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Digestive system Dr. Carmen E. Rexach Physiology Mt San Antonio College Functions • Motility – – – – ingestion mastication deglutition peristalsis • Secretion – exocrine – endocrine • Digestion • Absorption Tunics Innervation • Autonomic nervous system • Parasympathetic (vagus nerve) – motility – secretion • Sympathetic – decreases peristalsis – secretion – contraction of sphincters Esophagus • Collapsible tube • first 1/3 – Skeletal muscle • last 2/3 – Smooth muscle • gastroesophageal sphincter Stomach • Function – food storage – initiates protein digestion – kills bacteria – moves food to SI – intrinsic factor – Digestion and absorption • proteins • alcohol/aspirin Gastric Glands • Goblet cells – mucus • Parietal cells – HCl • Chief cells – pepsinogen • Argentaffin cells – Seratonin (correct) • G cells – gastrin pH • 3 functions of pH in stomach – denature ingested protein – convert pepsinogen to pepsin – destroy bacteria Protective mechanisms • Impermeability of gastric mucosa to CO2 and NH3 • alkaline mucus • tight junctions • rapid cell division and replacement • protective effects of prostaglandins • In doudenum – bicarbonate from pancreatic juice – Brunner’s glands • Secrete mucinous alkaline solution Peptic ulcers – Gastric ulcers • Weakened defense mechanisms – Duodenal ulcers • Increased acid and pepsin production – Helicobacter pylori • Major factor in both types • Present in 100% of gastric ulcer patients and 95% of duodenal ulcer patients – Exceptions: Those whose ulcers are the result of overuse of NSAIDS = inhibits prostaglandin production • Produces NH3 and urease – damages epithelium and allows H+ to permeate • Also linked to increased acid secretion Small intestines • Length • 3 divisions – duodenum – jejunum – ileum • absorption – doudenum + jejunum – ileum • system of folds – microvilli, villi, plicae circularis Villi • Columnar epithelium • goblet cells • crypts of Lieberkuhn villus microvilli capillaries – Glands that secrete intestinal juice at base of the villi • brush border – surface area – enzymes – enterokinase nerve arteriole Lymph duct lacteal vein Muscularis artery mucosa Intestinal motility • Major types of contractions – peristalsis – segmentation • Pacesetter potential – smooth muscle cells – can lead to a/p – parasympathetic and sympathetic influences • Relaxation – NO Large Intestine • Structure • Haustra • Fluid and electrolyte absorption – 90% in SI – passive osmosis – water secretion • Defecation Liver Liver lobules: functional unit Functions of liver • • • • • • • Exocrine Endocrine Clotting functions Plasma proteins Organic metabolism Cholesterol metabolism Excretory and degradative functions Exocrine & Endocrine • Synthesis and secretion of bile salts – 250-1500ml/day • Adds bicarbonate rich solution to bile • Secretes IGF-1 = promotes cell division • Forms T3 from T4 • Secretes Angiotensinogen • Metabolizes hormones • Secretes immune cytokines Clotting and Plasma proteins • Produces prothrombin & fibrinogen • Produces bile salts needed for vitamin K absorption • Produces plasma albumin, acute phase proteins, binding proteins, and lipoproteins Organic metabolism • Converts plasma glucose to glycogen and triglycerides • Converts amino acids to fatty acids • Produces triglycerides and secretes them as lipoproteins • Gluconeogeneisis and glycogenolysis • Converts fatty acids into ketones • Produces urea Cholesterol metabolism/excretory & degradative functions • Synthesizes cholesterol • Secretes plasma cholesterol into bile • Converts plasma cholesterol into bile salts • Secretes bile pigments • Excretes toxins via bile • Destroys old erythrocytes • …and lots, lots more! Gallbladder & pancreas Hepatic ducts Common bile duct gallbladder Pancreas Pancreatic duct Duodenal papillae Doudenum Pancreas • Endocrine = islets of Langerhans – insulin and glucagon, etc. • Exocrine = pancreatic acini – pancreatic juice – composition • water • bicarb • digestive enzymes – role of enterokinase Neural and endocrine regulation • Long reflex – preconditioning • Short reflex – more local • GI hormones • Extrinsic controls – 3 phases (cephalic, gastric, intestinal) • some events without neural or endocrine control GI hormones • • • • Gastrin Secretin CCK glucose-dependent insolinotropic peptide (GIP) • Glucagon-like peptide 1 (GLP-1) • Guanylin Gastrin • endocrine cell location – antrum of stomach • stimulus for release – amino acids, peptides in stomach parasympathetic nervous system • actions – stimulates secretion of HCl and pepsinogen Secretin • Endocrine cell location – small intestine • stimulus for release – acid in small intestine • action – stimulate pancreatic bicarbonate secretion – potentiate CCK-stimulated pancreatic enzyme secretion Cholecystokinin (CCK) • Endocrine cell location – small intestine • Stimulus for release – amino acids, fatty acids in small intestine • Action – contraction and emptying of the gall bladder – stimulates release of pancreatic enzymes – (in the brain: acts as satiety hormone) Glucose-dependent insulinotropic peptide (GIP) • Endocrine cell location – small intestine • Stimulus for release – glucose, fat in the small intestine • Action – inhibits gastric emptying – potentiates insulin release Glucagon-like peptide 1 (GLP-1) • Endocrine cell location – ileum and colon • Stimulus for release – oral ingestion of nutrients – glucose, fats, amino acids (mixed meals) • Action – inhibits gastric motility – stimulates insulin secretion Guanylin • Endocrine gland location – ileum and colon • Action – stimulates intestinal Cl- release – result: increased NaCl and water in feces Cephalic phase • First 30 minutes – Smell, sight, and taste of food stimulates vagus nuclei of brain (long reflex) • vagus nerve stimulates: – parietal and chief cells – Gastrin secretion by G cells • Also stimulates parietal and chief cells Gastric phase • Begins when food arrives in stomach • Stimulus – distension – chemical composition of chyme • short polypeptides and amino acids in stomach – Positive feedback, cells secrete gastrin – HCl and pepsinogen are released • Glucose – No effect • Lipids – Inhibition of gastrin secretion – pH<2.5 negative feedback inhibition of gastrin Intestinal phase • Begins when chyme enters small intestines • neural inhibition of gastric emptying and acid secretion due to: – increased osmolality – stretching • enterogastrone – stimulus: fat in chyme – inhibits gastric acid secretion • GIP = insulin secretion if glucose is present • CCK = inhibits gastric emptying when chyme in duodenum Intestinal reflexes • Gastroileal reflex – increased gastric activity = increased motility of ileum = increased chyme through sphincter • ileogastric reflex – ileal distension = decreased gastric motility • intestino-intestinal reflex – one segment overdistends = rest relax Regulation pancreatic juice & bile • Neural & hormonal • Pancreatic juice – secretin • pH <4.5 in duodenum • stimulates bicarbonate production by pancreas • cAMP as a second messenger – CCK • fat content in chyme • production of pancreatic enzymes • Ca++ as second messenger • Secretion of bile – continuous secretion – gallbladder contraction under influence of CCK and secretin Digestion and absorption of carbohydrates • Mouth – salivary amylase • Stomach – enzymes denatured • Small intestines – pancreatic amylase = maltose, maltriose – brush border enzymes = monosaccharides – secondary active transport with Na+ – Into capillaries in villus Mouth: Salivary amylase Digestion of Carbohydrates Stomach: Enzymes denatured Duodenum: Pancreatic amylase Brush border enzymes Absorption of carbohydrates Digestion and absorption of proteins • Mouth – nothing • Stomach – Pepsin = short chain polypeptides • Small Intestines – – – – – trypsin, chymotrypsin, elastase carboxypeptidase aminopeptidase = from brushborder across into blood Whole proteins • babies • botulinum toxin Free amino acids Dipeptides tripeptides Digestion of proteins Stomach: Pepsin and HCl Small intestines: Trypsin, chymotrypsin Carboxypeptidase Brush border: aminopeptidases Protein absorption aa Na+ -Amino acids move into enterocytes by countertransport, in exchange for two Na+. -They are absorbed into a capillary bed on the basal side of the cell and taken to the liver via the hepatic portal system. Digestion and absorption of lipids • • • • Emulsification pancreatic lipase Mixed micelles to brushborder transport – across into epithelial cells – can be moved inside cell – chylomicron into lacteal Digestion of Fat Small intestines: Pancreatic lipase lacteal Lipid transport • From lymphatics to thoracic duct • Triglycerides removed by lipoprotein lipase on endothelial membranes • Free fatty acids and glycerol into tissues • Leftovers to liver – – – – Remnant particles contain cholesterol Combined with apoproteins to produce VLDL’s Deliver triglycerides to other organs Later converted to LDL’s when triglyceride has been removed Lipoproteins • What are they? – Lipid & protein complexes – Transport cholesterol & triglycerides in blood – Protein allows hydrophobic lipids to remain in suspension • Five classes: Based on density, molecular weight, size, chemical composition • Chylomicrons • VLDL • IDL • LDL – High levels associated with increased risk CVD • HDL – Low levels associated with increased risk of CVD – Best profile = high HDL, low LDL Treatment for morbid obesity Lapband Roux-en-Y Gastric Bypass