CH 23 THE DIGESTIVE SYSTEM

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1 CH 23 THE DIGESTIVE SYSTEM This system is responsible for processing nutrients, which are required in order to produce energy and chemical building blocks via cellular respiration for cellular growth, maintenance, and repair. Multicellular organisms require special systems for processing and delivering nutrients to all cells of the body. Food intake is regulated by two sensations: Hunger Appetite Hunger/appetite are regulated by the hypothalamus, which contains a laterally located appestat center and a medially located satiety center. BASIC ACTIVITIES INVOLVED IN FOOD PROCESSING Ingestion Propulsion Deglutition Segmentation Peristalsis Digestion Absorption Defecation DIGESTION INCLUDES TWO FORMS OF PROCESSING Mechanical Physical Mastication Chemical 53

2 ORGANS OF THE DIGESTIVE SYSTEM Digestive system consists of two sets of organs: the alimentary canal (GI tract) and the accessory structures. 1 THE ALIMENTARY CANAL (THE GI TRACT) These are the structures that actually physically touch/handle the food from the time it is ingested until it is eliminated. The GI tract is a continuous tube that extends approximately 9m (30 ) (cadaver measurement somewhat shorter in living persons) from the mouth to the anus. Organs include mouth, pharynx, esophagus, stomach, small intestine, and large intestine. 2 - ACCESSORY ORGANS Accessories are structures that do not physically touch/handle the food but rather provide assistance for the GI organs. These structures are located outside the tract (except for the tongue) and all produce or store secretions that aid in the chemical breakdown of food. They include the pancreas, liver, gallbladder, salivary glands, teeth, and tongue. NOTE: in some classifications the mouth is listed as an accessory, even though it directly handles food. GENERAL HISTOLOGY OF THE GI TRACT The wall of the GI tract, especially from the esophagus to the anal canal, has the same basic arrangement of tissues all the way through: TUNICS (COATS) OF THE DIGESTIVE SYSTEM 1 - MUCOSA The mucosal layer is the innermost epithelial layer of the GI tract. The type of epithelium varies within the GI tract. EX: it starts as stratified squamous in the oral cavity and changes to columnar in the stomach. Lamina Propria Muscularis Mucosae 54

3 2 SUBMUCOSA CT layer (dense CT and elastic fibers) that binds the mucosa to the muscularis mucosae. Submucosal Plexus Formerly Meissner s plexus. 3 MUSCULARIS EXTERNA The outermost layer of smooth muscle. Possible layers include: Circular Oblique Longitudinal Myenteric Plexus Formerly plexus of Auerbach. 4 SEROSA (VISCERAL PERITONEUM) Serous membrane composed of CT and simple squamous epithelium. PERITONEUM This is the largest serous membrane of the body and it consists of a layer of simple squamous epithelium (mesothelium) and some underlying supporting CT. Sections of this membrane include: Parietal Visceral Mesentery Sigmoid Mesocolon Falciform Ligament Lesser Omentum Greater Omentum 55

4 REVIEW OF THE GI TRACT ORGANS 1 BUCCAL (ORAL) CAVITY Bolus Salivary Glands Saliva Salivary Amylases Soft Palate Uvula Hard Palate Lingual Frenulum Bad breath (halitosis) is caused by the presence of hundreds of oral anaerobic bacterial species, which produce a range of chemical compounds, including putrescine, cadaverine, hydrogen sulfide, mercaptan, skatole, isovaleric acid, and indole. These compounds create the odor associated with an unkempt mouth! GUSTATION (TASTE) Taste buds are the sensory structures for taste and they are located primarily in the oral cavity. We have 10,000 or more buds of taste and most of them are positioned on our tongue, a few on the inner surface of the cheeks and a couple on the soft palate, pharynx, and epiglottis. The basic taste sensations and their locations are: Sweet Salty Sour Bitter Umami Taste buds are found primarily in stumpy-structures called papillae, which are extensions of the lamina propria. Each bud consists of epithelial cells of three types: 56

5 Supporting Cells Basal Cells Receptor Cells There are four types of papillae: Filiform Fungiform Circumvallate Foliate 2 - ESOPHAGUS UES LES Formerly gastroesophgeal sphincter or cardioesophageal sphincter. Gastroesophageal Junction 3 - STOMACH Chyme Oblique Muscle Layer Four Regions: Cardiac Cardiac Sphincter Fundic Body Pyloric Pyloric Sphincter Stomach Curvatures: 57

6 Greater Lesser Omenta Lesser Omentum Greater Omentum Rugae Gastric Pits The gastric pits contain gastric glands. Gastric glands include: Zymogen Cells Pepsinogen Parietal Cells HCL Intrinsic Factor Mucous Cells Enteroendocrine Cells Gastrin REGULATION OF STOMACH FUNCTIONS Medulla Parasympathetic Vagus Nerve (#X) An infant s stomach secretes rennin (a/k/a chymosin; an active enzyme in a complex of enzymes called rennet ); which aids in digestion of milk proteins (changes milk proteins to curds and whey), as well as gastric lipases. 58

