T1 Phys - Topic 9 - GI - Apr 2020 PDF

Module #9 – The Gastrointestinal System the organs and collectively perform digestion and absorption Digestion the breaking down of food into molecules small enough to be absorbed Absorption the passage of molecules through the plasma membranes of the cells lining the stomach and intestines into the...

Module #9 – The Gastrointestinal System the organs and collectively perform digestion and absorption Digestion the breaking down of food into molecules small enough to be absorbed Absorption the passage of molecules through the plasma membranes of the cells lining the stomach and intestines into the blood and lymph Structures of the Gastrointestinal System 1) gastrointestinal tract a.k.a. alimentary canal the continuous tube from the mouth to the anus 2) accessory digestive organs teeth tongue salivary glands liver gallbladder pancreas p. 194 Functions of the Gastrointestinal System 1) 2) 3) 4) 5) 6) ingestion secretion mixing and propulsion digestion absorption elimination 1) Ingestion eating 2) Secretion cells in the walls of the GI tract and accessory organs secrete water, acid, buffers, and enzymes into the lumen of the tract 3) Mixing and Propulsion muscles in the organ wall rhythmically contract and relax to mix the food and secretions together and to move the mixture through the system 4) Digestion a) mechanical b) chemical Mechanical Digestion the tongue mixes food the teeth grind and cut food the stomach and small intestine churn/mix food Chemical Digestion enzymes help break larger nutrients into smaller ones (which can then be absorbed) enzymes are secreted by the salivary glands, tongue, stomach, pancreas, small intestine 5) Absorption taken in through the membranes of cells lining the stomach and small intestines enters the blood or lymphatic systems circulated throughout the body 6) Elimination a.k.a. defecation feces: wastes, undigested substances, unabsorbed substances, bacteria, cells sloughed off from the GI tract lining p. 195 New Terminology     mastication: chewing deglutition: swallowing emesis: vomiting gustation: taste The Mouth and Teeth teeth: cut/grind food, mix it with saliva, make food more manageable to swallow (mastication/mechanical digestion) tongue: skeletal muscle covered in mucous membrane salivary glands (major ones) parotid: inferior and anterior to the ear, between the skin and the masseter submandibular glands: floor of the mouth sublingual glands: beneath the tongue saliva: water, solutes, enzymes bolus: a soft, flexible mass that is swallowed (i.e. passes into the pharynx) p. 196 The Pharynx a.k.a. throat skeletal muscle lined with mucous membrane muscle contractions move the bolus from the mouth into the pharynx into the esophagus The Esophagus skeletal and smooth muscle a collapsible muscular tube sitting posterior to the trachea runs from the lowest segment of the pharynx through the diaphragm (the esophageal hiatus) to the superior aspect of the stomach functions to secrete mucous and transport the bolus to the stomach peristalsis: wave‐like contractions of the smooth muscle lining the walls of the GI tract that move the bolus along the tract (from the esophagus to the anus) passage of food from the pharynx into the esophagus is controlled by the upper esophageal sphincter passage of food from the esophagus into the stomach is controlled by the lower esophageal sphincter (LES) p. 197 The Layers of the GI Tract the wall of the GI tract from the lower esophagus to the anal canal has the same 4 layered arrangement from deep to superficial: mucosa submucosa muscularis serosa Mucosa a mucous membrane consisting of: epithelium a) epithelial cells b) exocrine cells (secrete mucous, liquid) c) enteroendocrine cells (secrete hormone) lamina propria a) CT (areolar) b) blood and lymphatic vessels (for absorption) c) mucosa‐associated lymphatic tissue (MALT) (for immune function/protection) muscularis mucosa a) a smooth muscle layer that creates folds in the mucosa p. 198 Submucosa a CT layer that binds the mucosa to the muscularis layer contains many blood and lymphatic vessels (for nutrient transport) contains neurons of the enteric nervous system for GI control Muscularis skeletal muscle: mouth, pharynx, upper 2/3rds of the esophagus, external anal sphincter smooth muscle: everything else generally in 2 layers circular fibres longitudinal fibres contractions help: break down food mix food with secretions move food through the GI tract contractions (frequency and strength) are controlled by the enteric nervous system Serosa serous membrane outermost layer of the organs in the abdominal cavity a.k.a. visceral peritoneum p. 199 The Peritoneum 2 layers: visceral peritoneum (a.k.a. the serosa) parietal peritoneum between the 2 layers is the peritoneal cavity some organs (kidneys, ascending and descending colons, duodenum, pancreas) are covered by the peritoneum on their anterior side only – they are called retroperitoneal the peritoneum contains large folds that bind organs to one another and to the walls of the abdominal cavity p. 200 The Stomach a J‐shaped enlargement of the GI tract sits immediately below the diaphragm runs from the esophagus to the small intestine can expand to hold several litres food enters the stomach through the LES the stomach mixes food for 2‐4 hours the soupy mix of food in the stomach is called chyme little – no absorption happens in the stomach (water, ions, some fats, medications, alcohol) once food particles are small enough, they pass through the pyloric sphincter into the small intestine the wall of the stomach has the same 4 layered arrangement with some additions: 1) specialized cells 2) additional layer of smooth muscle 1) the mucosal layer has specialized cells that secrete: a) b) c) d) mucous (mucous neck cells) intrinsic factor and HCl‐ (parietal