Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Secretory Functions of the Alimentary Tract Physiology III, Tri 4 Guyton & Hall, Chapt. 64 I. General principles of alimentary tract secretion A. anatomical types of glands 1. mucous cells = goblet cells a. single-cell mucous glands b. surface epithelium c. respond to epithelial stimulation d. extrude mucus for lubrication to protect surfaces 2. pits - invaginations of the epithelium a. crypts of Lieberkühn - small intestine b. deep c. contain specialized secretory cells 3. tubular glands - stomach and upper duodenum 4. complex glands: a. compound acinous glands 1. salivary glands 2. pancreas b. liver c. provide secretions for digestion or emulsification of food d. lie outside the walls of alimentary tract e. contain millions of acini lined with secreting glandular cells f. acini feed into system of ducts that empty into alimentary tract B. stimulation of alimentary tract glands 1. mechanical presence of food 2. enteric nervous system a. tactile stimulation b. chemical irritation c. distention of gut wall 3. nervous reflexes stimulate both mucous cells on the epithelial surface and deeper glands in the mucosa C. ANS stimulation of secretion 1. parasympathetic stimulation a. increase rate of secretion b. upper portion of tract 1. salivary glands 2. esophageal glands 3. gastric glands 4. pancreas 5. Brunner's glands in the duodenum c. innervated by vagus & other cranial parasympathetic nerves d. distal portion of large intestine e. innervated by pelvic parasympathetic nerves f. (secretion in the remainder of the small intestine and in the 1st 2/3 of the large intestine occurs mainly in response to local neural and hormonal stimuli) 2. sympathetic stimulation a. slight to moderate increase in secretion in some parts b. constriction of blood vessels - leads to reduced secretion from glands D. regulation of glandular secretion by hormones 1. stomach and intestines 2. from gastrointestinal mucosa 3. stimulated by the presence of food in gut lumen 4. carried by blood to glands 5. increases output of gastric juice and pancreatic juice 6. hormonal stimulation causes constriction of gallbladder to empty bile 7. gastrointestinal hormones are polypeptides or polypeptide derivatives E. basic mechanism of secretion by glandular cells 1. all are not known 2. nutrient material needed for formation of the secretion must diffuse or be actively transported from the capillary into the base of the glandular cell 3. many mitochondria located inside the cell near its base use oxidative energy for formation of adenosine triphosphate (ATP) 4. energy from the ATP, along with appropriate substrates provided by the nutrients, is them used for synthesis of the organic substances; this synthesis occurs almost entirely in the endoplasmic reticulum and Golgi complex. The ribosomes adherent to the reticulum are specifically responsible for formation of the proteins that are to be secreted. 5. secretory materials are transported through the tubules of the endoplasmic reticulum, passing in about 20 min. all the way to the vesicles of the Golgi complex, which lies near the secretory ends of the cells 6. in the Golgi complex, materials are modified, added to, concentrated, and discharged into the cytoplasm in the form of secretory vesicles, which are stored in the apical ends of the secretory cells 7. vesicles remain stored until nervous or hormonal control signals cause the cells to extrude the vesicular contents through the cell's surface exocytosis F. water and electrolyte secretion 1. nerve stimulation has a specific effect on the basal portion of the cell membrane to cause active transport of chloride ions to the interior 2. resulting increase in electronegativity induced inside the cell by excess chloride ions then causes positive ions also to move to the interior of the cell 3. excess of both negative and positive ions inside the cell creates an osmotic force that pulls water to the interior - increasing the hydrostatic pressure inside the cell and causing the cell itself to swell 4. pressure in the cell results in minute ruptures of the secretory border of the cell and causes flushing of the water, electrolytes, and organic materials out of the secretory end of the glandular cell and into the lumen of the