Chapter 24: The Digestive System

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1 Bio40C Human A&P Welcome to Bio40C Dr. Ann Reisenauer Dr. R Class schedule Text: Principles of Anatomy and Physiology, 12th edition My faculty website Lecture outlines, lab assignments, presentations, extra credit How to contact me: reisenauerann@deanza.edu Office hours: TuTh 12-12:30 pm in SC2106 or by appt Chapter 24: The Digestive System Overview of the digestive system Digestive organs & accessory organs Layers of the GI tract Upper GI tract 2 groups of organs compose the digestive system 2 groups of organs compose the digestive system The human digestive system consists of: A digestive tube the alimentary canal or GI tract Accessory organs that secrete digestive enzymes Alimentary canal or Gastrointestinal (GI) tract mouth, most of pharynx, esophagus, stomach, small intestine, and large intestine Accessory digestive organs teeth, tongue, salivary glands, liver, gallbladder, and pancreas

2 Digestion includes 6 basic processes Ingestion Secretion of water, acid, buffers, and enzymes into lumen Mixing and propulsion Digestion Mechanical digestion churns food Chemical digestion breaks it down Layers of the GI Tract Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Wall of GI tract from lower esophagus to anal canal has the same 4 basic layers Submucosa artery vein Layers of the GI Tract Wall Layers of the GI Tract Wall Mucosa: Innermost layer of cells produces Lumen Serosa Muscularis Absorption of nutrients into blood or lymph Defecation elimination of feces nerve lymph supply vessel Mucosa made of 2 layers of smooth muscle that move food along the GI tract mucus that protects the lining digestive enzymes Submucosa: Muscularis: inner circular outer longitudinal Outer lining that is part of the peritoneum Peristalsis Coordinated waves of contractions of the circular and longitudinal layers of the muscularis Push bolus forward Neural regulation of the GI tract Enteric nervous system (ENS) Mixing movements Intrinsic set of nerves - brain of gut Neurons extend from esophagus to anus Neurons of ENS are arranged in 2 plexuses Segmental contractions muscularis Mixing and propulsion: peristalsis vs segmentation serosa Serosa: layer of loose connective tissue that contains blood vessels, lymphatic vessels and nerves. Myenteric plexus located between smooth muscle layers of the muscularis Controls GI tract motility Submucosal plexus controls secretions

3 ENS regulation of the GI tract Motor neurons supply the longitudinal and circular smooth muscles of the muscularis Sensory neurons supply the mucosal epithelium Functions Chemoreceptors activated by chemicals in food Stretch receptors activated when food distends the wall of GI tract ANS regulation of the GI tract Autonomic nervous system regulates the ENS Vagus (X) nerves supply parasympathetic fibers to most of GI tract Increase GI secretion and motility by stimulating the ENS Sympathetic nerves decrease GI secretions and motility by inhibiting the ENS Emotions (anger, anxiety) slow digestion because they stimulate the sympathetic nerves that supply the GI tract Both the enteric nervous system and ANS regulate the GI tract Peritoneum Largest serous membrane of the body Divided into Parietal peritoneum lines wall of cavity Visceral peritoneum covers some organs and is their serosa Space between is peritoneal cavity Peritoneal Folds Peritoneal folds weave between the viscera and bind the organs together and to walls of abdominal cavity 5 major peritoneal folds: Greater omentum falciform ligament lesser omentum mesentery mesocolon Peritoneal Folds fatty apron

4 Mouth The salivary glands Oral cavity Formed by cheeks, hard and soft palates, and tongue Oral cavity proper extends from gums and teeth to the fauces (opening between oral cavity and pharynx or throat ) hard palate nasopharynx Secrete saliva Saliva soft palate epiglottis 3 major salivary glands Saliva Mumps Saliva Mostly water 99.5% 0.5% solutes mucus, immunoglobulin A, lysozyme (bactericidal), and salivary amylase (enzyme that digests starch), ions, dissolved gases, urea, uric acid Not all salivary glands produce the same saliva Salivation is entirely under nervous control Controlled by ANS Parasympathetic stimulation promotes secretion of saliva Sympathetic stimulation decreases salivation An inflammation and enlargement of the parotid salivary glands caused by infection with the mumps virus (myxovirus). Vaccine developed in 1967 Tongue Teeth Tongue hard palate Accessory digestive organ Skeletal muscle covered by mucous membrane Functions Maneuvers food for chewing and swallowing Speech Lingual glands secrete salivary lipase nasopharynx Accessory digestive organ soft palate adapted for mechanical digestion 3 major regions crown, neck, and root Composition epiglottis functions to keep mucus membranes of mouth and throat moist lubricates and dissolves foods begins digestion of carbohydrates primarily made up of dentin, a calcified connective tissue Enamel - protective Cementum - attaches root to periodontol ligament Pulp - connective tissue containing blood vessels, nerves, lymphatic vessels Parotid Submandibular Sublingual

