THE DIGESTIVE SYSTEM

Transcription

1 THE DIGESTIVE SYSTEM I. Overview II. Esphagus and Stmach III. Small Intestine IV. Large Intestine V. Liver, Gallbladder and Pancreas VI. Regulatin f the Digestive System I. Overview The majr parts f the digestive system: Salivary glands Pharynx Esphagus Stmach Small Intestine Large Intestine Rectum Accessry digestive rgans: liver, gallbladder, pancreas The majr layers f the gastrintestinal tract: Mucsa: inner layer lines the gastrintestinal tract simple clumnar epithelilium Submucsa: bld vessels glands nerve plexuses (Meissner s plexus) Muscularis: peristalsis nerve plexus (Myenteric plexus) Sersa: Outer layer f cnnective tissue

2 Functins f the GI system Mtility: mvement thrugh the GI tract Digestin: breakdwn f fd r chime Secretin and absrptin: acrss and epithelial layer either int the GI tract (secretin) r int bld (absrptin) Strage and eliminatin: II. Esphagus and Stmach Esphagus Frm pharynx t stmach Salivary glands release mucus fr lubricatin, antimicrbial agents, and amylase t digest starch. epiglttis cvers respiratry tract during swallwing At end f esphagus is the lwer esphageal sphincter (LES) Prpulsin f fd ccurs thrugh peristalsis: cntractin ccurs behind the blus f fd and relaxatin ccurs ahead f the blus f fd. Stmach Functins: 1. stre fd 2. initiate digestin f prteins 3. kill bacteria with the strng acidity (lw ph f the gastric juice) 4. make chyme Parts f the stmach: a. Fundus b. bdy c. pylric regin (pylric sphincter) material passed frm the stmach t the small intestine is called the chyme. The gastric glands f the stmach cntain several types f cells:

3 Cell Type Parietal cells Chief cells Gblet cells Enterchrmaffinlike (ECL) cells D cells G cells Secretins HCl; intrinsic factr pepsingen mucus histamine;sertnin Smatstatin Gastrin ph f gastric juice is 2. The lw ph f gastric juice: 1. denatures ingested prteins 2. ptimum ph fr pepsin activity is at ph 2.0, weak pepsingen enzymes digest each ther t frm pepsin The stmach digests nly prteins, but nt fats and carbhydrates There is basically n absprtin in the stmach Acid secretin by parietal cell: H + transprt 1. H + is cnverted t CO 2 (bld) 2. CO 2 diffused int parietal cell 3. CO 2 is cnverted back t H + 4. H + is transprted int the GI lumen by a H + -K + -ATpase Cl - transprt 1. Cl - is transprted int the parietal by a Cl - /HCO 3 - transprter 2. Cl - diffused int the GI lumen via a Cl - channel

4 III. Small Intestine small intestine is frm the pylric sphincter t the ilececal valve 12ft in length, small in diameter cmpared t large intestine regins f the small intestine dudenum: absprtin f carbhydrates, lipids, amin acids, Ca 2+, irn jejuneum: abspritn f carbhydrates, lipids, amin acids, Ca 2+, irn ileum: absrptin f bile salts, vitamin B 12, water electrlytes. Clumnar epithelial cells Villi/ micrvilli: increases surface area fr absrptin Cre f villus bld capillaries: absrptin f mnsaccharides, amin acids lymphatic vessels (central lacteal): absrptin f fats Brush brder enzymes: dissacharidase, peptidase, phsphatase. Absrptin in the Small Intestine Calric cntent f fd is derived mainly frm: carbhydrates (50%) prteins (11-14%) lipids (36%-39%) Carbhydrates Begins as starch (plysaccharide) and then eventually digested int mnsacharides fr absrptin. Amylase: Starch digestin begins in the muth (salivary amylase), and then cntinues in the dudenum (pancreatic amylase). Amylase digestin f starch prduces maltse (disaccharide) and maltrise (trisaccharide) and lisaccharides. Brush brder enzymes: hydrlyze maltse, maltrise, and ligsaccharides, sucrse, lactse t mnsaccharides fr absrptin. The three absrbable mnsaccharides are glucse, galactse, and fructse. Transprt acrss epithelial layer 1. Lumen side: Na + ctransprter with mnsaccharides 2. Bld side: passive diffusin via a transprter

5 Prteins Stmach: Smewhat digested t shrt-chain plypeptides by pepsin Dudenum, jejunum: Digested t amin acids, di-peptides, tri-peptides by pancreatic juice enyzmes Transprt acrss epithelial layer Fats 1. Lumen side: Na + c-transprter with amin acids, di-peptides, tri-peptides 2. Bld side: passive diffusin via a transprter Absrptin f fats takes place in the dudenum and are transprted int the lymphatic system. 1. Fat drplets, mainly cmprised f triglycerides are first emulsified by bile salts (see later sectin fr discussin f bile salts). Emulsificatin makes the fat drplets smaller, making them mre easily digested enzymatically. 2. Pancreatic lipase digests the smaller, emulsified fat drplets int free fatty acids and mnglycerides. 3. The free fatty acids and mnglycerides frm micelles which migrate twards the brush brder membrane. The micelles cntain bile salts, lecithin, chlesterl and 4. The free fatty acids and mnglycerides leave the micelle and enter the epithelial cell. 5. Inside the epithelial cell the free fatty acids and mnglycerides cmbine with prtein t frm chylmicrns (lipid + prteins). 6. The chylmicrns are secreted int the lymphatic system. III. Large Intestine large intestine is frm the ilececal valve t the anus parts f the large intestine: ascending cln, transverse cln, descending cln, sigmid cln, rectum, anal canal clumnar epithelial cells, gblet cells, scattered lymphcytes, lympathic ndules cntains n villi invlved in absrptin f water, electrlytes, vitamins. Cntains bacteria which serve a number f functins absrptin f vitamins (B and K) prduce small fatty acids used as energy by GI epithelial cells help breakdwn indigestible mlecules final water cntent f feces is abut 200 ml