gland G. lubricating & protective properties of mucus and its importance in the GI tract 1. mucus a. thick secretion composed mainly of water, electrolytes, and glycoproteins b. adherent qualities c. prevents contact of food particles with the mucosa d. low resistance for slippage - particles slide e. causes adherence to fecal matter f. strongly resistant to digestion by the GI enzymes g. amphoteric properties - pH buffer h. contain bicarbonate ions - neutralizes acids 2. mucus allows easy slippage of food along GI tract and prevents excoriative or chemical damage to epithelium II. Secretion of Saliva A. principal glands (tubuloalveolar) 1. parotid secretory end-pieces serous cells 2. submandibular 3. sublingual intercalated duct 4. buccal striated duct 5. lingual glands excretory duct 6. other glands B. daily rate of secretion = 1000 - 1500 ml C. types of saliva secretion 1. serous a. ptyalin 1. an -amylase mucous cells 2. salivary amylase b. for digestion of starches c. produced by parotid, submandibular and sublingual glands 2. mucous a. mucin (glycoprotein) b. for lubrication c. produced by buccal, submandibular and sublingual glands Mucous Cells 3. muramidase a. lysozyme b. acts on muramic acid of the cell wall c. bacteria can not maintain osmotic pressure Serous Cells ** 4. lingual lipase a. fat digesting enzyme b. digest milk fat c. important in infants Na HCO3ClK+ 5. lactoferrin a. protein b. binds with iron 6. epidermal growth factors 7. immunoglobulin - IgA 8. ABO blood factors (in persons who are secretors) D. pH of saliva = 6-7.4 (excellent for ptyalin activity) E. ions in saliva: 1. potassium (30 mEq/L, 7X plasma) 2. bicarbonate ion (50 - 70 mEq/L, 2-3X plasma) 3. sodium & chloride (15 mEq/L, 1/7 - 1/10 of plasma) 4. concentration of ions change with maximal salivation and aldosterone secretion F. stages of salivary secretion: 1. stage 1 - from acini a. secrete primary secretion b. contains ptyalin and/or mucin c. ions - similar to extracellular fluid 2. stage 2 - from salivary ducts (amylase) a. Na+ actively reabsorbed from all salivary ducts b. K+ actively exchanged for sodium c. chloride passively reabsorbed d. bicarbonate ion secreted by ductal epithelium End Piece ClHCO3Na+ K+ 3. oral hygiene a. flow of saliva helps to wash away bacteria and food b. contains antibacterial agents: 1. thiocyanate ions 2. proteolytic enzymes - ex. lysozyme 3. protein antibodies G. nervous regulation of salivary secretion 1. salivary glands controlled mainly by parasympathetic nervous signals from the superior and inferior salivatory nuclei of brain stem a. at juncture of medulla and pons b. excited by taste and tactile stimuli c. excitation from tongue, mouth and pharynx d. "sour taste" = "sapid taste" & smooth objects copious secretion of saliva e. rough objects reduce/inhibit salivation 2. salivatory nuclei and appetite area of anterior hypothalamus effect salivation 3. reflexes from stomach and upper intestines a. irritating foods b. nausea c. GI abnormality 4. sympathetic stimulation BF saliva a. less powerful than parasympathetic b. originates from the superior cervical ganglia c. travels along blood vessels to salivary glands 5. blood supply to glands a. acts as secondary factor b. secretion requires nutrition c. parasympathetic nerve signals dilate blood vessels d. saliva dilates blood vessels e. kallikrein 1. dilates vessels 2. splits a blood protein to bradykinin (vasodilator) H. functions of saliva: 1. chewing 2. swallowing 3. speech 4. dissolves components of food 5. enhances taste 6. cleans oral tissue (brushing teeth is still necessary) III. Esophageal Secretion A. mucoid B. provide lubrication for swallowing C. simple mucous glands in body of esophagus D. compound mucous glands in the gastric end and initial portion of esophagus 1. prevents mucosal excoriation (initial segment) 2. protect esophageal wall from digestion by gastric juices ( distal segment) E. initiates starch digestion F. aids in excretion of heavy metals 1. iron 2. lead 3. others IV. Gastric Secretion A. mucous-secreting cells 1. line entire surface of stomach between glands 2. secret viscid mucus - mainly insoluble 3. coats mucosa with gel layer of mucus 4. protection for stomach - lubrication for food 5. alkaline pH 6. stimulus: a. food b. irritation