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Chapter 16 The Digestive System 15 - 1 16-1: The Digestive System • Consists of digestive tract & accessory organs • Functions of digestive system – Ingestion—food enters digestive tract – Mechanical processing—moving food – Digestion—chemical breakdown of food – Secretion—release enzymes, acids – Absorption—move nutrients – Excretion—remove waste products 15 - 3 Organization of Digestive Tract • Digestive tract has four layers: – Mucosa—inner lining; has folds to increase surface area for absorption – Submucosa—blood vessels, nerves – Muscularis externa—smooth muscle cells; push materials along digestive tract – Serosa—outer layer 15 - 4 15 - 5 Movement of Digestive Materials • Muscle cells in digestive tract trigger waves of contraction • Peristalsis – Waves of contractions propels materials from one part of digestive tract to another • Segmentation – Movements that churn & fragment digested materials 15 - 6 15 - 7 16-1 Checkpoint 1. Identify the organs of the digestive system. 2. List and define the six primary functions of the digestive system. 3. Name the layers of the digestive tract from superficial to deep. 4. Which is more efficient in propelling intestinal contents: peristalsis or segmentation? 16-2: The Oral Cavity • Mouth opens into the oral cavity – Senses & analyzes food, begins digestion of carbs & lipids • Cheeks, lips, vestibule, gingivae form boundaries of oral cavity • Hard & soft palate form roof of oral cavity; tongue forms floor 15 - 10 The Tongue • Manipulates materials inside the mouth – Also senses temperature, taste Salivary Glands • Parotid, sublingual, submandibular glands • Produce saliva containing water, mucins, enzymes – Begins digestion of carbs 15 - 11 Teeth • Perform mastication (chewing) • Parts of tooth: – Root anchors tooth; crown is top of tooth – Enamel covers surface of tooth – Dentin composes tooth – Pulp cavity contains blood vessels & nerves 15 - 12 • Types of teeth: – Incisors—used in biting/cutting – Canines—used in tearing – Bicuspids & molars—used in crushing/grinding • First teeth—deciduous teeth (baby teeth) – Replaced by permanent teeth – Wisdom teeth may emerge later 15 - 14 16-2 Checkpoint 1. Name the structures associated with the oral cavity. 2. The digestion of which nutrient would be affected by damage to the salivary glands? 3. Which type of tooth is most useful for chopping off bits of raw vegetables? 16-3: Pharynx, Esophagus The Pharynx • Passageway for air, food, liquid The Esophagus • Moves substances from mouth to stomach • Meets stomach at lower esophageal sphincter (cardiac sphincter) Swallowing • Food compacted into a bolus • Phases of swallowing: – Buccal—moves bolus to back of mouth, triggers swallowing reflex – Pharyngeal—pushes bolus through pharynx into esophagus – Esophageal—peristalsis pushes 15 - 17 bolus into stomach 16-3 Checkpoint 1. Describe the function of the pharynx. 2. Name the structure connecting the pharynx to the stomach. 3. What process is occurring when the soft palate and larynx elevate, the glottis closes, and the food bolus moves past the structures? 16-4: The Stomach • Stores & breaks down food – Food bolus becomes chyme • Regions of stomach – Cardia—by esophagus – Fundus—upper bulge of stomach – Body—majority of stomach – Pylorus—connects stomach to small intestine; ends in pyloric sphincter – Rugae—folds to expand stomach 15 - 20 The Gastric Wall • Gastric pits in stomach wall contain gastric glands – Release gastric juice containing acid & enzymes • Stomach protected from gastric juice by mucosal barrier 15 - 21 CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. 15 - 22 Regulation of Gastric Activity • Cephalic phase – Begins with sight/thought of food – Prepares stomach for digestion • Gastric phase – Food arrives at stomach – Gastric juices released, contents mixed • Intestinal phase – Chyme enters small intestine 15 - 23 Digestion in the Stomach • Proteins & carbohydrate digestion continues in stomach • Most nutrient absorption does not take place in stomach – Nutrients are broken down; some absorption occurs 15 - 24 16-4 Checkpoint 1. Name the four main regions of the stomach. 2. Discuss the significance of the low pH in the stomach. 16-5: The Small Intestine • Digests & absorbs 90% of nutrients • Segments of small intestine – Duodenum—closest to stomach – Jejunum—middle segment – Ileum—last, longest segment; connects to large intestine at ileocecal sphincter The Intestinal Wall • Intestinal lining has folds – Each fold covered with fingerlike villi to increase surface area for absorption Intestinal Movements • Chyme is pushed along intestine by peristalsis (takes about 5 hrs) • Intestinal juice & hormones assist in digestion & absorption 15 - 27 15 - 28 16-5 Checkpoint 1. What purpose do the folds and the villi of the small intestine serve? 2. Name the three segments of the small intestine from proximal to distal. 16-6: Pancreas, Liver, Gallbladder The Pancreas • Lies behind stomach • Produces pancreatic juice that contains enzymes – Carbohydrases—break down carbs – Lipases—break down lipids – Nucleases—break down nucleic acids – Proteases—break down proteins The Liver • Liver divided into lobes – Gallbladder contained in liver • Lobes contain liver lobules composed of hepatocytes – Hepatocytes release bile to break down fat & aid in fat digestion 15 - 31 15 - 32 • Liver functions – Metabolic regulation—monitor glucose & nutrients levels – Blood regulation—detoxifies & filters blood – Bile production The Gallbladder • Stores & concentrates bile before its release into small intestine – Concentrated bile forms gallstones 15 - 33 15 - 34 16-6 Checkpoint 1. Does a high-fat meal raise or lower the level of bile released into the small intestine? 2. What is the purpose of the pancreas in the digestive system? In the endocrine system? 16-7: The Large Intestine • Reabsorbs water & vitamins, stores feces • Segments of large intestine – Cecum—first segment; location of appendix – Colon segments—ascending colon, transverse colon, descending colon, sigmoid colon – Rectum—stores feces; ends in anal canal 15 - 37 Functions of the Large Intestine • Reabsorption water, bile salts, vitamins, & organic wastes • Movements of the large intestine – Mass movements push materials along intestine – Diarrhea results from rapid movements • Defecation – Distention of rectal wall triggers defecation reflex 16-7 Checkpoint 1. Identify the four segments of the colon. 2. What are some structural differences between the large intestine and the small intestine? 3. A narrowing of the ileocecal sphincter would cause problems with movement between what two organs? 16-8: Digestion & Absorption • Carbohydrates break down into di- & monosaccharides • Lipids break down into fatty acids & glycerol • Proteins break down into amino acids • Fat-soluble & water-soluble vitamins absorbed during digestion 16-8 Checkpoint 1. An increase in which component of a meal would increase the number of fatty acids and glycerol in the small intestine? 2. Why is diarrhea potentially life threatening but mild constipation is not?