6 Summary f Water transprt in GI tract Small Intestine Amunt f water entering Ingestin: 1.5 liters secretins: 7-9 liters Ttal: liters Amunt reasbrbed liters Large Intestine Rectum liters liters 200 ml Diarrhea is caused by many prblems. The end result in a decrease in water absrptin, s the stls are very watery. This can lead t severe dehydratin. Chlera: Na + secretin int GI lumen Celiac Sprue: damage t GI wall Lactse intlerance: smlarity f the GI lumen Defecatin reflex: pening f the external anal sphincter due t pressure in the rectum. III. Liver, Gallbladder and Pancreas A. Liver and Gallbladder Anatmy Cnnected t gallbladder via bile duct and then t small intestine Cntains sinusidal capillaries which are permeable t mst substances Unusual vasculature: G.I. capillaries Liver vena cava. This allws filtratin f ingested substances. Enterhepatic circulatin: frm liver via bile duct t small intestine and then frm small intestine back thrugh prtal vein t liver Majr functins 1. prductin and secretin f bile 2. detxicatin f bld 3. secretin and strage f glucse 4. prductin f albumin

7 Liver clears substances via the bile duct in a similar manner t the way the kidney clears substances int the nephrn. Prductin and secretin f bile Cmpnents f bile: bile pigment r bilirubin: remves hemglbin breakdwn prducts bile salts: adds in fat absrptin phshphlipids, chlesterl, inrganic ins. Gallbladder stres bile. Bile entering gallbladder is cntrlled by the sphincter f Odii. B. Pancreas Endcrine versus excrine functin: endcrine: invlves secretin int bld (inside the bdy, end): insulin and glucagns (endcrine functin nt discussed in lecture) excrine: invlves secretin int GI system (utside the bdy, ex). Pancreatic juice cntains: water: H 2 O bicarbnate: HCO 3 - amylase: digests starch trypsin: digests prtein lipase: digests fatty acids I. Cntrl f the Digestive System Digestive system is cntrlled by: autmatic activity autnmic nerves hrmnes Innervatin f the gastrintestinal tract parasympathetic: rest and digest sympathetic: fight and flight enteric nervus system: intrinsic nervus system in GI system

8 Autnmic Branch parasympathetic sympathetic Effect n GI system mtility, pen valves mtility, clse valves Three Phases in Cntrl f Gasric Functin A. Cephalic Phase B. Gastric Phase C. Intestinal Phase A. Cephalic Phase: Regulatin by the vagus nerve: lasts apprximately 30 minutes. The vagus nerve is activated by sight, smell, taste f fd. Activatin f the vagus nerve: 1. indirectly causes the parietal cells t secrete HCl 2. directly stimulates chief cells t secreate pepsingen t digest prteins A. Gastric Phase Stimulated by 1. distensin f the stmach (i.e. amunt f chyme) 2. chemical nature f the chyme The gal f this phase is t release acid and prtelytic enzymes int the stmach. Feedback lps A psitive feedback lp ccurs in which peptides cause acid and pepsingen t be released and this in turn causes mre peptides in the stmach, which causes acid and pepsingen t be released, etc.. A negative feedback lp ccurs in which the lw ph f the stmach inhibits gastrin secretin by the G cells which results in less acid secretin. Stimulatin f HCl secretin: 1. Vagus nerve and amin acids in the stmach lumen stimulate gastrin release by G-cells 2. Gastrin stimulates histamine release by ECL cells

9 3. Histamine stimulates HCl secretin by parietal cells. Stimulus fr gastric phase peptides (particularly phenylalanine and tyrptphan) stimulate pepsingen and acid secretin glucse and fats d nt stimulate acid secretin. A. Intestinal phase Inhibitin f gastric activity due t: neural reflex: stretch f the dudenum inhibits gastric mtility and secretin hrmne: fat in the chyme stimulates an inhibitry hrmne. It is nt clear what this hrmne is. Ptential candidates include gastric inhibitry peptide, smatstatin, chlecystkinin, glucagn-like. Cntrl f Intestine Chyme in dudenum stimulates 1. gastric inhibitin 2. pancreatic secretin 3. bile secretin Pancreatic juice secretin is cntrlled by secretin and chlecystkinin (CCK). Secretin: stimulated by a drp in dudenal ph results in HCO 3 - secretin by pancreas and bile secretin Chlecystkinin: stimulated by fats and prteins in dudenum results in pancreatic secretin f enzymes and bile secretin.