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Chapter 23A Digestive System Slides by Barbara Heard and W. Rose. figures from Marieb & Hoehn 9th ed. Portions copyright Pearson Education Ch. 23: Digestive System Introduction/Overview Functional Anatomy Physiology of Digestion & Absorption Digestive System Introduction/Overview Digestive System Organs Digestive Processes Basic Concepts Relations & Common Features of Digestive Organs Functional Anatomy Physiology of Digestion & Absorption Digestive System Organs (Fig 23.1) Alimentary Canal = path through Mouth to anus Digests food and absorbs fragments Mouth, pharynx, esophagus, stomach, small intestine, and large intestine Accessory organs = "off-to-the-side" helpers Teeth, tongue, gallbladder Digestive glands Salivary glands Liver Pancreas Figure 23.1 Alimentary canal and related accessory digestive organs. Mouth (oral cavity) Tongue* Parotid gland Sublingual gland Submandibular gland Salivary glands* Pharynx Esophagus Stomach Pancreas* (Spleen) Liver* Gallbladder* Transverse colon Small intestine Anus © 2013 Pearson Education, Inc. Duodenum Jejunum Ileum Descending colon Ascending colon Cecum Sigmoid colon Rectum Appendix Anal canal Large intestine Digestive Processes • Six essential activities 1. 2. 3. 4. 5. 6. Ingestion Propulsion Mechanical breakdown Digestion Absorption Defecation © 2013 Pearson Education, Inc. Figure 23.2 Gastrointestinal tract activities. Ingestion Mechanical breakdown • Chewing (mouth) • Churning (stomach) • Segmentation (small intestine) Digestion Food Pharynx Esophagus Propulsion • Swallowing (oropharynx) • Peristalsis (esophagus, stomach, small intestine, large intestine) Stomach Absorption Lymph vessel Small intestine Large intestine Blood vessel Mainly H2O Feces Defecation © 2013 Pearson Education, Inc. Anus Figure 23.3 Peristalsis and segmentation. From mouth Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, moving food along the tract distally. © 2013 Pearson Education, Inc. Segmentation: Nonadjacent segments of alimentary tract organs alternately contract and relax, moving food forward then backward. Food mixing and slow food propulsion occur. Basic Concepts • Gut lumen is "outside" • Digestion control mechanisms try to control chemical environment in lumen to optimize absorption • Digestion is provoked by mechanical & chemical stimuli (sensors in walls): stretch, pH, osmolarity, which can cause secretion and propulsion via reflexes (Fig. 23.4) • Control by local as well as remote reflexes (Fig 23.4) • Short reflexes: via enteric nerve plexus • Long: via CNS © 2013 Pearson Education, Inc. GI Tract Regulatory Mechanisms 1. Mechanoreceptors and chemoreceptors – Respond to stretch, changes in osmolarity and pH, and presence of substrate and end products of digestion – Initiate reflexes that • • © 2013 Pearson Education, Inc. Activate or inhibit digestive glands Stimulate smooth muscle to mix and move lumen contents GI Tract Regulatory Mechanisms 2. Intrinsic and extrinsic controls – Short reflexes - enteric nerve plexuses (gut brain) respond to stimuli in GI tract – Long reflexes respond to stimuli inside or outside GI tract; involve CNS centers and autonomic nerves – Hormones from cells in stomach and small intestine stimulate target cells in same or different organs to secrete or contract © 2013 Pearson Education, Inc. Figure 23.4 Neural reflex pathways initiated by stimuli inside or outside the gastrointestinal tract. External stimuli (sight, smell, taste, thought of food) Central nervous system Long reflexes Visceral afferents Internal (GI tract) stimuli Extrinsic visceral (autonomic) efferents Chemoreceptors, osmoreceptors, or mechanoreceptors Local (intrinsic) nerve plexus ("gut brain") Effectors: Smooth muscle or glands Short reflexes Gastrointestinal wall (site of short reflexes) Lumen of the alimentary canal © 2013 Pearson Education, Inc. Response: