GI Anatomy and Physiology 2024 PDF

GASTROINTESTINAL ANATOMY AND PHYSIOLOGY Diane O’Meara, Physician Associate Mouth Teeth 32 Incisors (8) Cuspid (4) Premolars (8) Molars (12) Oral vestibule Gingiva Superior/Inferior labial frenula Lingual frenulum Uvula Palatine tonsil Hard and soft palates Phary...

GASTROINTESTINAL ANATOMY AND PHYSIOLOGY Diane O’Meara, Physician Associate Mouth Teeth 32 Incisors (8) Cuspid (4) Premolars (8) Molars (12) Oral vestibule Gingiva Superior/Inferior labial frenula Lingual frenulum Uvula Palatine tonsil Hard and soft palates Pharynx Nasopharynx Oropharynx Laryngopharynx Tongue Hard palate Soft palate Vallecula Epiglottis Oesophagus Upper oesophageal sphincter (UOS) Separates the pharynx from the oesophagus Lower oesophageal sphincter (LOS) Gastro-oesophageal junction (GOJ) Separates the oesophagus from the stomach- high resting tone prevents gastric reflux Diaphragm surrounds Upper endoscope generally measures 40cm from incisors to GOJ Stomach Fundus Body Antrum Pyloric sphincter Small Bowel Duodenum First and shortest Approx 30cm Duodenal bulb Liver and pancreas drain into Ligament of Treitz separates Small Bowel Duodenum First and shortest Jejunum Second Ileum Third and longest Ileocaecal valve separates it from colon Liver Location: Right upper quadrant, protected by the ribs Functions: Digestion Metabolic Storage Immune Detoxification Clotting factors Pancreas Exocrine Function (digestion) Acinar cells secrete digestive enzymes into main pancreatic duct Main pancreatic duct enters the duodenum through the Ampulla of Vater Endocrine Function (bloodstream signalling) Islets of Langerhans Beta cells- insulin Alpha cells- glucagon Liver and Pancreas Biliary Tree Common bile duct (CBD) Gallbladder Cystic duct Pancreatic duct Head and tail of the pancreas Ampulla of Vater Common duct formed by the Pancreatic duct and CBD Sphincter of Oddi Sphincter closing the bile duct and pancreatic duct at the duodenal entrance Colon Caecum Appendix Ascending colon (hepatic flexure) Transverse colon (splenic flexure) Descending colon Sigmoid Rectum Anus Lengths Tongue 150 cm 50 cm Oesophagus Small Bowel Stomach Colon 550 to 600 cm Layers of the GI Tract Mucosa Epithelium Lamina propria Muscularis mucosae Submucosa Muscularis (peristalsis) Circular Contracts behind a food bolus Prevents food from travelling backwards Longitudinal Contracts in front of a food bolus Shortens tract Serosa Mesentery Attaches bowel to the posterior abdominal wall Connection for Blood vessels Lymphatic vessels Nerves Arteries Arteries Coeliac artery Superior mesenteric artery (SMA) Inferior mesenteric artery (IMA) Hepatic artery no copyright infringement is intended Veins leading to the Inferior Vena Cava (IVC) Renal veins Ovarian or testicular veins Left adrenal vein Common iliac veins Hepatic vein Phrenic veins (diaphragm) no copyright infringement is intended Veins leading to the Hepatic Portal Vein Hepatic Portal Vein Splenic vein Inferior mesenteric vein Superior mesenteric vein no copyright infringement is intended Hepatic circulation Hepatic Portal vein 70% of the blood supply to the liver 50% of the oxygen Hepatic Artery 30% of the blood supply to the liver Hepatic Vein Drains liver, joins IVC no copyright infringement is intended Hepatic circulation Branches divide Blood flows slowly through the hepatic sinusoids Into hepatic venules Eventually into hepatic vein Re-join systemic circulation at the inferior vena cava no copyright infringement is intended Liver Removes toxins ingested with food Metabolises medicines Kupffer cells clear microorganisms no copyright infringement is intended Portal hypertension Increased pressure in the portal system due to increased resistance in the hepatic system Cirrhosis or thrombosis can cause ‘portal hypertension’ Oesophageal and rectal varicies can form when portal blood is re-routed Ascites can form as a result of high pressure and low albumin no copyright infringement is intended Lymph Lymph Fluid from cells and tissues that doesn’t