L23 - GI System: Digestion and Absorption

• Define Digestion

  mechanical and chemical breaking down of food so it can be chemically convertes and absorbed by cells to maintain bodily functions
• What are the main sites of digestion?

  Saliva - amylaseStomach - HCl, pepsinSmall intestine - main siteLarge intestine - bacteria

  
• What causes gas?

  Side effect of bacterial digestion in large intestine
• Secretion in Salivary glands- name three salivary glands- what do they secrete?

  - parotid (mostly serous cells)-sublingual (mucus cells) -submandibular- secrete amylase (carbohydrates). Inactivated in acidic conditions- lingual lipase (fats)- the older you get, the less lingual lipase - important in newborns. Can work at low pH- lysozymes: saliva is antibacterial

  
• Secretion in stomach- what are the 4 main cell types in stomach wall?- what do they secrete?

  1) Parietal cells:- secretes HCl- low pH cleaves pepsinogen to pepsin for protein digestion2) Chief Cells- secrete pepsinogen (proform of pepsin)3) Mucous cell- secrete mucous/bicarbonate for protection/lubrication4) G cell- secrete gastrin: signals to parietal cells to release HCl

  
• What is pepsin?

  exopeptidase formed from pepsinogen that denatures protein
• Gastric Acid Secretion- where does it occur?


  - occurs in cannuliculi of parietal cells in gastric glands- G cells produce gastrin- gastrin stimulates parietal cells to release HCl- provides low pH for optimum pepsin activity- enterochromaffin cells secrete histamine which also stimulates HCl production
• How is carbonate formed?where?(and then released from mucous neck cells?)

  - in parietal cell- CO2 from blood combines with H2O to form H2CO3- H2CO3 (carbonic acid) unstable, so loses H+- HCO3- formed, and actively pumped out of parietal cell to neutralise HCl when necessary

  
• Gall bladder and pancreatic Secretion- what happens in gallbladder?- what does pancreas produce?- what do duct cells produce?

  - bile is concentrated in gallbladder, then passes down bile duct into small intestine- pancreas produces enzymes eg amylase and lipase- duct cells produce bicarbonate (HCO3-) to neutralise HCl

  
• Name some proteases and their proforms- where produced?- where are proforms converted into active forms

  Trypsin - from trypsinogenChymotrypsin - from chymotrypsinogen- produced in pancreas- converted in small intestine

  
• Control of Digestive Process- what triggers secretion of digestive juices?- how is HCl released?- what do intestines secrete?


  - stretch receptors signal to brain, which triggers secretion of digestive juices- g cells release gastrin, stimulating parietal cells to release HCl- intestine releases secretin and cholecystokinin (CKK), stimulating secretion of bile and pancreatic juice

  
• What's the main site of absorption?How's it adapted?

  small intestine- villi and microvilli give large SA- surrounded by mesentery: membrane containing blood vessels and lymphatics for easy transport

  
• What's mesentary?

  Membrane surrounding small intestinehighly vascularised, and lymphatics
• How are sugars/amino acids absorbed?

  Transported directly into blood from intestinal lumen, via active/passive transport

  
• How are lipids absorbed?

  Via lymphatic system, which drains into blood stream in lymph nodes/lacteals

  
• Protein Digestion

  - chief cells secrete pepsinogen (inactive)- HCl cleaves pepsinogen into pepsin due to low pH- higher pH of small intestine inhibits pepsin from further breakdown

  
• Why are proteases generated in proform?

  To prevent them from denaturing essential cell proteins, eg carrier proteins

  
• How are proteins absorbed once digested?

  - small peptides/amino acids diffuse across epithelial cells via carrier proteins

  
• How are carbohydrates digested?

  - mostly degraded by salivary amylase (or pancreatic amylase when secreted into small intestine)- need to be broken into monosaccharides for absorption

  
• What are these disaccharides broken down into? By which enzymes?MaltoseSucroseLactose

  glucose + glucose (maltase)glucose + fructose (sucrase)glucose + galactose (lactase)

  
• Fat digestion- main site?- what's required?- key enzyme?

  - small intestine- emulsification by bile required- lipase

  
• How are bile salts produced?- what's bile formed from?- where?

  - formed from cholesterol (diet)- synthesised in liver, stored in gallbladder

  
• What 4 acids does bile contain?

  cholic,chenodeoxycholic,lithocolic,deoxycholic

  
• What does bile do?

  Emulsifies fat into components eg triglycerides, fatty acids.These are absorbed through villi, then reform as triglycerides

  
• Define chylomicron- how are they formed?- major benefit?- where do they go?

  chylomicron = triglyceride + protein + cholesterol- formed by aggregation of triglycerides and cholesterol- benefit = soluble in water- water-soluble chylomicron enters lacteal, where it's transported to blood

  
• Oral Drug Absorption- what factors affect rate?- what is absorption affected by?- where?

  - physicochemical properties, formulation, route of administration- luminal pH, SA, perfusion (passage of blood through blood vessels), bile, mucous- small intestine

  
• Define enteric coating

  = when a drug is coated in a polymer resistant to stomach pH, so goes through stomach unaltered.Insoluble at low pH

  
• Why is enteric coating used?


  - protects med from stomach acid- local release into intestines- drug release is controlled with pH- protects pt from gastric distress/nausea- slightly delayed release