of mucosa Cells within the Gastric Glands and their Secretory Products Gland Area Cell Secretory Products Cardiac Mucous Mucus, HCO3-, pepsinogens (group II) Oxyntic Pyloric Endocrine Parietal (oxyntic) Chief Mucous neck Enterochromaffin Endocrine Mucous G cell Enterochromaffin Other endocrine HCL, intrinsic factor Pepsinogens (groups I and II) Mucus, HCO3 -, pepsinogens (group I and II) Serotonin Mucus, HCO3-, pepsinogens (group II) Gastrin Serotonin B. tubular glands 1. oxyntic (gastric) glands - body and fundus of stomach (80% of stomach) a. acid forming b. secrete HCl, pepsinogen, intrinsic factor and mucous c. cell types: 1. mucous neck cells a. mucus b. pepsinogen 2. peptic (chief) cells a. pepsinogen 1. several different types 2. activated by HCl 3. forms pepsin a. active proteolytic enzyme b, active in high acid environment c. pH 1.8 -3.5 optimum d. > pH 5, is inactive e. necessary for protein digestion Gastric Pit Surface Mucous Cells Oxyntic Cell Isthmus Mucous Neck Cells Neck Endocrine Cell Base Chief Zymogenic Cells 3. parietal (oxyntic) cells a. HCL 1. pH = 0.8 2. formed in membranes of canaliculi 3. fig. 64-6 b. intrinsic factor - B12 d. other enzymes: 1. gastric lipase a. = tributyrase b. acts on butterfat 2. gastric amylase a. digestion of starches b. minor role 3. gelatinase - liquefy proteoglycans in meats 4. intrinsic factor a. essential for vit. B12 absorption in ileum b. deficiency = pernicious anemia 2. pyloric glands - antral portion of stomach a. mucous (lubrication) b. pepsinogen c. gastrin - controls gastric secretion d. cell types: 1. structurally similar to oxyntic glands 2. few peptic cells 3. no parietal cells 4. mostly mucous cells C. regulation of gastric secretion by nervous and hormonal mechanisms 1. acetylcholine a. bind with receptor on secretory cells b. excites all secretory cells in gastric glands c. released at parasympathetic nerve endings with vagi stimulation 2. gastrin a. bind with receptor on secretory cells b. stimulates secretion of acid by parietal cell c. little effect on other cells d. secreted by gastrin cells (G cells) in pyloric glands e. large peptide 1. 6 different molecules 2. 4 AA at end are active site f. secreted in 2 forms: 1. G-34 a. large form b. contains 34 amino acids 2. G-17 a. smaller form b. contains 17 amino acids c. most abundant form g. absorbed into the blood h. carried to oxyntic glands in body of stomach i. stimulates parietal cells (strongly) and peptic cells (weakly) j. increases HCl secretion by parietal cells k. stimulated by vagi l. may HCO3- release from liver and pancreas m. gastric motility n. pressure of LES o. open ileocecal valve 3. histamine a. bind with receptor on secretory cells b. stimulates secretion of acid by parietal cell c. little effect on other cells d. amino acid derivative e. formed continually in gastric mucosa f. in the presence of acetylcholine will enhance acid secretion g. necessary cofactor for exciting significant acid secretion h. type H2 receptors on parietal cells 4. other substances that stimulate gastric secretions: a. amino acids b. caffeine c. alcohol d. food in stomach - causing distention Neurocrine Paracrine Endocrine Histamine H2 ACH Gastrin Atropine Ca++ Cimetidine Proglumide ATP camp pK protein ADP protein H 5. nervous stimulation: a. ½ of signals originate in the dorsal motor nuclei of vagi 1. pass by way of vagus nerves 2. to enteric nervous system 3. then to gastric glands 4. release ACH b. ½ of signals are generated by local reflexes 1. occur within the wall of stomach 2. release ACH c. in pyloric glands 1. intermediate neuron serves as final path 2. secretes gastrin-releasing peptide as neuro-transmitter 3. probably the peptide bombesin 6. signals originate in a. brain b. stomach 1. long vagovagal reflexes a. from stomach mucosa to brain stem b. back to stomach through vagus nerves 2. short reflexes a. originate locally b. transmitted through local enteric nervous system 7. stimuli: a. distention of stomach b. tactile stimuli on surface of stomach mucosa c. chemical stimuli 1. amino acids 2. peptides 3. acid d. when acetylcholine, gastrin, and histamine are all present, copious amounts of acid are secreted D. regulation of pepsinogen secretion 1. stimuli: a. stimulation of peptic cells by ACH from vagus