enter the capillary system Fats and proteins White blood cells Lymphatic vessels Transport the above Connect lymph nodes Lymphatic drainage Right lymphatic duct Drains the right upper Thoracic duct Drains the left upper and bilateral lower Nerves/Neural control of the GI system Enteric Nervous system Voluntary Nerves Enteric=gut Lips, tongue, mastication, pelvic Sensory and motor neurones floor muscles and external anal sphincter Has some autonomy Receives signals from extrinsic Autonomic Nerves nerves (voluntary and autonomic) Sympathetic Fight or flight Decrease motility and secretion Increase sphincter tone Parasympathetic Rest and digest(!) Increase motility and secretion GI Physiology Overview Review molecules and Digestion, absorption and mechanical actions involved utilisation of in gastrointestinal function Carbohydrates Journey from the start to the Fats end of the GI tract Proteins Types of molecules in the GI tract Enteric Hormones Control gastric secretion and motility Mediate communication between different parts of GI system or CNS Enzymes Amylases - Break big carbohydrates (CHO) down to smaller CHO Oligosaccharidases - Break small CHO down to even smaller CHO Peptidases - Break down proteins Lipases - Break down fats Special Molecules Mouth Mastication Mechanical breakdown Mixing with saliva Saliva Salivary amylase Digests polysaccharides (starches) into monosaccharides and disaccharides Parotid gland through the parotid/Stensen’s duct Mucus and Serous Fluid Binds food particles, acts as lubricant Sublingual gland and submandibular gland Swallowing/deglutition Oropharyngeal phase Food is moved back through the mouth by the tongue Space between the tongue and anterior hard palate expands and food collects in the valleculae The epiglottis closes off the trachea The tongue moves backwards and pharyngeal wall forward, forcing food down the oesophagus Swallowing/deglutition Oesophageal phase Upper oesophageal sphincter opens Food moved down with peristaltic action (and gravity) Lower oesophageal sphincter opens to allow food to pass into the stomach  Swallow studies Normal barium swallow https://radiopaedia.org/cases/85944 Morgan M, Normal oesophageal peristalsis. Case study, Radiopaedia.org (Accessed on 15 Sep 2024) https://doi.org/10.53347/rID-85944 Stomach Production of Gastric Juice Digestive enzymes Acidic and alkaline secretions pH 1.5-3.5 Regulatory molecules Mechanical Mixing to create Chyme (food particles and gastric juice) Propulsion into the duodenum through the pylorus Absorption Water, some salts, lipid soluble drugs, alcohol (also in small intestine) Gastric pits and cell types Stomach Enteroendocrine Cells Release hormones into the blood stream in response to stimuli Gastrin Stimulates gastric juice production Histamine Stimulates gastric juice production Bind to receptors on parietal cells, stimulating them to release HCl Stomach Enteroendocrine Cells Ghrelin ‘Hunger hormone’ Regulates appetite Increased prior to a meal Targets the hypothalamus Stomach- Chief Cells Pepsinogen Secreted into the gastric pit Becomes pepsin in the presence of HCl Digests protein into polypeptides Stomach- Parietal Cells Intrinsic Factor Required for B-12 absorption in ileum Hydrochloric acid (HCl) Creates optimal pH for enzyme activity Activates pepsinogen Denatures proteins Kills ingested bacteria ‘Proton pump’  Parietal Cells Mucous Basolateral Surface Histamine H2 Receptor Gastric Pit Gastrin Gastric Lumen Proton Pump Acetylcholine HCO3- + H+  CO2 + H2O  Parietal Cells Mucous Basolateral Surface Histamine H2 Receptor Gastric Pit Gastrin Cl- HCl Gastric Lumen H+ K+ Proton Pump Acetylcholine HCO3- + H+  CO2 + H2O HCO3- Cl- Vomiting Complex reflex managed by parts of the medulla oblongata The chemoreceptor trigger zone (CTZ) Receives stimuli from signals in the blood Communicates with the vomiting centre Multiple receptors including 5HT3 (serotonin), D2 (opiates), Histamine (target for motion sickness medications) The vomiting centre directs motor responses Close