Week 35- The GI tract part 2

• Functions of the many cell types of the Stomach gastric glands involved in digestion

  • Parietal cells secrete HCl to help digestion of proteins and kill bacteria
  • HCl secretion is stimulated by gastrin, histamine and neurotransmitters
  • Parietal cells secrete HCl into lumen and HCO3- into blood
  • HCl activates pepsin to digest proteins
• Hormonal regulation of gastric acid (HCL) secretion

  1. Amino acids in stomach lumen stimulate gastrin secretion

  2. Gastrin stimulates histamine release from ECL cells

  3. Histamine stimulates parietal cells to secrete HCL

  4. Gastrin and histamine also stimulate pepsinogen production from chief cells

  5. Vagus nerve also stimulates gastrin secretion and stomach muscle contraction
• Digestion of proteins in the small intestine

  1. Endopeptidases (trypsin, chymotrypsin) cut internal peptide bonds

  2. Exopeptidases (carboxypeptidases, aminopeptidases) cut bonds at ends
• Transport of monosaccharides across intestinal epithelium

  1. Monosaccharides absorbed by co-transport with Na+

  2. Monosaccharides move out of epithelial cells by facilitated diffusion

  3. Monosaccharides enter blood capillaries and hepatic portal vein
• Transport of lipids across intestinal epithelium

  1. Bile salts form micelles with fatty acids and monoglycerides

  2. Micelles release contents into epithelial cells

  3. Fatty acids and monoglycerides converted to triglycerides in chylomicrons

  4. Chylomicrons exocytosed and enter lacteals of intestinal villi
• Chylomicron


  Lipoprotein particle that transports dietary lipids to liver, adipose, cardiac, and skeletal muscle tissue
• Chylomicrons consist of triglycerides (85-92%), phospholipids (6-12%), cholesterol (1-3%) and proteins (1-2%)
• Chylomicrons are one of the five major groups of lipoproteins that enable fats and cholesterol to move in the bloodstream
• Peptic ulcers


  Erosions of the mucous membranes in the stomach or duodenum caused by hydrochloric acid (HCl)
• Helicobacter pylori


  Bacterial infection associated with peptic ulcers, can also lead to stomach cancer
• Lactose intolerance


  Inability to digest milk sugar (lactose) due to lack of lactase enzyme
• Appendicitis


  Inflammation of the appendix, can be life-threatening if it ruptures
• Pathogenesis of acute appendicitis

  1. Obstruction of appendix lumen allows bacterial growth

  2. Increased luminal pressure blocks blood flow and causes swelling and pain

  3. More bacterial growth under anaerobic conditions leads to acute appendicitis
• Coeliac disease


  Autoimmune response to gluten protein alpha-gliadin, destroys intestinal villi
• Sources of GI tract infections and methods of spread


  • Pathogens excreted in faeces and spread through contaminated food, fluids or fingers

  Bacterial toxins that enhance chloride and water secretion causing diarrhea
• Diseases of the GI tract

  • Inflammation (gastritis, IBD, appendicitis)

  Microorganism entry (diarrhea, emetic toxins, fever/malaise)

  Ulceration (Helicobacter pylori)

  Obstruction (ileus, hernia, polyps/cancer)

  Malabsorption (celiac, pernicious anemia)