nerves & enteric nerves b. stimulation of peptic secretion by acid in stomach (indirectly) c. acid pepsinogen secretion 2. when pH falls below 3.0 , stimulation for gastric secretion becomes blocked pepsinogen secretion 3. pH 3.0 = optimal pH for peptic enzymes E. phases of gastric secretion (phases fuse together) 1. cephalic phase a. occurs before food enters the stomach b. results from: 1. sight 2. smell 3. thought 4. taste c. appetite stimulation d. neurogenic signals originate in cerebral cortex or appetite centers of amygdala or hypothalamus e. transmitted through the dorsal motor nuclei of vagi to stomach f. accounts for about 20% of gastric secretion 2. gastric phase a. food in stomach excites: 1. vagovagal reflexes 2. local enteric reflexes 3. gastrin mechanism b. accounts for 70% of gastric secretion 3. intestinal phase a. food in small intestine stimulates stomach to secret gastric juice b. duodenal mucosa secretes gastrin in response to distention or chemical stimuli c. amino acids and other hormones reflexes play minor roles in secretion of gastric juice Stimulation of Gastric Acid Secretion Phase Stimulus Pathway Cephalic Chewing, Swallowing vagus n. to: 1. Parietal cells 2. G cells Gastric Gastric Intestinal Protein digestion products in Stimulus to parietal cell Local and vagovagal reflexes to: 1. Parietal cells 2. G cells ACH Gastrin ACH Gastrin 1. Intestinal G Gastrin cells 2. Intestinal enterooxyntin F. inhibition of gastric secretion by intestinal factors 1. chyme stimulates gastric secretion during intestinal phase of secretion and inhibits during gastric phase a. presence of food in small intestine initiates an enterogastric reflex b. reflex inhibits stomach secretion that is initiated by: 1. distention of small bowel 2. presence of acid in upper intestine 3. presence of protein breakdown products 4. irritation of mucosa 2. presence of acid, fat, protein breakdown products, hyperosmotic or hypo-osmotic fluids, or irritating factor in upper small intestine causes release of several hormones: a. secretin - opposes stomach secretion b. gastric inhibitory peptide c. vasoactive intestinal polypeptide d. somatostatin 3. inhibition of gastric secretion by intestinal factors slows the release of chyme from the stomach when small intestine is already filled 4. during interdigestive period small amount of nonoxyntic gastric juice is secreted a. mainly mucus b. contains little pepsin c. almost no acid d. increases with emotional stimuli G. chemical composition 1. gastrin a. polypeptide b. terminal five amino acids same as cholecystokinin c. activity resides in terminal 4 amino acids 2. cholecystokinin a. polypeptide b. terminal five amino acids same as gastrin c. activity resides in terminal 8 amino acids 3. secretin a. polypeptide c. all the amino acids in secretin are essential 4. enzymes -secreted in inactive or zymogenetic form a. pepsin 1. proteolytic enzyme 2. pepsinogen (zymogenetic) major enzyme 3. pepsinogen HCl pepsin 4. found in peptic and mucous cells of the gastric glands 5. pepsinogen is activated by HCl 6. pH < 3.0 is needed for activation of pepsinogen b. gelatinase - helps to liquefy c. gastric lipase 1. tributyrase (AKA) 2. acts on tributyrin (=butterfat) 3. does not breakdown triglycerides d. gastric amylase - has minor role in digestion of starch e. renin 1. chymosin (AKA) 2. not the same as in the kidney 3. not part of the human system 4. originally thought to be in the human GI tract 5. casein (a protein) paracasein 6. HCl produces curdling of mile in humans in order to prolong time of in the stomach for protein digestion Mechanisms for Inhibition of Gastric Acid Secretion Region Stimulus Mediator Inhibit Gastrin Release Antrum Acid (pH<3.0) None, direct + Duodenum Acid Secretin Bulbogastrone Nervous reflex + + Duodenum Hyperosmotic Jejunum solutions Unidentified enterogastrone Fatty acids, Gastric + inhibitory peptide Monoglycerides Cholecystokinin + Unidentified enterogastrone Inhibit Acid Secretion + + + + + + + V. Pancreatic secretion A. pancreas 1. weighs about 100 gms 2. "sweet breads" = delicacy 3. secretes 1200 - 1500 ml/day of clear, odorless fluid 4. 