nasal cavity, glottis (opening to trachea) Relax LOS Contract diaphragm and abdominal wall Vomiting Stimuli In the GI tract Drugs, toxins Distention Vagus nerve Outside the GI tract Higher brain centres - sights, sounds, odours, tastes, emotions Inner ear- Labyrinths of the inner ears with changes in motion Mechanical stimulus Back of the pharynx Leaving the stomach Carbohydrates Intestinal phase fastest Begins when chyme leaves the Proteins stomach Involves coordination of the Fatty Foods Pancreas 3-6 hours Liver and gall bladder Small intestine (duodenum) Liver - Bile Hepatocytes make bile Send it down the common bile duct (CBD) Bile in the CBD builds up CBD and fills gallbladder while the Sphincter of Oddi (SOD) is closed Bile contains Bile salts Bilirubin- yellowish green SOD is a sphincter closing the liquid bile duct (and pancreatic duct) Cholesterol at the duodenal entrance Water  Bile salts (liver) Reduce surface tension/break fat globules into droplets (emulsification) Enhance fatty acid, cholesterol, Vit A, D, E, K absorption CBD Cholecystokinin CCK (duo) Hormone secreted by duodenal wall cells Triggered by fatty food in the duodenum Inhibits gastrin Stimulates gallbladder (gb) contraction Stimulates pancreatic secretion  Pancreatic Enzymes Pancreatic amylase Pancreatic lipase CBD Peptidases Enzymes secreted in zymogen granules, to be activated Higher pH Including Trypsin Neutralises chyme Allows digestive enzymes to work better Small Intestine Absorption Plicae circulares On the intestinal wall Villi On the inner wall of small intestine More in the duodenum and jejunum Microvilli Cells Increase surface area to aid in absorption Digestion and Absorption of Starch and glycogen Carbohydrates Amylase Disaccharides- Maltotriose, Sucrose, Lactose Carbohydrates Salivary < Pancreatic amylase Maltase Sucrase Lactase Break down starch and glycogen into disaccharides SGLT Disaccharidases Glucose Brush border enzymes Fructose Break down disaccharides into Galactose monosaccharides Monosaccharides Sodium-glucose co-transporter (SGLT-1) Capillary Lacteal Digestion and Absorption of Carbohydrates Enter capillary via facilitated diffusion Glucose is either Stored as glycogen in the liver Glucose Utilised in the citric acid cycle, oxidative phosphorylation ATP production Capillary Lacteal Digestion and Absorption of Protein, polypeptides Proteins Pepsin, Trypsin Oligopeptides Proteins Amino Acids Digested into oligopeptides Oligopeptides digested into amino acids Amino Acids Amino acids enter the cell via a Na+ co-transporter Amino acids absorbed into the blood for Capillary transport to other cells Lacteal  Triglycerides and phospholipids Digestion and Absorption of Water, alkaline pH, mixing Emulsion (fat droplets) Fats Lipase Fatty acids, glycerol An emulsion is created and Bile salts broken down by lipase Micelles Bile salts, phospholipids cholesterol help form micelles: Hydrophobic core Hydrophilic surface Absorbed across cell membrane to form chylomicrons Chylomicron Chylomicrons are secreted into the basolateral space and Capillary carried in the lacteals Lacteal Vitamins and Minerals Iron Vitamin B12 Haem-iron easily absorbed (animal Binds intrinsic factor (from parietal sources) cells) Non Heam iron - Ferric Fe3+ (in Absorbed in the terminal ileum plants) Source- meat, eggs, milk Ferrous Fe2+ (absorbed) HCl and Vitamin C promote Vitamins A, D, E, K conversion to Fe2+ Fat soluble vitamins Absorbed in the duodenum Require adequate bile acid secretion Folic Acid Absorbed in the jejunum Source- green leafy vegetables Large Intestine/Colon Major function – reabsorb water Undigested, unabsorbed material turns to faeces Contains up to 500 species of bacteria Large Intestine/Colon Elimination of faeces Movement of faecal material into to rectum signals parasympathetic neurons in spinal cord Defecation reflex Rectum contracts Internal sphincter relaxes Brainstem and thalamus notified External sphincter relax at a convenient time defecation-reflex1-n.jpg (720×540) (slideserve.com)