BIOM2012 GIT - Lecture 1 (4) PDF

BIOM2012: Systems Physiology Module: Gastrointestinal System Ben Weger, PhD ([email protected]) Institute of Molecular Bioscience Physiology of Circadian Rhythms  BIOM2012 GI Module Lecture 1: Overview of dige...

BIOM2012: Systems Physiology Module: Gastrointestinal System Ben Weger, PhD ([email protected]) Institute of Molecular Bioscience Physiology of Circadian Rhythms  BIOM2012 GI Module Lecture 1: Overview of digestion and absorption processes in the GIT, phases of gastric secretions, secretions of the Gastrointestinal system and their functions, digestive enzymes Lectures 2 & 3: The remaining GI system secretions and their functions, the intestinal phase of gastric secretion, motility in the GIT, the enteric nervous system Lecture 4: Liver anatomy and structure, functions of the liver, circadian clock and the Gastrointestinal system The Gastrointestinal system What is the Gastrointestinal system or Digestive system? It consists of the Gastrointestinal tract (GIT) or Alimentary Canal A long, continuous hollow tube that starts at the mouth and ends at the anus Accessory organs that aid in digestion Primary function (simplified): 1. Extract nutrients that we take in as food 2. Excrete waste products Organs of the Digestive System Major Organs of the Accessory Organs of Digestive Tract the Digestive System Oral Cavity (Mouth): 30 - 40 s Teeth Pharynx (throat): Tongue Esophagus: 4 - 10 s Salivary Glands Stomach: Liver 1-4h Small Intestine Gallbladder 4-6h (Duodenum, Jejenum, Ileum ) Pancreas Large Intestine 7-15 h (Colon, Cecum/Appendix, Colon Rectum, Anus) Rectum 4 Organs of the Digestive System Major Organs of the Digestive Tract Oral Cavity (Mouth): Food Ingestion, mechanical digestion with accessory Pharynx organs (teeth and tongue), moistening, mixing with salivary secretions. Pharynx (throat): Muscular propulsion of materials into the Bolus esophagus. Esophagus: Transport of materials to the stomach. Stomach: Engages in both chemical digestion (via digestive enzymes) and mechanical digestion (muscular Chyme “juice” contractions). Colon Small Intestine (duodenum, jejenum and ileum): Enzymatic digestion, absorption of water, organic substrates, vitamins, and ions. Nutrients Large Intestine (colon, rectum and anus): Dehydration and compaction of indigestible Faeces Rectum materials in preparation for elimination. 5 Organs of the Digestive System Accessory Organs of the Digestive System Teeth: Mechanical digestion by chewing (mastication). Tongue: Assists mechanical digestion with teeth; sensory analysis. Salivary Glands: Secretion of lubricating fluid containing enzymes that break down carbohydrates. Liver: Secretion of bile (important for lipid digestion), storage of nutrients, and many other vital functions. Gallbladder: Storage and concentration of bile. Pancreas: Exocrine cells secrete buffers and digestive enzymes; endocrine cells secrete hormones. 6 Smooth and skeletal muscle sphincters control the passage of food along the GIT Upper Esophageal Sphincter: Allows food to pass from the mouth into the esophagus. Lower Esophageal Sphincter: Lets food pass from the esophagus into the stomach and allows air to escape from the stomach when burping. Pyloric Sphincter: Opens to allow partially digested food to pass from the stomach into the duodenum. Ileocecal Sphincter: A muscle valve that separates the small intestine from the large intestine. Internal Anal Sphincter: An involuntary muscle that prevents stool from leaking out. External Anal Sphincter: A voluntary muscle that allows to control the passage of stool. 7 Sphincter of Oddi Sphincter of Oddi is a valve which allows bile and pancreatic juice to flow into the small intestine. It ensures that the right amounts are released when food passes into the duodenum. Anatomy of the wall of the GI tract – a common organizational theme The anatomy of the GI tract wall varies along its length but shares common organizational themes across all segments: Mucosa: Epithelial layer with connective tissue (lamina propria), capillaries, neurons, immune cells, and smooth muscle. Submucosa: Loose connective tissue with larger blood vessels, may have Lumen glands. Muscle Layer: Two smooth layers (inner circular, outer longitudinal) with neurons in between. Muscle Serosa: Outer connective tissue with layer squamous epithelial cells. 9 Basic Function of the GI Tract 6 steps and four processes Ingestion (eating/intake) Secretion (e.g., enzymes, mucus, chemicals, hormones) Digestion (mechanical + chemical) Absorption Motility (mixing and propulsion) Defecation Four processes of the digestive system in different parts of the GI tract  Digestive enzymes Digestive enzymes are enzymes that break down polymeric macromolecules into their smaller building blocks, to facilitate their absorption by the GIT. Carbohydrates Proteins Fats (triglycerides) DNA, RNA Proteases, Lipase Peptidases Nuclease Mono/Di- mono Amino Acids, glycerides, Fatty Peptides glycerol acids Nucleotides Where in the GIT do these enzymes act? Stomach Stomach Oral Cavity/stomach Small Intestine Small Intestine Small Intestine Small Intestine  Digestive enzymes Digestive enzymes are classified based on their substrates: proteases and peptidases split proteins into small peptides and amino acids. lipases break triglycerides into fatty acids and glycerol or monoglycerides (which is glycerol attached to a single fatty acid) diglycerides (which is glycerol attached to two fatty acids) amylases digest starch/glycogen into smaller molecules yielding maltose, which in turn is cleaved into two glucose molecules by maltase nucleases split DNA/RNA into nucleotides. Fun fact: How much DNA do we eat per year? Not examinable https://padletuq.padlet.org/bweger/git-n5g2hmcn9ncwlise 16 Organs of the Digestive System Major Organs of the Accessory Organs of Digestive Tract the Digestive System Oral Cavity (Mouth): Teeth Pharynx (throat): Tongue Esophagus: Salivary Glands Stomach: Liver Small Intestine Gallbladder Large Intestine Pancreas Colon Rectum 17 Digestive process in the oral cavity Digestion: Carbohydrates Absorption: None There are three pairs of major salivary glands: the parotid glands, the submandibular glands, and the sublingual glands In the Mouth: Saliva contains salivary amylase mainly from parotid glands, starting starch breakdown into maltose. Only about 5% of starches are hydrolyzed before swallowing. In the Stomach: Digestion continues for up to 1 hour until food mixes with gastric acid. Salivary amylase becomes inactive in acidic conditions (pH < 4.0). By this point, app. 30% of starches are digested. 