Small Intestine Lecture Notes Fall 2024 PDF

1502 Small Intestine Fall 2024 Lecture 16 Monday, Sept 30: 9am Dr. L. Al-Nakkash [email protected] 1  Learning Objectives 1. Examine the structur...

1502 Small Intestine Fall 2024 Lecture 16 Monday, Sept 30: 9am Dr. L. Al-Nakkash [email protected] 1  Learning Objectives 1. Examine the structure of the SI 2. Identify the differences in SI motility. 3. Identify the effects of PNS & SNS innervation. 4. Examine the processes involved in the absorption of carbohydrate. 5. Examine the processes involved in the absorption of protein. 6. Examine the processes involved in the absorption of fat. 7. Examine the processes involved in the absorption of iron. 8. Describe the role of the Ileocecal juncture. 9. Discuss the pathophysiology: diarrhea 2 © L. Al-Nakkash 2024  Small Intestine duodenum Site where absorption occurs 3 segments Jejunum What are the roles of the small intestine: ______________ Mix contents ileum ______________ Propel chyme forward Expose contents to lumen wall _____________ © L. Al-Nakkash 2024 1. Examine the structure of the SI 3  Structure of Small Intestine Structure lends itself to function Villi Size? Villi in duodenum longest Villi in terminal Ileum shortest What about surface area? large surface area for ↑ absorption © L. Al-Nakkash 2024 1. Examine the structure of the SI 4  Segmentation – Primary means of motility during digestion of meal. – Mixes & slowly propels chyme. – oscillating contractions of circular smooth muscle layer along length of intestine. – Between contracted segments there are relaxed areas with the chyme bolus. – After brief time period: contracted areas → relax & new contraction rings occur in previously relaxed areas. Motility is not always a BIG forward movement- MIXING is a major form of motility © L. Al-Nakkash 2024 2. Identify the differences in SI motility. 5  Initiation & Control of Segmentation Initiated by SI pacesetter cells within GIT walls Duodenum segments – due to local distension (i.e., when chyme enters it coming from the stomach) Ileum segments – due to gastrin = GASTROILEAL REFLEX ________________________. & what is the major stimulus for gastrin release? Chyme in stomach Contents take 3-5 hours to move through SI. A good thing - allows maximum absorption of nutrients. Note – the rate of segmentation contractions in duodenum > ileum (proximal faster than distal) © L. Al-Nakkash 2024 2. Identify the differences in SI motility. 6  Migrating Motility Complex – Peristaltic Waves Occur: when most of meal is absorbed: segmentation contractions stop. MMC takes over. At next meal: MMC stops & segmentation starts. proximal distal MMC: – an in-between meal motility. – repetitive/peristaltic wave. – wave that lasts a short distance » Waves occur every 1.5 hours. Note – this is more of a clean-up type motility © L. Al-Nakkash 2024 2. Identify the differences in SI motility. 7  Innervation of the Small Intestine PNS – Stim → ↑ contraction of intestinal Smooth Muscle Releases: Ach motilin Secreted from upper duodenum during fasting ↑ GI motility – starts MMC’s SNS – – Stim → ↓ contraction of intestinal Smooth Muscle © L. Al-Nakkash 2024 3. Identify the effects of PNS & SNS innervation. 8  Digestion of Carbohydrates ~50% of typical US diet Starch – Salivary amylase starts digestion in mouth Inactivated in stomach by gastric secretions – Starch Pancreatic-amylase disaccharides lactose maltose sucrose lactase maltase BBE sucrase Brush Border Glucose+galactose Glucose + fructose glucose enzyme Monosaccharides are absorbed at SI BBM (brush border membrane) © L. Al-Nakkash 2024 4. Examine the processes involved in the absorption of carbohydrate 9  Carbohydrate Absorption 1 Na+ Lumen 2 cotransport carrier Na+- and energy- dependent Facilitated secondary diffusion active transport 3 3 4 Energy Facilitated diffusion