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Chapter 24, part 1 The Digestive System
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Learning Objectives Identify the organs of the digestive system and their major functions Outline the mechanisms that regulate digestion Describe the anatomy of the organs and accessory organs of the digestive system Discuss the functions of the major structures and regions of the digestive system and discuss the regulation of their activities
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Learning Objectives Explain the significance of the large intestine in the absorption of nutrients Describe the events involved in the digestion of organic and inorganic nutrients Summarize the effects of the aging process on the digestive system
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SECTION 24-1 The Digestive System
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The Digestive system includes:
The muscular digestive tract Various accessory organs
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Figure 24.1 The Components of the Digestive System
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Functions of the digestive system
Ingestion Mechanical processing Digestion Secretion Absorption Excretion
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The digestive system organs and the peritoneum
Mesenteries Sheets of serous membranes that support portions of the digestive tract Greater omentum lies anterior to abdominal viscera Provides padding, protection, insulation, and energy reserves Lesser omentum
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Figure Mesenteries Figure 24.2a
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Figure Mesenteries Figure 24.2b
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Figure Mesenteries Figure 24.2c
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Figure Mesenteries Figure 24.2d
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Histological organization of the digestive tract
Mucosa lines digestive tract (mucous epithelium) Moistened by glandular secretions Lamina propria and epithelium form mucosa Submucosa Layer of dense irregular connective tissue
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Histological organization of the digestive tract
Muscularis externa Smooth muscle arranged in circular and longitudinal layers adventitia Serosa Serous membrane covering most of the muscularis externa
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Figure 24.3 The Structure of the Digestive Tract
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Movement of digestive materials
Visceral smooth muscle shows rhythmic cycles of activity Pacemaker cells Peristalsis Waves that move a bolus Segmentation Churn and fragment a bolus
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Figure Peristalsis Figure 24.4
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Control of the digestive system
Movement of materials along the digestive tract is controlled by: Neural mechanisms Parasympathetic and local reflexes Hormonal mechanisms Enhance or inhibit smooth muscle contraction Local mechanisms Coordinate response to changes in pH or chemical stimuli
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Figure 24.5 The Regulation of Digestive Activities
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SECTION 24-2 The Oral Cavity
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The mouth opens into the oral or buccal cavity
Its functions include: Analysis of material before swallowing Mechanical processing by the teeth, tongue, and palatal surfaces Lubrication Limited digestion
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Oral cavity Lined by oral mucosa
Roof of cavity = hard and soft palates Floor of cavity = tongue Uvula guards opening to pharynx
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Figure The Oral Cavity Figure 24.6a, b
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The tongue primary functions include: Mechanical processing
Assistance in chewing and swallowing Sensory analysis by touch, temperature, and taste receptors
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Tongue movements involve
Extrinsic and intrinsic tongue muscles Innervated by the hypoglossal nerve
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Salivary glands (three pairs)
Parotid, sublingual, and submandibular Saliva watery solution electrolytes, buffers, glycoproteins, antibodies, enzymes Functions include: Lubrication, moistening, and dissolving Initiation of digestion of complex carbohydrates PLAY Animation: Mastication Flythrough
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Figure 24.7 The Salivary Glands
Figure 24.7a, b
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Teeth Function in mastication of bolus Contact of occlusal surfaces
Contain three layers Enamel covering crown Dentin forms basic structure Root coated with cementum Periodontal ligaments hold teeth in alveoli
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Eruption of teeth 20 primary teeth AKA deciduous teeth
32 teeth of secondary dentition
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Figure Teeth Figure 24.8a, b
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Figure 24.9 Primary and Secondary Teeth
Figure 24.9a, b
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SECTION The Pharynx
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The pharynx Common passageway for food, liquids, and air
Lined with stratified squamous epithelium Pharyngeal muscles assist in swallowing Pharyngeal constrictor muscles Palatal muscles
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SECTION 24-4 The Esophagus
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The esophagus Carries solids and liquids from the pharynx to the stomach Passes through esophageal hiatus in diaphragm The wall of the esophagus contains mucosal, submucosal, and muscularis layers
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Histology of the esophagus
Distinctive features of the esophageal wall include Nonkeratinized, stratified squamous epithelium Folded mucosa and submucosa Mucous secretions by esophageal glands A muscularis with both smooth and skeletal muscle portions Lacks serosa Anchored by an adventitia
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Figure The Esophagus Figure 24.10a-c
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Swallowing (deglutition)
Buccal phase Pharyngeal phase Esophageal phase
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Figure 24.11 The Swallowing Process
Figure 24.11a-h
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SECTION The Stomach
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Functions of the stomach
Bulk storage of undigested food Mechanical breakdown of food Disruption of chemical bonds via acids and enzymes Production of intrinsic factor
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Anatomy of the stomach Cardia – superior, medial portion
Fundus – portion superior to stomach-esophageal junction Body – area between the fundus and the curve of the J Pylorus – antrum and pyloric canal adjacent to the duodenum
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Stomach anatomy Phloric Sphincter Guards exit from stomach Rugae
Ridges and folds in relaxed stomach
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Figure The Stomach Figure 24.12b
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Histology of the stomach
Gastric glands Parietal cells Intrinsic factor, and HCl Chief cells Pepsinogen Pyloric glands Mucous secretion containing several hormones Enteroendocrine cells G cells secrete gastrin D cells secrete somatostatin
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Figure 24.13 The Stomach Lining
Figure 24.13a, b
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Figure 24.13 The Stomach Lining
Figure 24.13c, d
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Figure 24.14 The Secretions of Hydrochloric Acid
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Regulation of gastric activity
