L21 - GI Structure

• Define GI system

  Network of organs and specialised cells that allows us to transform food into energy and nutrients
• What are 3 main sections of GI tract?

  1) GI tract/Alimentary canal. 2) Accessory organs. 3) Mesentery
• What's included in GI Tract/Alimentary Canal?

  mouth, oesophagus, stomach, small intestine, large intestine, anusdivided into upper/lower GI tract
• What are the accessory organs?

  Help GI function and digestion; not part of alimentary canal- salivary glands- liver- gallbladder- pancreas

  
• What's Mesentery?

  - membranous tissue surrounding organs in lower GI tract- holds intestines in place- provides blood vessels and lymphatics - crucial for absorption in small intestine

  
• Mouth- structures and function- what do pharynx and epiglottis do?

  - teeth, tongue, mucous membranes- function = mastication- Pharynx + epiglottis prevent food travelling to the lungs- Pharynx responsible for swallowing- Epiglottis closes over larynx on swallowing

  
• Oesophagus- Structure/Function?- What is food bolus?

  - long tube- involuntary movement of food bolus- transports food bolus to stomach- no protection from HCl- food termed food bolus once past the pharynx

  
• Stomach- Structure/Function- what does it contain?- What are the main cell types in stomach wall?- What's the role of sphincters?

  - muscular sac after oesophagus- fat emulsification- HCl, pepsin- Parietal, gastric chief, mucous neck, G, D cells- Sphincters = ring of smooth muscle that close to stop movement of food (eg to prevent backflow into oesophagus)

  
• What are the three layers of the stomach wall?


  Mucosa - glands secrete mucusSubmucosa - glands and blood vesselsMuscularis - smooth muscle
• Stomach cells: Role of Mucous neck cell

  - secretes mucus and bicarbonate- protects from HCl
• Stomach cells: Role of parietal cells

  - secrete HCl to lower stomach pH (optimum condition to activate pepsin
• Stomach cells: Role of Enterochromaffin-like cells


  - secrete histamine- modulate neuronal signalling in ENS (eg in response to infection)
• Stomach cells: Role of Chief Cells

  - secrete pepsinogen (inactive form of pepsin). Cleaved into pepsin by low pH due to HCl secreted by parietal cells- secrete gastric lipase to break down fatty acids
• Stomach cells: Role of D cells

  - secretes somatostatin = peptide hormone which inhibits HCl production
• Stomach cells: Role of G cells

  - stimulate HCl production
• Why is it important that the stomach is acidic?(are weakly acidic drugs mostly ionised or unionised at low pH?)

  - Pepsinogen cleaved to its active form pepsin at low pH (for protein breakdown)- to enable activation of protease enzymes- weakly acidic drugs are mostly unionised at low pH
• Small Intestine:- Structure/Function- what 3 sections is it divided into?- what are the main cell types?


  - main site of absorption- main site of carbohydrate, protein and fat breakdown- longest section- Duodenum, jejunum, ileum- goblet, paneth, enteroendocrine, epithelial

  
• Small Intestine: Role of Epithelial Cells

  - increase SA for absorption (on villi, contain microvilli- releases enzymes involved in digestion

  
• What are Cyrpts of Lieberkuhn?

  pits between villi that contain stem cells for regeneration

  
• Small intestine: Role of paneth cells

  Defense - produce lysozymes (antimicrobial enzymes)- Phagocytic- Role in immunity

  
• Small Intestine: Role of goblet cells

  - secretes mucus

  
• Small intestine: role of Enteroendocrine Cell

  - secretes hormones eg secretin, cholecystokinin (inhibits gastric emypting, triggers gallbladder to contract and release bile)

  
• Large Intestine:- structure/function

  - ascending, transverse, descending, sigmoid limbs- reabsorption of water/ions- bulk movement of faeces- haustral shuttling

  
• Salivary Glands- structure/function- 3 main types?

  Three pairs: parotid, submandibular, sublingual- secrete saliva:- mucus - lubrication- amylase/lipase for digestion of starch/fatty acids

  
• Liver-structure/function

  - accessory organ- bile production and excretion- main site of drug metabolism

  
• Gallbladder- structure/function

  - coupled with liver: sits under visceral surface- stores and concentrates bile. Delivers it to small intestine

  
• Pancreas- structure/function

  - produces insulin and glucagon from alpha/beta islets of Langerhans- produces lipase, protease, amylase to digest fats, proteins, carbohydrates

  
• What are the six main functions of the GI system?

  ingestion, secretion, movement, digestion, absorption, excretion
• Define ingestion


  the process of taking in food/drink via mouth

  
• Define secretionGive examples of where and what

  release of substances which contribute to digestion- salvary glands secrete saliva (salivary amylase, mucus)- liver and gallbladder secrete bile- parietal cells in stomach secrete HCl

  
• What are the two types of movement that occur in the GI tract?

  segmentation and peristalsis
• Define segmentation


  contraction of circular muscles in GI tract

  
• Define peristalsis

  contraction of longitudinal muscleswavelikemain way food moved through GI tract

  
• Define digestion- what are the 2 types?

  breakdown of food into smaller components that can be absorbed into bloodstream- mechanical and chemical
• What is mechanical digestion?How does it occur?

  physically breaking down food into smaller particleseg via mastication, peristaltic contractions, gastric acids

  
• What is chemical digestion?

  use of enzymes secreted by accessory organs to break down food
• Define absorptionWhere's the main site? How is it adapted

  the process through which nutrients, water and electrolytes enter the bloodSmall intestine - villi and microvilli increase SA

  
• Define excretion

  The process by which metabolic waste is eliminated from the body- water is reabsorbed in large intestine, anything left behind is excretes
• What's the innate immune system?

  - many pathogens enter through mouth- HCl kills bacteria- Gut micriobiota
• What's the acquired immune system?

  - GALT - Gut Associated Lymphoid Tissue- Peyer's patches, mesenteric lymph nodes