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Lecture 9

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Biological Sciences
Connie Soros

1 Lecture 9 The Digestive System (based on Chapter 25) Introduction The digestive system consists of: The digestive tract (one continuous tube from mouth to anus) Accessory organs of digestion Functions of the Digestive System Ingestion Bringing food and liquids into the mouth Mechanical processing Chewing and swallowing food Digestion Chemical breakdown of food into nutrient form Secretion Secretion of products by the lining of the digestive tract Secretion of products by the accessory organs of digestion Absorption The movement of nutrients from the small intestine to the bloodstream Excretion The removal of waste products from the digestive tract Compaction Progressive dehydration of organic wastes The Components of the Digestive System Digestive Tract Mouth - begins the process of mechanical digestion Esophagus - passage tube for food to enter the stomach Stomach - enzymatic breakdown of food Small intestine - enzymatic breakdown of food absorption of nutrients into the bloodstream Large intestine - removes solid waste reabsorbs water into the bloodstream to prevent dehydration houses bacteria that produce vitamin K for blood clotting processes Anus - Removal of solid waste Accessory Organs Oral cavity - teeth, tongue, mechanical and chemical breakdown of food Salivary glands - produce an enzyme to begin digesting food Pancreas - produces numerous enzymes that enter into the small intestine to digest food Liver - produces bile for the emulsification of fat in the small intestine Gallbladder - stores bile Histological Organization of the Digestive Tract There are four major layers of the digestive tract The Mucosa The inner lining of the digestive tract The mucosa of the small intestine makes up folds called plicae Plicae increase the surface area for increased absorption The Submucosa Surrounds the muscularis mucosae Large blood vessels and lymphatics are in this layer The Muscularis Externa Surrounds the submucosa © 2012 Pearson Education, Inc. 2 Dominated by smooth muscle fibers The Serosa Covers the muscularis externa Outermost layer of the digestive system Movement of digestive materials through the digestive tract The muscularis externa propels material through the digestive tract This is called peristalsis Material is churned and fragmented and at the same time is propelled through the digestive tract This is called segmentation See Figs 25.3a and 25.3b for peristalsis and segmentation The Peritoneum The serosa (visceral peritoneum) is continuous with the parietal peritoneum The abdominal organs lie in association with the peritoneal membrane Intraperitoneal Organs Organs that lie within the peritoneal cavity Organs are surrounded completely by the visceral peritoneum Examples: stomach and liver Retroperitoneal Organs Organs are covered by the visceral peritoneum on their anterior surface These organs lie deep to the visceral peritoneum Examples: kidneys, ureters and abdominal aorta Structures within the Oral Cavity tongue, uvula, salivary glands,teeth Salivary Glands There are three pairs of salivary glands Parotid Sublingual Submandibular All three glands produce salivary amylase, which partially digests carbohydrates The Esophagus The bolus moves down the esophagus toward the stomach via peristaltic action The esophagus passes through the diaphragm by passing through the esophageal hiatus The esophagus has an upper esophageal sphincter and a lower esophageal sphincter Histology of the Esophageal Wall The esophageal wall is made of: Mucosa lining Submucosa Smooth muscle layer (muscularis mucosae) Muscularis externa The esophagus does not have a serosa layer The Stomach The stomach consists of: lesser curvature, greater curvature, fundus, body, pylorus The stomach also consists of: longitudinal muscles, circular muscles, oblique muscles, gastric rugae Histology of the Stomach Lamina propria – layer of loose connective tissue directly beneath the epithelium of mucous membranes © 2012 Pearson Education, Inc. 3 o Structures within the lining of the stomach Mucous surface cells: produce copious amounts of mucus to protect the lining of the stomach Gastric pits: produce cells to continuously replace lost stomach cells Mucous neck cells: produce mucus to lubricate the food entering the stomach Parietal cells: secrete intrinsic factor and hydrochloric acid Intrinsic factor: facilitates the absorption of vitami12B from the small intestine into the bloodstream, which is used during erythropoiesis Hydrochloric acid: kills microorganisms and activates pepsinogen Chief cells: secrete pepsinogen, which is converted to pepsin via the action of hydrochloric acid Enteroendocrine cells: these are cells of the stomach that produce hormones. The G cells produce the hormone gastrin. Gastrin causes the parietal and chief cells to release their products Regulation of the Stomach Food enters the stomach and the stomach stretches Stretching causes the G cells to release gastrin Gastrin causes the parietal and chief cells to release their products The Small Intestine Features of the small intestine Approximately 20 feet in length Approximately 1 to 2 inches in diameter Consists of: Duodenum: 10 inches long; receives digestive enzymes from the pancreas, bile from the liver and gallbladder Jejunum: 8 feet long; most of the digestion and absorption occurs in the jejunum Ileum: 12 feet long Histology of the Small Intestine The lining consists of: Plicae Each plica consists of numerous microvilli (villi) Within each villus are capillaries Villi will absorb the digested nutrients from the lumen of the small intestine into the capillaries Intestinal crypts Appear at the base of the villi
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