Digestive System - Lecture notes 10 PDF

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PET3322 22 October 2020 Lecture 10- Digestive System Process  Mouth  esophagus  small intestine  large intestine  anus  Ingestion  Mechanical breakdown o Chewing (mouth) o Churning (stomach)  Propulsion o Swallowing o Peristalsis  Digestion o Segmentation (small intestine)  More active than peristalsis  Absorption o Blood circulation  Defecation Enteric Nervous System  Gut “brain”  Located in sheaths of tissue lining the esophagus, stomach, small in, colon  Contains more neurons than spinal cord  Communicates extensively with each other  Enteric neurons make up bulk of two main interconnecting intrinsic nerve plexuses o Submucosal nerve plexus  Regulates glands and smooth muscle in mucosa  Dilate or constrict o Myenteric nerve  Controls GI tracts motility Organs  Mouth: salivary glands o Composed of two secretory cells  Serous cells: produce watery secretion, enzymes, ions, bit of mucin  Mucous cells: produce mucus  Lubricating o Parotid and submandibular (near ear) glands contain mostly serous cells, but sublingual gland consists mostly of mucous cells o Functions of saliva  Cleanse mouth  Dissolves food chemicals for taste  Moistens food  Begins breakdown of starch with amylase o Salivary glands  Mostly water (97-99%)

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 Hypo-osmotic Slightly acidic- pH 6.75 to 7 Salivary amylase and lipase Converts nitrated in food to nitric oxide to protect against microorgs  Provide more blood to digestive system Control of salivation  1500mL/day can be produced to keep mouth moist  Activated by parasympathetic nervous system when… o Smell/sight of food o Upset GI

Stomach o Temporary storage tank that starts chemical breakdown of protein digestion o Highly stretchable  Empty stomach has ~50 mL volume but can expand to 4L o Converts bolus of food to paste-like chyme o When empty, stomach mucosa forms many folds called rugae o Stomach is always open to esophagus- no sphincter o Pyloric sphincter- muscle that separates stomach from duodenum o Gastric glands  Millions of holes- gastric pits  Cells make and release digestive enzyme  Cells all make different enzymes, work together  Parietal cell o Secrete…  HCl: denatures protein, activate pepsin, breaks down plant cell walls, kill many bacteria  pH 1.5-3.5  intrinsic factor: glycoprotein required for absorption of vit B12 in small intestine  Chief cell o Secrete…  Pepsinogen: inactive enzyme, activated by HCl and pepsin itself  Positive feedback mechanism  Important for protein digestion  Lipases  Digest ~15% of lipids  Enteroendocrine cell o Release helping enzymes for small intestine o Secrete chemical messengers to lamina propria  Serotonin, histamine, CCK- go to gall bladder  Paracrine  CCK- contraction of gall bladder to release bile o Important for lipid digestion

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Somatostatin (acts as paracrine) and gastrin- go to bloodstream and stomach  Hormones Regulation of gastric secretions  Cephalic phase (reflex) o Occurs before food enters stomach o Triggered by aroma, taste, sight, thought o Vagus nerve stimulates gastric glands to get ready for digestions  Gastric phase o Occurs once food hits stomach o Lasts 3-4 hours and provides 2/3 of gastric juice releases o Stimulation  Distension activates stretch receptors, initiating both long and short reflexes  Chemical stimuli  Release of gastrin initiates HCl release from parietal cells and stimulates enzyme secretion  Buffering action of ingested proteins causes pH to rise- more gastrin o Inhibition  Low pH inhibits gastrin secretion  Occurs between meals and during digestion o Negative feedback o More protein = more HCl = low pH  Intestinal phase o Begins with brief stimulatory component followed by inhibition o Stimulation  Partially digested food enters duodenum  Brief release of intestinal gastrin  Encourages gastric glands to continue secretory activity o Inhibition  Four main factors in duodenum  1- distention of duodenum due to entry of chyme  2-4: presence of acidic, fatty, hypertonic chyme  Inhibitory effects protect intestine from being overwhelmed by too much chyme or acidity  Achieved in two ways  Enterogastric reflex: duodenum inhibits acid secretion in stomach by o Short reflex