cells) pepsinogen and gastric lipase (chief cells) gastrin (a hormone secreted by G cells)) stimulates gastric juice production strengthens LES contraction increases stomach peristalsis relaxes the pyloric sphincter e) secretions from mucous, parietal and chief cells (G cells not included) are collectively called gastric juice 2) the muscularis has an additional (oblique) layer of smooth muscle to facilitate mixing p. 201 p. 202 The Small Intestine ~ 3 m long and 2.5 cm diameter large surface area for digestion and absorption 3 regions duodenum jejunum ileum circular folds: ridges in the mucosa/submucosa that increase surface area the mucosal layer has villi – finger‐like projections that increase surface area each villus is covered in epithelium cells of the epithelium: absorptive cells (digestion/absorption) goblet cells (mucous) Paneth cells (kill bacteria) enteroendocrine cells (secrete hormones) absorptive cells have microvilli (brush border) each villus has an arteriole, a venule and a lacteal p. 203 is chyme mixed with p. 204 intestinal and pancreatic juices absorbed nutrients pass into either: the blood stream (carbohydrates, proteins, water, electrolytes, water‐soluble vitamins) and go to the liver the lymphatics (lipids) via a lacteal remaining food passes through the ileocecal valve/sphincter into the large intestine p. 205 The Pancreas sits posterior to the stomach has both endocrine and exocrine functions exocrine function: secretes pancreatic juice (water, salts, sodium bicarbonate, enzymes) into the duodenal lumen juices are secreted into the proximal duodenum via 2 main ducts: a common duct formed by the pancreatic duct joining the common bile duct from the liver/gallbladder accessory duct p. 206 The Liver liver sits inferior to the diaphragm in the right superior region of the abdominal cavity 2 lobes: larger right and smaller left receives blood flow from 2 sources: hepatic artery – oxygenated blood hepatic portal vein – deoxygenated blood containing absorbed nutrients, medications, microbes and toxins blood flows past hepatocytes (liver cells) the hepatocytes: absorb oxygen, nutrients and toxins release nutrients (needed by other cells) blood eventually flows into the hepatic vein hepatocytes also manufacture and secrete bile p. 207 Other Functions carbohydrate metabolism helps maintain blood sugar levels low blood sugar – hepatocytes break down glycogen and release glucose into the blood high blood sugar – hepatocytes remove glucose from the blood and store it (as glycogen and fats) can convert some amino acids, lactic acid, and other sugars into glucose lipid metabolism hepatocytes store fat, break it down (to make ATP), and make cholesterol protein metabolism makes amino acids available for energy can convert amino acids into carbs or fats synthesizes most plasma proteins drug & hormone processing detoxifies/inactivates substances (drugs, alcohol, hormones) bilirubin elimination vitamin & mineral storage vitamin D activation p. 208 The Gallbladder a pear‐shaped organ sitting in the posterior surface of the liver stores and concentrates bile Bile a yellow/brown/olive liquid made by hepatocytes collects into larger and larger vessels function: emulsification (breakdown) of fats p. 209 p. 210 The Large Intestine food enters from the small intestine (ileum) through the ileocecal valve/sphincter 4 main regions of the large intestine: cecum colon rectum anal canal functions: completion of absorption, vitamin K production, formation and elimination of feces Cecum small pouch attached to the ileocecal valve attached to the cecum is the appendix contents move from the cecum into the colon Colon divided into 4 portions: ascending (to the hepatic flexure) transverse (to the splenic flexure) descending (to the left iliac crest) sigmoid (to the rectum at ~ S3) as food accumulates in the ascending colon, haustra fill up/distend which stimulates smooth muscle contraction and food moves into the next haustra regular peristalsis occurs but more slowly with each meal, mass peristalsis moves feces from the transverse colon into the rectum Rectum travels inferiorly to the anal canal Anal Canal terminal portion of the large intestine elimination control: internal (involuntary control) and external (voluntary control) anal sphincters p. 211 The Defecation Reflex distension of the rectum ultimately gives the urge to go and relaxes the internal anal sphincter p. 212 Divisions of the GI Tract upper GI: mouth, pharynx, esophagus, stomach, duodenum lower GI: most of the intestines and the anus p. 213 p. 214 The Control & Process of Digestion 3 overlapping phases: cephalic gastric intestinal 1) Cephalic Phase the thought, smell, sight, initial taste of food activates the NS salivary and gastric glands are stimulated to prepare the mouth and stomach for digestion 2) Gastric Phase once food reaches the stomach: stretch receptors are stimulated by the change in the stomach shape chemo receptors are stimulated by the change in stomach pH these changes: increase peristalsis and gastric juice production stimulate gastrin release small amounts of chyme pass through the pyloric sphincter into the duodenum food leaving the stomach decreases the volume of the stomach and removes some of the stimulus from stretch receptors food leaving the stomach allows the stomach to become more acidic decreasing stretch receptor stimulation and normalizing pH removes some of the stimulus for gastrin release 3) Intestinal Phase chyme in the duodenum causes enteroendocrine cells to release hormones: cholecystokinin (CCK) secretin these hormones: stimulates the gallbladder to eject bile into the duodenum (CCK) secretin decrease gastric juice production (CCK/secretin) slow food coming through the pyloric valve (CCK/secretin) increase pancreatic juice secretion (CCK/secretin) p. 215