7.7 - 8.2 pH, alkaline for the neutralization of acid form the GI tract 5. function: a. endocrine 1. insulin 2. glucagon 3. somatostatin b. exocrine - GI secretions 6. watery HCO- solution a. CO2 is taken up by pancreas and mixed with H2O b. Na and H move counter c. Na is added to bicarb solution B. pancreatic enzymes - secreted by acini 1. proteolytic enzymes: a. synthesized in the inactive forms trypsinogen, chymotrypsinogen and procarboxypolypeptidase b. become active in the intestinal tract c. trypsinogen 1. endopeptidase activated by enterokinase (secreted by intestinal mucosa) and trypsin to form trypsin Trypsinogen Enterokinase Proelastase Chymotrypsinogen Trypsin Elastase Procarboxypeptidase d. proelastase (endopeptidase) e. chymotrypsinogen 1. endopeptidase Chymotrypsin Carboxypeptidase 2. is activated by trypsin to from chymotrypsin f. procarboxypolypeptidase 1. A&B forms 2. is activated by trypsin to form carboxypolypeptidase 3. exopeptidase g. trypsin 1. most abundant 2. split whole and partially digested proteins into peptides but does not cause release of individual amino acids g. chymotrypsin 1. split whole and partially digested proteins into peptides but do not cause release of individual amino acids h. carboxypolypeptidase 1. split some peptides into individual amino acids i. elastases j. nucleases k. trypsin inhibitor 1. stored in cytoplasm of glandular cells 2. prevents activation of trypsin 3. prevents digestion of pancreas (autodigestion) 4. w/o trypsin inhibitor, pancreatic secretions digest the entire pancreas within a few hours = acute pancreatitis a. often lethal b. shock c. not lethal - pancreatic insufficiency l. ribonucleases & deoxyribonucleases 1. breaks down RNA & DNA 2. prevents ingestion & absorption of DNA & RNA supplements 2. pancreatic amylase a. digests carbohydrates b. hydrolyzes 1. starches 2. glycogen 3. other carbohydrates 4. not cellulose c. forms disaccharides and few trisaccharides 3. pancreatic fat digestion a. pancreatic lipase 1. main enzyme for fat digestion 2. hydrolyzes fat into fatty acids and monoglycerides 3. needs bile and calcium ions 4. main enzyme that attacks fat b. cholesterol esterase 1. main enzyme for fat digestion 2. hydrolyzes cholesterol esters c. phospholipase a. main enzyme for fat digestion b. splits fatty acids from phospholipids **** c. only non-proteolytic enzyme that requires activation by trypsin C. secretion of bicarbonate ions 1. secreted by epithelial cells of small ductules and larger ducts leading from the acini 2. mechanism: a. CO2 diffuses to interior cell from blood b. CO2 combines with water to form carbonic acid ( in the presence of carbonic anhydrase) c. carbonic acid dissociate into bicarbonate ion and hydrogen ions d. bicarbonate ions are actively transported into lumen of duct e. hydrogen ions are exchanged with sodium ions in the blood f. sodium ions are transported into pancreatic duct - providing electrical neutrality for secreted bicarbonate ions g. movement of sodium and bicarbonate ions from blood to lumen creates osmotic gradient h. water moves into pancreatic duct to create isosmotic bicarbonate solution D. regulation of pancreatic secretion 1. stimuli: a. diet - change in diet may effect secretions b. acetylcholine 1. released from parasympathetic vagus nerve endings 2. released from other cholinergic nerves in enteric nervous system 3. stimulate acinar cells more than ductal cells c. cholecystokinin 1. released by duodenal and upper jejunal mucosa 2. stimulated by the presence of food 3. stimulate acinar cells more than ductal cells d. secretin 1. released by duodenal and upper jejunal mucosa 2. stimulated by highly acid food in small intestine 3. stimulates large quantities of sodium bicarbonate by ductal epithelium 4. almost no stimulation of enzyme secretion 2. multiplicative effects - the effects of stimuli occurring simultaneously is multiplied vs. the stimuli occurring alone E. phases of pancreatic secretion - same as gastric secretion 1. cephalic a. vagus nerve b. smell, sight, thought are stimuli c. same nervous signals that cause secretion in the stomach d. acetylcholine released by vagal nerve endings e. produce 20% of total pancreatic secretion 2. gastric a. same nervous (vagus) signals that cause secretion in the stomach b. acetylcholine released by vagal nerve endings c. produce 5-10% of total pancreatic secretion 3. intestinal a. main control b. stimulated