to form microvilli; this brush border of the mucosa increases the absorptive surface area.

  3. Movements of the small intestine include rhythmic segmentation, pendular movement, and peristalsis.


Large Intestine (pp. 636-639)

  1. The large intestine absorbs water and electrolytes from the chyme and passes fecal material out of the body through the rectum and anal canal.

  2. The large intestine is divided into the cecum, colon, rectum, and anal canal.

    1. The appendix is attached to the inferior medial margin of the cecum.

    2. The colon consists of ascending, transverse, descending, and sigmoid portions.

    3. Haustra are bulges in the walls of the large intestine.

  3. Movements of the large intestine include peristalsis, haustral churning, and mass movement.


Liver, Gallbladder, and Pancreas (pp. 639-648)

  1. The liver is divided into right, left, quadrate, and caudate lobes. Each lobe contains liver lobules, the functional units of the liver.

    1. Liver lobules consist of plates of hepatic cells separated by modified capillaries called sinusoids.

    2. Blood flows from the periphery of each lobule, where branches of the hepatic artery and hepatic portal vein empty, through the sinusoids and out the central vein.

    3. Bile flows within the hepatic plates, in bile canaliculi, to the biliary ductules at the periphery of each lobule.

  2. The gallbladder stores and concentrates the bile; it releases the bile through the cystic duct and common bile duct into the duodenum.

  3. The pancreas is both an exocrine and an endocrine gland.

    1. The endocrine portion, consisting of the pancreatic islets, secretes the hormones insulin and glucagon.

    2. The exocrine acini of the pancreas produce pancreatic juice, which contains various digestive enzymes.

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