The Digestive System

38.1a The mucosa is the innermost lining of the digestive tract (Applegate, 2000, 330). In some regions the surface is folded to increase surface area. The mucosa contains different types of cells that secrete mucus, digestive enzymes, and hormones. Ducts from other glands penetrate the mucosa to open into the lumen. In the stomach and intestines, the epithelium is simple columnar for secretion and absorption, but in the mouth and anus, where protection against abrasion is required, the epithelium is stratified squamous. The mucosa is a mucous membrane consisting of epithelium lying over a layer of loose connective tissue known as the lamina propria, and a thin layer of smooth muscle called the muscularis mucosa.

b The submucosa consists of a thick layer of loose connective tissue containing blood vessels and lymphatic vessels, nerves, and some glands. Blood vessels provide nourishment to the surrounding tissues, and the blood and lymph vessels carry away absorbed nutrients resulting from digestion. The nerves of the submucosa form a network known as the submucosal plexus, or Meissner's plexus, and provide autonomic nerve impulses to the muscle layers of the digestive tract.

c The muscularis externa layer of the digestive tract consists of two layers of muscle fibers, the inner layer of which are arranged in a circular fashion around the diameter of the digestive tract, and an outer layer of muscle fibers arranged longitudinally along the length of the tract (2

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e liver (Appelgate, 2000, 344). If the small intestine is empty, the sphincter of Oddi where the common bile duct enters the duodenum is closed and gall backs up through the cystic duct into the gallbladder. When chyme containing fat enters the duodenum, the hormone cholecystokinin stimulates the gallbladder to contract and the sphincter of Oddi to open, permitting bile to flow from the gallbladder through the cystic duct and common bile duct into the duodenum. 38.22 Three structures always found in the portal triad region of the liver are a branch of the hepatic portal vein, a branch of a hepatic duct, and a branch of the hepatic artery (342). 38.23 Kupffer cells of the liver line the sinusoids (344). 38.24 The function of Kupffer cells is to remove bacteria, damaged red blood cells, and other particles from the blood. 38.25 The liver is dark red in the living animal because the oxygenated blood brought to the liver by the hepatic artery mixes with the deoxygenated blood brought to the liver from the digestive tract by the hepatic portal vein as the blood from both sources flows through the sinusoids toward the central vein. 38.26 The acinar cells of the pancreas secrete digestive enzymes (344). Ducts carry the secretion

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