7 4 SMALL INTESTINE The major portions of digestion and absorption occur in the small intestine. The small intestine originates at the pyloric sphincter of stomach and extends to the ileocecal valve of the large intestine. Three Regions: Duodenum Duodenal Glands Formerly Brunner s glands. Jejunum Ileum Intestinal Crypts Formerly crypts of Lieberkuhn. Intestinal Glands Villi Sing: Villus Lacteals Plica Circularis Peyer s Patches Mucosa-associated. DIGESTION IN THE SMALL INTESTINE A MECHANICAL DIGESTION Two Types of Movement: Peristalsis Segmentation B CHEMICAL DIGESTION CARBOHYDRATES Salivary Amylases Intestinal Amylases 59

8 Pancreatic amylases PROTEINS Stomach Pepsin Small intestine Trypsinogen Enterokinase Trypsin Chymotrypsinogen Chymotrypsin LIPIDS Liver Bile salts Gall Bladder Bile Pancreas Pancreatic Juice Pancreatic lipases Intestinal lipases Emulsification Micelles Mucosal Cells Chylomicrons Lacteals 60

9 5 - LARGE INTESTINE (COLON) Regions of the Colon: Caecum Appendix Formerly the vermiform appendix. Ileocaecal Valve Ascending Colon Transverse Colon Descending Colon Sigmoid Colon Rectum Anal Canal Internal Sphincters External Sphincters Colon-related terms: Tenia (Taenia) Coli Pl: Taeniae or teniae Haustra Enteric Bacteria Indole and Skatole Hemoglobin Reduction: Bilirubin Urobilinogen Stercobilin Defecation 61

10 ACCESSORY ORGANS FOR DIGESTION 1 PANCREAS The pancreas is called a mixed gland because it has endocrine as well as exocrine regions. A Endocrine Islets of hormone-producing cells. Pancreatic Islets Formerly the islets of Langerhans. Alpha Cells Beta Cells Delta PP Epsilon B - The exocrine portion of the pancreas consists of acinar cells, which produce pancreatic juice. CCK Hepatopancreatic Duct Vagus Nerve (#10) 2 - LIVER Two Lobes Falciform Ligament Lobules Septum Central Vein Triads Hepatocytes Stellate Cells Formerly Kupffer or hepatic macrophages. Bile 62

11 Bile Pigments 3 GALLBLADDER Liver Fossa CCK Hepatopancreatic Duct 4 SALIVARY GLANDS Three Pairs: Parotid Submandibular Sublingual Serous Demilunes Mucous Cells 63

12 DISORDERS/DISEASES Barrett s esophagus Cirrhosis A pathological change in the esophageal epithelium from stratified squamous to metaplastic columnar. Predisposes the patient to aggressive esophageal cancer (adenocarcinoma). Replacement of functional liver cells by non-functional CT. Cleft palate/cleft lip Congenital defect. The palatine bones fail to fuse properly. Constipation/diarrhea Diverticulosis Food poisoning Improperly formed stools, either too hard or too loose. Motility disorders. Small herniations of the mucosa usually in the sigmoid colon. If irritated, condition is diverticulitis. Most common contaminants are Campylobacter, E. coli, and Salmonella. Gastritis Geographical Tongue GERD Halitosis Hepatitis Hiatus hernia Hiccups Lactose Intolerance Mumps Inflammation of the underlying layers of the stomach wall. If uncorrected, it usually leads to gastric ulcers. Note: The causative agent of most peptic and gastric ulcers is Helicobacter pylori. Benign migratory glossitis, Glossitis areata exfoliativa. Red patches on the surface of the tongue bordered by grayish white. Fissures may be present. The papillae are missing from the reddish areas and overcrowded in the grayish white borders. Gastroesophageal reflux disease. Consistent regurgitation of stomach acid into the esophagus, causing tissue damage over time. Reflux. Bad breath is caused by the presence of hundreds of oral anaerobic bacterial species, which produce a range of chemical compounds, including putrescine, cadaverine, hydrogen sulfide, mercaptan, skatole, isovaleric acid, and indole. These compounds create the odor associated with an unkempt mouth! Inflammation of the liver; which is most often (but not always) caused by one of six hepatitis viruses. Usually the causative agent is Hepatitis-B. Hiatal Hernia. A structural abnormality involving the protrusion of part of the stomach above the diaphragm. Repeated, involuntary spasms of the diaphragm. The exact cause is still unknown. Genetic. Insufficient lactase production. Lactose production usually declines with age in normal persons but situation is aggravated for persons with the genetic predisposition. Viral inflammation of the parotid salivary glands. Peritonitis Inflammation of the peritoneum. Tooth/Gum disease Periodontal diseases. Vomiting (Emesis) Presence of bacterial toxins in the stomach (also excessive alcohol, spicy foods, etc) sends sensory impulses to the emetic center of the medulla. 64