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o Long reflexes (SNS and vagus nerve) Enterogastrones: duodenal enteroendocrine cells release hormones that inhibit gastric secretions o Secretin, CCK

o Regulation of gastric emptying  Stomach empties fully in 4 hours  Carb-rich chyme moves quickly through duodenum  Increase fatty chyme entering duodenum can increase time for emptying to 6 hours or more  Duodenum can prevent overfilling by controlling how much chyme enters  Duodenal receptors respond to stretch and chemical signals  Enterogastric reflex and enterogastrones inhibit gastric secretion and duodenal filling Accessory organs o Liver  Largest gland in the body: ~3lbs  Hepatocytes have increased rough and smooth ER, Golgi apparatus, peroxisomes, and mitochondria  Hepatocyte functions  Produce bile  Process bloodborne nutrients  Store fat-soluble vitamins (A, D, E, K)  Perform detoxification o Convert ammonia to urea (urea cycle)  Bile  Produced by liver, stored by gall bladder  Yellow-green, alkaline solution contains… o Bile salts: cholesterol derivative that function in fat emulsification and absorption o Bilirubin: pigment formed from heme waste product  Bacteria breakdown in intestine to stercobilin that gives brown color of feces o Cholesterol, triglycerides, phospholipids o Gallbladder  Thin-walled muscular sac on ventral surface of liver  Functions to store and concentrate bile  Contains honeycomb folds that allow it to expand as it fills  Muscular contractions release bile via cystic duct, flows into bile duct o Pancreas  Locations- deep greater curvature of stomach  Releases pancreatic juice for neutralization  Composition o Watery, alkaline solution to neutralize acidic chyme coming from stomach

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o Electrolytes o Digestive enzymes  Proteases: secreted in inactive form to prevent selfdigestion  Trypsinogen  Amylases  Lipases  Nucleases Control by small intestine  Bile and pancreatic ducts unite in wall of duodenum  Hormonal regulations o CCK: stimulates secretion by pancreatic acinar cells and gall bladder contraction o Secretin: stimulates secretion by pancreatic duct cells  Week stimulator of bile secretion

Small intestine o Major organ of digestion and absorption  Extends from stomach (pyloric sphincter) to large intestine  Duodenum, jejunum, ileum o Blood supply  Superior mesenteric artery brings blood supply  Veins (carrying nutrient-rich blood) drain into liver o Nerve supply  Parasympathetic innervation via vagus nerve  Sympathetic innervation from thoracic nerves o Chyme from stomach contains partially digested carbs and protein and undigested fats o Sources of enzymes for digestion  Bile, bicarbonate, dig enzymes are imported from liver and pancreas o Takes 3-6 hours to absorb all nutrient and most water o Absorption  Process of moving substances from lumen of gut into body  Tight junctions ensure molecules pass through epithelial cells rather than between them  Material enter cell through apical membrane and exit through basolateral membrane  Lipid molecules can be absorbed passively through membrane  Other polar molecules are absorbed by active transport  Most nutrients are absorbed before chyme reaches ileum o Carb digestion  Begins in the mouth with salivary amylase  Starch and glycogen broken down into oligosac by pancreatic amylase  Further broken down by other intestinal enzymes  Final product = monosaccharides (glucose, fructose, galactose)  Only thing that can be absorbed  Glucose and galactose are absorbed via cotransport with Na+

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Fructose passes via facilitated diffusion All monosaccharides leave cells via facilitated diffusion  Transported to the liver o Protein digestion  Broken down into large polypeptides by pepsin- activated by HCl  pH = 1.5-2.5  Pancreatic enzymes (trypsin) break down further  Brush border enzymes break small peptides into amino acids  Amino acids absorbed via cotransport with Na+ o Lipid digestion  Starts in small intestine  Binds with bile salt  Steps  Emulsification: bile salts emulsify large fat globules into small ones o Lipids cannot be digested or absorbed into bloodstream w/o bile  Digestion: pancreatic lipases break down fat  Micelle formation: products from digestion are coated with bile salts and lecithin  Diffusion: products leave micelles and cross epithelial membrane via diffusion  Chylomicron formation: lipids are converted back into triglycerides and packaged with lecithin and lipoproteins o Forms chylomicron  Chylomicron transport: chylomicrons are exocytosed from basolateral side and enter lymphatic system o Emptied into venous blood at thoracic duct o Once in blood, chylomicrons are broken into free fatty acids and glycerol by lipoprotein lipase so they can be used by cells o Short-chain fatty acids can diffuse directly into the blood  Slow process...