Change in contractile or secretory activity Peritoneum and Peritoneal Cavity • Peritoneum - serous membrane of abdominal cavity – Visceral peritoneum on external surface of most digestive organs – Parietal peritoneum lines body wall • Peritoneal cavity – Between two peritoneums – Fluid lubricates mobile organs © 2013 Pearson Education, Inc. Figure 23.5a The peritoneum and the peritoneal cavity. Abdominopelvic cavity Vertebra Dorsal mesentery Parietal peritoneum Ventral mesentery Visceral peritoneum Peritoneal cavity Alimentary Liver canal organ Two schematic cross sections of abdominal cavity illustrate the peritoneums and mesenteries. © 2013 Pearson Education, Inc. Peritoneum and Peritoneal Cavity • Mesentery - double layer of peritoneum – Routes for blood vessels, lymphatics, and nerves – Holds organs in place; stores fat • Retroperitoneal organs posterior to peritoneum • Intraperitoneal (peritoneal) organs surrounded by peritoneum © 2013 Pearson Education, Inc. Figure 23.5b The peritoneum and the peritoneal cavity. Mesentery resorbed and lost Alimentary canal organ Alimentary canal organ in a retroperitoneal position Some organs lose their mesentery and move, becoming retroperitoneal, during development. © 2013 Pearson Education, Inc. Homeostatic Imbalance • Peritonitis – Inflammation of peritoneum – Causes by e.g., piercing abdominal wound, perforating ulcer, ruptured appendix – Peritoneal coverings stick together, localizing infection – Dangerous and lethal if widespread – Treated with debris removal and antibiotics © 2013 Pearson Education, Inc. Blood Supply: Splanchnic Circulation • Branches of aorta serving digestive organs – Hepatic, splenic, and left gastric arteries – Inferior and superior mesenteric arteries • Hepatic portal circulation – Drains nutrient-rich blood from digestive organs – Delivers it to the liver for processing © 2013 Pearson Education, Inc. Figure 23.6 Basic structure of the alimentary canal. Intrinsic nerve plexuses • Myenteric nerve plexus • Submucosal nerve plexus Glands in submucosa Mucosa • Epithelium • Lamina propria • Muscularis mucosae Submucosa Muscularis externa • Longitudinal muscle • Circular muscle Mesentery © 2013 Pearson Education, Inc. Nerve Artery Gland in mucosa Vein Duct of gland outside Lymphatic vessel alimentary canal Serosa • Epithelium (mesothelium) • Connective tissue Lumen Mucosa-associated lymphoid tissue Histology: all alimentary canal organs have same basic 4 layers From lumen out (Fig 23.6): • Mucosa = mucous membrane = epithelial layer of cells Secrete: mucus, digestive enz, hormones Absorb broken down compounds into blood Protect against infection • Submucosa Connective tissie with blood vessels & lymphatics for absorption, lymph nodes, elastic fibers (shape retention) • Muscularis externa Inner circular muscles Outer longitudinal muscles • Serosa The peritoneum already mentioned © 2013 Pearson Education, Inc. Figure 23.6 Basic structure of the alimentary canal. Intrinsic nerve plexuses • Myenteric nerve plexus • Submucosal nerve plexus Glands in submucosa Mucosa • Epithelium • Lamina propria • Muscularis mucosae Submucosa Muscularis externa • Longitudinal muscle • Circular muscle Mesentery © 2013 Pearson Education, Inc. Nerve Artery Gland in mucosa Vein Duct of gland outside Lymphatic vessel alimentary canal Serosa • Epithelium (mesothelium) • Connective tissue Lumen Mucosa-associated lymphoid tissue Enteric nervous system • Submucosal plexus In submucosa Controls glandular secretions & thin submucosal muscular layer • Myenteric nerve plexus Between circular & longitudinal muscle layers Controls GI tract motility © 2013 Pearson Education, Inc. Enteric Nervous System • Linked to CNS via afferent visceral fibers • Long ANS fibers synapse with enteric plexuses Sympathetic impulses inhibit digestive activities Parasympathetic impulses stimulate digestive activities © 2013 Pearson Education, Inc.