by chyme entering intestine ** c. secretin (important one) 1. stimulates copious amounts of secretion 2. polypeptide with 27 a.a., mol. wt. = 3400 3. present in S cells of mucosa of upper small intestine 4. prosecretin = inactive form 5. chyme of < pH 4.5 entering the duodenum stimulates release and activation of secretin 6. HCl, major stimulator of secretin release 7. secretin enters blood and travels to pancreas 8. causes pancrease to secrete bicarbonate ion in water and sodium fluid with few enzymes 9. as more acid enters the duodenum, more bicarb is released 10. provides neutral or slightly alkaline environment for optimum pancreatic enzyme activity d. cholecystokinin stimulates enzyme secretion 1. polypeptide with 33 amino acids 2. stimulated by the presence of food in the upper small intestine a. proteoses and peptones - protein breakdown products b. long-chain fatty acids c. HCl 3. released from I cells in the mucosa of the duodenum and upper jejunum 4. travels via the blood to pancreas 5. stimulates secretion of large quantities of digestive enzymes by the acinar cells 6. accounts for 70 - 80% of pancreatic enzyme secretion after a meal VI. Secretion of Bile by the Liver A. bile Bile Salts = steroid acids precursor = cholesterol cholic acid chenodeoxycholic acid glycine or taurine glycocholic acid taurocholic acid 1. secreted @ of 600-1200 ml/day 2. contains: a. no digestive enzymes b. water c. bile salts d. bilirubin e. cholesterol f. fatty acids g. lecithin h. sodium i. potassium j. calcium k. chloride l. bicarbonate ion 3. components of bile become more concentrated with secretion 4. starts to empty about 30 minutes after a meal 5. purpose: a. production of micelles for aiding absorption b. fat digestion 1. emulsification of large fat particles - increase surface area 2. transport and absorption of digested fat end products via micelles to and through intestinal mucosal membrane c. excretion of blood waste products 1. bilirubin - end-product of hemoglobin destruction 2. cholesterol B. stages of bile secretion 1. initial portion a. secreted by liver hepatocytes b. contains large amounts of bile acids, cholesterol, and organic constituents c. secreted into the minute bile canaliculi 2. second stage a. bile flows peripherally toward the interlobular septa b. canaliculi empty into terminal bile ducts and them larger ducts c. bile flows to hepatic duct and common bile duct d. bile empties into duodenum or is diverted to gallbladder via cystic duct e. as bile flows through ducts second portion is added 1. watery 2. sodium and bicarbonate ions secreted by secretory epithelial cells lining ductules 3. increases bile by 100% 4. stimulated by secretin C. storage and concentration of bile 1. continual secretion of bile is stored in the gallbladder until needed in the duodenum 2. maximum volume of gallbladder is 30-60 ml 3. up to 12 hours of bile secretion can be stored in the gallbladder 4. gallbladder concentrates bile that is stored 5. bile constitutes: a. bile salts - accounts for half of total solutes b. bilirubin c. cholesterol d. lecithin e. electrolytes of plasma 6. in the concentrating process of the gallbladder, large portions of electrolytes (except calcium ions) are reabsorbed by the gallbladder mucosa D. emptying of the gallbladder 1. stimulated by food in upper GI tract 2. rhythmical contractions occur in the wall of the gallbladder 3. sphincter of Oddi relaxes a. cholecystokinin causes relaxation b. peristaltic waves from gallbladder c. relaxation phase of intestinal peristaltic waves 4. cholecystokinin - most potent stimulus of gallbladder contractions 5. stimulated by ACH-secreting nerve fibers from vagi and enteric nervous system 6. in the presence of fat, the gallbladder will empty in about 1 hour 7. 90% of all bile is reabsorbed in distal ilium E. bile salts 1. 0.6 grams of bile salts formed daily 2. cholesterol, from diet or formed by the liver, is precursor of bile salts 3. actions of bile salts: a. detergent action on fat particles 1. decreases surface tension 2. allows for agitation to break fat globules 3. emulsifying or detergent function b. help with absorption 1. fatty acids 2. monoglycerides 3. cholesterol 4. other lipids 5. form micelles F. secretion of cholesterol; most common gallstone formation 1. bile salts are formed in the hepatic cells from cholesterol 2. 