18 Organs of the Digestive System Major Organs of the Accessory Organs of Digestive Tract the Digestive System Oral Cavity (Mouth): Teeth Pharynx (throat): Tongue Esophagus: Salivary Glands Stomach: Liver Small Intestine Gallbladder Large Intestine Pancreas Colon Rectum 19 Stomach secretion from gastric glands Digestion: Proteins and fats (minimal) Absorption: alcohol, aspirin (lipid soluble substances) Secretions: Acid HCl (hydrochloric acid) Decreases pH; kills bacteria and pathogens; denature proteins; activates pepsin (pepsinogen → pepsin) Mucus Viscous secretion composed primarily of water, bicarbonate (HCO3-) and glycoproteins called mucins protects wall of stomach from acid GI hormone Gastrin regulates digestive activity by stimulating acid production and promoting stomach muscle contractions Digestive enzymes remove only one fatty acid from each Triglyceride Gastric secretion and food processing is traditionally divided into 3 phases: Cephalic Gastric Intestinal Phase Phase Phase  Gastric secretion and food processing is traditionally divided into 3 phases: Cephalic Gastric Intestinal Phase Phase Phase sight, smell, taste, Presence of food Presence of food Stimulus: or thought in the stomach in the duodenum 1. Cephalic Phase Triggered by: The sight, smell, taste, or thought of food before it even enters the stomach. Mechanism: The brain (specifically, the hypothalamus and medulla) stimulates the vagus nerve (CN X), which sends signals to the stomach to begin the secretion of gastric juices and gastrin. Outcome: This phase prepares the stomach for digestion by stimulating the release of gastric juice production (e.g., hydrochloric acid (HCl), and pepsinogen), as well as increasing gastric motility. Take-home message from this slide: Each phase has also input from the CNS The parasympathetic nervous system stimulates gastric secretion, while the sympathetic nervous system generally inhibits gastric secretion. 2. Gastric Phase Triggered by: The presence of food in the stomach, which causes stomach distension (stretching) and the presence of proteins and amino acids. Gastric phase Mechanism: The distension activates stretch receptors, and partially digested proteins stimulate the release of gastrin. pH > 4 gastrin release ↑ pH < 3 gastrin release ↓ Increased gastrin leads to an increase in gastric juice production (including HCl and pepsinogen) and enhances stomach contractions. Outcome: This phase is responsible for the majority (about 60-70%) of gastric secretion, ensuring the proper breakdown of ingested food. The intestinal phase will be discussed in the next lecture! 26 Organs of the Digestive System Major Organs of the Accessory Organs of Digestive Tract the Digestive System Oral Cavity (Mouth): Teeth Pharynx (throat): Tongue Esophagus: Salivary Glands Stomach: Liver Small Intestine Gallbladder Large Intestine Pancreas Colon Rectum 27 The Pancreas Composition of pancreatic juice - Watery, alkaline solution (pH 8) - Electrolytes, primarily HCO3− (bicarbonate) → neutralize stomach acid - 1200–1500 ml/day is produced - Digestive enzymes Proteases Converts triglycerides into monoglycerides and fatty acids Nucleases Pancreas Converts DNA/RNA into nucleotides Liver & Gallbladder The Gallbladder Gallbladder: A small, pear-shaped, thin-walled muscular sac located on the underside (ventral surface) of the liver. Primary Function: Stores and concentrates bile by absorbing water and electrolytes. Structure: Contains many honeycomb folds, allowing it to expand as it fills with bile. Bile Release: When stimulated (by CCK), the gallbladder contracts, releasing bile into the cystic duct, which then flows into the common bile duct and into the duodenum. Gall bladder - Bile Bile: is produced in the liver Contains bile salts that are amphipathic (hydrophobic and hydrohilic regions) Bile facilitates lipid absorption and digestion by Emulsification: Bile salts surround fat droplets; hydrophobic side binds to fat, hydrophilic side interacts with water, breaking them into smaller droplets (emulsion droplets). Ce Increased Surface Area: Enhances fat exposure to enzymes (e.g., pancreatic lipase) for digestion. Cell of small intestine Micelle Formation: Bile salts form micelles to transport digested fats for absorption.  GI Lecture 1 Learning objectives Now make sure you have a solid understanding of Organs of the GIT Digestion and absorption processes in the GIT GI tract secretions and their function (digestive enzymes) Phases of gastric secretions (cephalic and the gastric phase)  Gastrointestinal Lecture 1: Test your knowledge Following this GI lecture, you should be able to… Describe the organs of the GIT and their functions Name digestive enzymes, their functions, where they act, and organ of secretion Describe two phases of gastric secretion (trigger, mechanism)  [email protected] COMMONWEALTH OF AUSTRALIA Copyright Regulations 1969 WARNING This material has been reproduced and communicated to you by or on behalf of the University of Queensland pursuant to Part VB of the Copyright Act 1968 (the Act). The material in this communication may be subject to copy right under the Act. 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