The Digestive System Functions of the Digestive System
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The Digestive System Functions of the Digestive System • Ingestion • Mechanical processing • Digestion • Secretion • Absorption • Excretion 1 Histological organization • In lab • Know it for both lecture and lab tests Movement of Material • Visceral smooth muscle • Pacesetter cells • Peristalsis – Longitudinal movement 2 Second type of movement • Segmentation – Churning – Mixing • Nervous tissue of GI tract 3 Control of Digestive Tract • Motility & secretion • Neural mechanisms – Many neurons – Short & long reflexes • Hormones - 18! • Local mechanisms – Prostaglandins – histamine Stomach • Temporary storage of food • Mechanical breakdown of food • Chemical digestion • Intrinsic factor secretion – intestinal absorption of vitamin B12 4 Stomach lining Gastric pits • Mucous cells – Secrete mucus – Provides protection from low pH Gastric glands • Parietal cells • Chief cells 5 Gastric pits • Mucous cells – Secrete mucus – Provides protection from low pH Gastric glands • Parietal cells • Chief cells Chief cells • secrete pepsinogen • Pepsin – proteolytic enzyme • 2 enzymes secreted by infants – Rennin – Gastric lipase 6 Pyloric glands • Mucous secretion • Enteroendocrine cells G cells – secrete Gastrin • Increases activity of – Parietal cells – Chief cells • Increases gastric motility – D cells FYI only • Somatostatin • Inhibits gastrin secretion • Overcome by neural stimuli gastric digestion & absorption • carbohydrate and lipid digestion occurs until gastric pH hits 4.5 • proteins digested in stomach to polypeptides/short peptides by pepsin • no nutrient absorption occurs • other absorbed molecules… 7 Small Intestine digestion and absorption • duodenum 10 in – Duodenal (Brunner’s) glands • jejunum 8 ft – Prominent plicae and villi • ileum 12 ft – Peyer’s patches – Terminates at colon ileocecal valve Intestinal characteristics • increased surface area (2200 sq ft) – plicae – villi – microvilli • brush border • lacteals 8 Intestinal secretions • 1.8 liters of intestinal juice • moistens chyme • buffers gastric acid • intestinal juice secreted by – osmosis from mucosa – secretion by intestinal glands • Stimulated by touch and stretch receptors Intestinal peristalsis 1 - Weak peristalsis toward jejunum; continuous – Via local myenteric reflexes and pacesetter cells – pacesetter cells, p.317 or 328, spontaneously depolarize… – Gap junctions allow spread of signal analward…“law of the gut” 2 - Strong peristalsis, result of • parasympathetic stimulation • reflexes stimulated by gastric stretch – Gastroenteric reflex stimulates motility along small intestine – Gastroilieal reflex relaxes ileocecal valve – hormones enhance or suppress these reflexes 9 Pancreas • organization – islets – acini (s. is acinus)- acinar cells secrete enzymes – ducts - cuboidal epithelium secretes water & ions • pancreatic juice – alkaline mixture of above secretions – secreted into small intestine pancreatic physiology • pancreas: controlled by – duodenal hormones secretin and CCK – vagus nerve • secretin – secreted as chyme enters duodenum – targets duct cells – stimulates secretion of a watery buffer solution containing • bicarbonate & phosphate ions (buffers) • CCK – secreted as chyme enters duodenum – targets acinar cells – stimulates secretion of pancreatic enzymes 10 pancreatic enzymes • alpha-amylase • pancreatic lipase • nuclease • proteolytic enzymes – proteases & peptidases – trypsinogen, chymotrypsinogen, procarboxypeptidase, proelastase – trypsinogen activated by enteropeptidase, trypsin activates the others… Liver Functions; ~ 200 functions • metabolic regulation – carbohydrate/lipid/amino acid metabolism – waste removal – vitamin storage – mineral storage – drug inactivation 11 more liver functions: hematological regulation • phagocytosis and antigen presentation • plasma protein production • removal of hormones, worn RBCs • vitamin D synthesis • removal of antibodies • removal/storage of toxins • synthesis of bile • secretion of bile – Emulsification Biliary ducts 12 Gallbladder • stores/modifies bile • bile ejection stimulated by CCK – stimulates gall bladder contractions, ejects bile into duct – relaxes hepatopancreatic sphincter (sphincter of Oddi) • Bile used for lipid breakup, more on this to follow Emulsification Mechanical break- up of lipid droplets Bile acid 13 Regulation of gastric activity • controlled by NS • Regulated by – short reflexes of ENS – GI tract hormones • Three phases – Cephalic – Gastric – Intestinal Regulation of gastric activity 14 1 Cephalic phase • Initiated by – sight, smell, taste, thought of food • Parasympathetic vagal efferent impulses stimulate stomach • Prepares stomach for food – Gastric juice secreted • Mucus • Pepsinogen • HCl • Emotions may exacerbate or inhibit the cephalic phase 2 Gastric phase: 3-4 hrs • Initiated by: Arrival of food; stretch – Distension, increase in pH , protein, peptides, caffeine, low levels of alcohol • Responses to arrival of food: – Local: stretch causes histamine release • histamine stimulates parietal cells – Neural: stretch and elevated pH • Myenteric and submucosal plexus activated – Hormonal: neural impulses and peptides stimulate • gastrin secretion by G cells and duodenum 15 2 Gastric phase functions: 3-4 hrs • Submucosal plexus – Enhances gastric juice secretion • Myenteric plexus – Increased motility • Result: homogenize & acidify chyme via motility 5 • Initiate protein digestion • Resolves as pH drops, [protein] drops…less gastrin… 3 Intestinal phase • Controls rate of gastric emptying • Initiated by: chyme arrival in duodenum • Chyme presence causes 2 stimuli – stretch – decreased pH • Response to stimuli – Duodenal mucus production • From Brunner’s glands – Enterogastric reflex • Inhibits gastrin production • Constricts pyloric sphincter 16 Intestinal phase Hormone reflexes • CCK and GIP secretion – Stimulated by presence of Lipids and carbs • Secretin secretion – If pH is less than 4.5 – Inhibits parietal & chief cells – Stimulates pancreatic buffer secretion – Stimulates hepatic bile secretion • Intestinal Gastrin secreted – Proteins present in chyme The Large Intestine • Cecum – Appendix • Colon • Rectum – Anus 17 The Large Intestine • Physiology – absorption • Water, ions, vitamins, bile salts – vitamin synthesis by coliform bacteria • K, biotin, B5 – excretion of organic wastes • Urobilin, stercobilin, • indole, skatole, ammonium ions, hydrogen sulfide from peptide breakdown…. • mass movement – stimulated by gastric/duodenal distension Defecation reflex 18 Time frame for digestion • swallowing: 4-8 sec food, 1-2 sec for liquid. • gastric digestion: ~4 hors for an average meal, up to 6 hours if high in fat. • small intestine: About 2 hrs. • large intestine: 12 to 24 hours. Digestive secretion and absorption of water 19 .