5 Teeth adapted for mechanical digestion Digestion in the mouth Incisors Adapted for cutting into food Cuspids Used to tear and shred food Premolars and molars Crush and grind food Mechanical digestion Chewing or mastication Food manipulated by tongue, ground by teeth, and mixed with saliva Forms bolus Chemical digestion Salivary amylase converts starches to disaccharides Acts in the mouth; inactivated by stomach acid Lingual lipase breaks down triglycerides Secreted by lingual glands of tongue Becomes activated in acidic environment of stomach Pharynx (throat) Extends from the internal nares to the esophagus 3 parts Nasopharynx Functions only in respiration Oropharynx Digestive and respiratory functions Laryngopharynx Digestive and respiratory functions Esophagus Muscular tube that connects the pharynx to the stomach Functions Secretes mucous Transports food No enzymes produced, no absorption Contains an upper and a lower esophageal sphincter Esophagus - anatomy Swallowing (deglutition) Histology (4 layers) Mucosa protects against wear and tear from food particles Submucosa Muscularis Superior 1/3 is skeletal muscle Middle 1/3 is skeletal and smooth muscle Inferior 1/3 is smooth muscle 2 sphincters upper esophageal sphincter (UES) regulates movement of food into esophagus lower esophageal sphincter (LES) regulates movement of food into stomach Superficial layer is adventitia rather than serosa attaches esophagus to surrounding structures Facilitated by saliva and mucus Involves mouth, pharynx, and esophagus 3 stages Voluntary bolus passed to oropharynx Pharyngeal involuntary passage through pharynx into esophagus Esophageal involuntary passage through esophagus to stomach Peristalsis pushes bolus forward

6 Swallowing (deglutition) Anatomy of the Stomach 3 stages Voluntary bolus passed to oropharynx Pharyngeal the tongue rises against the palate, the epiglottis seals off the entrance to the trachea Esophageal food moves to stomach by peristalsis 4 main regions Cardia, fundus, body, pylorus Lesser and greater curvatures Same 4 layers Mucosa Exocrine and endocrine cells Large folds called rugae Submucosa Muscularis additional 3 rd layer of smooth muscle Inner oblique layer Serosa part of visceral peritoneum Swallowing Copyright 2009, John Wiley & Sons, Inc. Anatomy of the stomach Muscularis esophagus 3 layers of muscle 2 sphincters gastroesophageal LES sphincter pyloric sphincter Mucosa Rugae Gastric pits 2-3 L per day Endocrine cells muscularis layer has three layers of muscle mucosa layer has rugae Secretions from several gastric glands flow into each gastric pit and then into the stomach Exocrine gland cells secrete gastric juice Functions of the Stomach Mixes saliva, food and gastric juice to form chyme Serves as mixing chamber and holding area for food Most expandable part of GI tract Secretes gastric juice Secretes gastrin into blood a. Stomach gastric pit Stomach controls movement of chyme into the small intestine HCl, digestive enzymes gastric gland cells that secrete gastric Copyright 2009, juice John Wiley & Gastric secretions Gastric secretions 3 types of exocrine gland cells Mucous neck cells mucous SEM 3,260 gastric pit Parietal cells HCl and intrinsic factor Mucous cells Mucus protects the mucosal cells lining the stomach Parietal cells Secrete HCl acidifies the stomach contents (ph of 2) Chief cells pepsinogen and gastric lipase Endocrine cells G cells gastrin, stimulates gastric activity Secrete intrinsic factor Kills microorganisms found in food Denatures proteins Activates pepsinogen Facilitates absorption of vitamin B12 in the ileum

7 Gastric secretions Gastric secretions Chief cells Secrete an inactive proenzyme, pepsinogen Protein digestion Rennin is secreted in infants Coagulates proteins in milk aids in milk digestion Gastric lipase fat digestion Endocrine function G cells secrete gastrin Released when food enters the stomach Stimulates secretion of gastric acid (HCl) by the parietal cells and aids in gastric motility Recap: Digestion by the stomach Mechanical digestion Mixing waves gentle, rippling peristaltic movements creates chyme Chemical digestion Carbohydrate Digestion Salivary amylase breaks down carbs to disaccharides until its inactivated by acidic gastric juice Lipid digestion Lingual lipase is activated by acidic gastric juice Gastric lipase Digest triglycerides into fatty acids and mono- or diglycerides Digestion by the stomach Protein digestion Parietal cells secrete H + and Cl - separately but net effect is HCl Kills many microbes Acidic ph denatures proteins Pepsin breaks down proteins into smaller peptides Enzyme is secreted as inactive precursor, pepsinogen Protein digestion Pepsin HCl Nutrient absorption in stomach Small amount of nutrient absorption Some water, ions, short chain fatty acids, certain drugs (aspirin) and alcohol Pepsinogen (inactive) Pepsin is an enzyme that degrades food proteins into peptides secreted by the chief cells The inactive pro-form, pepsinogen, is activated by HCl HCl allows pepsinogen to unfold and cleave itself into the active enzyme, pepsin Pepsin Digestive sys - swallowing and stomach 1:48

8 Stomach Ailments Peptic ulcer disease Peptic ulcers open sores that develop in areas exposed to acidic gastric juice the lining of the stomach, upper small intestine or esophagus. Causes Infection with the bacterium Helicobacter pylori Aspirin and other nonsteroidal antiinflammatory drugs Hypersecretion of HCl But NOT stress or diet Helicobacter pylori is the most frequent cause of PUD. This bacterium lives in the mucus and can cause gastric ulcers. H. pylori is especially welladapted for survival in the stomach. It produces an enzyme that, through a series of biochemical reactions, shields the bacterium from the acidity of the stomach Gastroesophageal reflux disease (heartburn) The lower esophageal sphincter fails to close adequately after food enters the stomach The stomach contents reflux into the inferior portion of the esophagus. HCl from the stomach contents irritates the esophageal wall resulting in heartburn.