1-2 gm/day cholesterol is secreted into the bile 3. cholesterol is insoluble in water 4. in the presence of bile salts and lecithin, cholesterol is converted into micelles, which are soluble 5. gall stones: a. precipitated cholesterol b. too much water absorption from bile c. too much absorption of bile salts and lecithin from bile d. inflammation of the epithelium of the gallbladder (ex. infection) e. increased cholesterol due to high fat diet &/or no-fat diet VII. Secretions of the Small Intestine A. Brunners glands 1. compound mucous glands 2. located in first few centimeters of duodenum 3. mainly between the pylorus and the papilla of Vater 4. secrete alkaline mucus (bicarbonate ions - neutralize acid) 5. stimulated by: a. tactile stimuli or irritation of mucosa b. vagal stimulation c. gastrointestinal hormones (mainly secretin) d. secretin is major stimulus for Brunner's glands 6. function - protection of the duodenal wall from digestion by gastric juice B. Crypts of Lieberkhn 1. small pits 2. located throughout small intestine 3. lie between the intestinal villi 4. covered with epithelium containing 2 cell types: a. goblet cells 1. secrete mucus 2. protect intestinal surfaces b. enterocytes 1. secrete water and electrolytes (in the crypts) 2. reabsorb water and electrolytes with the end products of digestion (over the surfaces of the villi) 5. secretions are formed by enterocytes of the crypts @ 1800 ml/day 6. secretions are almost pure extracellular fluid with pH of 6.5 - 7.5 7. secretion do not contain enzymes 8. fluid from crypts to villi supplies a watery vehicle for absorption 9. mechanism of secretion: a. active secretion of chloride into crypts b. active secretion of bicarbonate ions c. formation of electrical drag of sodium ions d. osmotic movement of water C. enzymes of the small intestine 1. final digestion occurs by membrane bound enzymes 2. peptidases 3. enzymes for splitting disaccharides into monosaccharides: a. sucrase b. maltase c. isomaltase = dextrinase d. lactase 4. intestinal lipase 5. glucoamylase - breaks maltoligosaccharides 6. tripeptidases 7. dipeptidase 8. lie in brush border 9. catalyze hydrolysis of food on the outside surface of the microvilli before absorption - absorption - provides aqueous solution for absorption 10. lifespan of intestinal epithelia cell = 5 days VIII. Secretions of the Large Intestine A. has many crypts of Lieberkhn with many goblet cells B. no villi C. almost no enzymes D. epithelial cells are mostly mucous cells that secrete only mucus E. mucus - main secretion 1. protects against excoriation 2. large amounts of bicarbonate ions 3. regulated by tactile stimulation of mucous cells and local nervous reflexes to mucous cells in crypts of Lieberkhn 4. secretion with parasympathetic (with extreme parasympathetic stimulation, ropy mucus bowel movement can occur @ of 1 every 30 min) 5. protects bowel from bacterial activity and provides adherent medium for fecal matter F. irritation (enteritis) 1. secretion of large quantities of water and electrolytes with normal alkaline mucus 2. acts to dilute irritating factors and cause rapid movement of feces toward anus 3. diarrhea - protective mechanism; loss of large quantities of water and electrolytes as irritant factor is washed away Factors Affecting Gallbladder Emptying & Bile Synthesis & Secretion Phase of Digestion Stimulus Mediating Factor Response Cephalic taste and smell of food; food in mouth and pharynx Impulses in branches of vagus nerve; Gastrin? rate of gallbladder emptying Gastric gastric distension impulses in branches of vagus nerve: Gastrin? rate of gallbladder emptying Intestinal fat digestion products in duodenum cholecystokinin acid in duodenum secretin rate of secretion of bicarbonaterich fluid by the bile duct epithelium (this effect strongly potentiated by cholecystokinin) Absorption of bile acids in the distal part of ileum High concentration of bile acids in portal blood stimulation of bile acid secretion inhibition of bile acid synthesis low rate of release of bile low concentration of bile acids in portal stimulation of bile acid synthesis; inhibition of bile acid secretion Interdigestive rate of bladder emptying; rate of bile acid secretion