Cephalic phase prepares stomach to receive ingested material Gastric phase begins with the arrival of food in the stomach Neural, hormonal, and local responses Intestinal phase controls the rate of gastric emptying
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Figure 24.15 The Phases of Gastric Secretion
Figure 24.15a
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Figure 24.15 The Phases of Gastric Secretion
Figure 24.15b
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Figure 24.15 The Phases of Gastric Secretion
Figure 24.15c
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Digestion and absorption in the stomach
Preliminary digestion of proteins Pepsin Permits digestion of carbohydrates Very little absorption of nutrients Some drugs, however, are absorbed
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SECTION 24-6 The Small Intestine and Associated Glandular Organs
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Small intestine Important digestive and absorptive functions
Secretions and buffers provided by pancreas, liver, gall bladder Three subdivisions: Duodenum Jejunum Ileum Ileocecal sphincter Transition between small and large intestine
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Figure 24.16 Regions of the Small Intestine
Figure 24.16a
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Histology of the small intestine
Plicae Transverse folds of the intestinal lining Villi Fingerlike projections of the mucosa Lacteals Terminal lymphatic in villus Intestinal glands Lined by enteroendocrine, goblet and stem cells
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Figure 24.17 The Intestinal Wall
Figure 24.17a
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Figure 24.17 The Intestinal Wall
Figure 24.17b, c
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Figure 24.17 The Intestinal Wall
Figure 24.17d, e
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Intestinal juices Moisten chyme Help buffer acids
Maintain digestive material in solution
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Small Intestine Duodenal glands (Brunner’s glands)
produce mucus, buffers, urogastrone Ileum aggregated lymphoid nodules (Peyer’s patches)
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Intestinal movements Peristalsis Segmentation Gastroenteric reflexes
Initiated by stretch receptors in stomach Gastroileal reflex Triggers relaxation of ileocecal valve
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The pancreas Pancreatic duct penetrates duodenal wall
Endocrine functions Insulin and glucagons Exocrine functions Majority of pancreatic secretions Pancreatic juice secreted into small intestine Carbohydrases Lipases Nucleases Proteolytic enzymes
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Figure The Pancreas Figure 24.18a-c
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The liver Performs metabolic and hematological regulation and produces bile Histological organization Lobules containing single-cell thick plates of hepatocytes Lobules unite to form common hepatic duct Duct meets cystic duct to form common bile duct
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Figure 24.19 The Anatomy of the Liver
Figure 24.19a
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Figure 24.19 The Anatomy of the Liver
Figure 24.19b, c
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Liver lobule is the basic functional unit of the liver
Hepatocytes form irregular plates arranged in spoke-like fashion Bile canaliculi carry bile to bile ductules Bile ductules lead to portal areas
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Figure 24.20 Liver Histology
Figure 24.20a, b
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The gallbladder Hollow, pear-shaped organ
Stores, modifies and concentrates bile PLAY Animation: Accessory Organ
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Figure 24.21 The Gallbladder
Figure 24.21a, b
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Coordination secretion and absorption
Neural and hormonal mechanisms coordinate glands GI activity stimulated by parasympathetic innervation Inhibited by sympathetic innervation Enterogastric, gastroenteric and gastroileal reflexes coordinate stomach and intestines
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Figure 24.22 The Activities of Major Digestive Tract Hormones
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SECTION 24-7 The Large Intestine
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Functions of the large intestine
Reabsorb water and compact material into feces Absorb vitamins produced by bacteria Store fecal matter prior to defecation
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The four areas of the colon are:
Ascending Transverse Descending Sigmoid
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Figure 24.23 The Large Intestine
Figure 24.23a
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Figure 24.23 The Large Intestine
Figure 24.23b, c
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The rectum Last portion of the digestive tract
Terminates at the anal canal Internal and external anal sphincters
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Histology of the large intestine
Absence of villi Presence of goblet cells Deep intestinal glands
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Physiology of the large intestine
Reabsorption in the large intestine includes: Water Vitamins – K, biotin, and B5 Organic wastes – urobilinogens and sterobilinogens Bile salts Toxins Mass movements of material through colon and rectum Defecation reflex triggered by distention of rectal walls
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Figure 24.25 The Defecation Reflex
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SECTION 24-8 Digestion and Absorption
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Processing and absorption of nutrients
Disassembles organic food into smaller fragments Hydrolyzes carbohydrates, proteins, lipids and nucleic acids for absorption
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Carbohydrate digestion and absorption
Begins in the mouth Salivary and pancreatic enzymes Disaccharides and trisaccharides Brush border enzymes Monosaccharides Absorption of monosaccharides occurs across the intestinal epithelia
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Lipid digestion and absorption
Lipid digestion utilizes lingual and pancreatic lipases Bile salts improve chemical digestion by emulsifying lipid drops Lipid-bile salt complexes called micelles are formed Micelles diffuse into intestinal epithelia which release lipids into the blood as chylomicrons
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Protein digestion and absorption
Low pH destroys tertiary and quaternary structure Enzymes used include pepsin, trypsin, chymotrypsin, and elastase Liberated amino acids are absorbed
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Absorption Water Nearly all that is ingested is reabsorbed via osmosis
Ions Absorbed via diffusion, cotransport, and active transport Vitamins Water soluble vitamins are absorbed by diffusion Fat soluble vitamins are absorbed as part of micelles Vitamin B12 requires intrinsic factor
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Figure 24.27 Digestive Secretion and Absorption of Water
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Figure 24.28 Ion and Vitamin Absorption by the Digestive Tract
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You should now be familiar with:
The organs of the digestive system and their major functions The mechanisms that regulate digestion The anatomy of the organs and accessory organs of the digestive system The functions of the major structures and regions of the digestive system and the regulation of their activities
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You should now be familiar with:
The significance of the large intestine in the absorption of nutrients The events involved in the digestion of organic and inorganic nutrients
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