Facilitated required diffusion Epithelial cell of villus Capillary © L. Al-Nakkash 2024 4. Examine the processes involved in the absorption of carbohydrate 10  Carbohydrate Absorption Salivary amylase 1 Dietary polysaccs Disacc (Starch & glycogen) Pancreatic amylase (maltose) Dietary disaccs Maltase, lactase, sucrase 2 (lactose & sucrose) Monosacc In BBM & maltose Na+-dependent Absorbed into Monosacc 3 SI epithelial cell (glucose & galactose) active transport & enter blood Facilitated 4 Monosacc(fructose) Absorbed into blood diffusion apical Sodium coupled- transport Facilitated transport Facilitated transport © L. Al-Nakkash 2024 4. Examine the processes involved in the absorption of carbohydrate 11  Brush Border Membrane Enzymes Enterokinase → what else is this known as?enteropeptidase – Activates pancreatic enzyme trypsinogen to produce? Trypsin, Chymotrypsin, Carboxypeptidase Aminopeptidases → – Complete protein digestion by hydrolyzing small peptide fragments into AA © L. Al-Nakkash 2024 5. Examine the processes involved in the absorption of protein 12  Protein Digestion & Absorption Trypsin Chymotrypsin Carboxypeptidase 13 © L. Al-Nakkash 2024 13 5. Examine the processes involved in the absorption of protein  Protein Digestion & Absorption Amino-acids transported in via Na+-amino acid co-transporters Di & tripeptides are absorbable on H+-dependent cotransporters Oligopeptides require BBE digestion Facilitated diffusion on BLM 14 © L. Al-Nakkash 2024 14 5. Examine the processes involved in the absorption of protein  Protein Digestion & Absorption Gastric pepsin AA & 1 Dietary proteins small peptides Pancreatic proteolytic enzymes Na+-dependent active transport Absorbed into 2 AA SI epithelial cells Or H+-dependent co-transporters For di- and tri-peptides Small peptide aminopeptidases 3 fragments AA in BBM 15 © L. Al-Nakkash 2024 5. Examine the processes involved in the absorption of protein  Fat Digestion & Absorption 1 Lipid emulsion Micelles 2 diffusion Lumen Lumen 3 Micelle Micelles Microvillus Fatty acids, 4 monoglycerides 4 5 Aggregate and Short or Passive absorption coated with lipoprotein 6 medium chain Basement Epithelial membrane cell of villus (Exocytosis) Central lacteal 7 Capillary © L. Al-Nakkash 2024 6. Examine the processes involved in the absorption of fat 16  Fat Digestion & Absorption 1 Dietary fat – emulsified via detergent action of bile salts → suspension of fat droplets. 2 Lipase – hydrolyzes TG → __________ MG + 2FFA These water-insoluble products – carried on inside of water-soluble 3 micelles. – formed from bile salts. 4 As micelles approach absorptive epithelial surface → MG & FFA leave micelles → passively diffuse through luminal plasma membrane. 5 MG & FFA → ___________to Re-esterified form TG in the cell. 6 TG → aggregate → coated with Apoproteins, _________ i.e. ApoB Produced by intestinal cells. → form water-soluble chylomicrons ___________ → extruded through basal membrane of SI epithelial cell – via exocytosis. © L. Al-Nakkash 2024 6. Examine the processes involved in the absorption of fat 17  Iron Absorption Fe2+ = ferrous 7 Iron not absorbed by cells Is absorbed Dietary Iron in feces 3+ Fe = ferric is not iron Absorbable Iron lost as cell is 1 iron sloughed Lumen 6 Iron absorbed into cell Ferritin – 4 pool of iron not absorbed into blood Epithelial 2 cell of villus 5 Iron absorbed into blood; Excess Plasma 3 bound to transferrin iron in blood 18 © L. Al-Nakkash 2024 7. Examine the processes involved in the absorption of iron  Iron Absorption Dietary iron → only some is in an absorbable form. 1 Dietary iron absorbed into SI epithelial cells Fe2+ = ferrous form 2 – only some of it is immediately needed for RBC production – transferred into blood In blood, absorbed iron → carried to bone marrow 3 → via transferrin (plasma protein carrier). Absorbed dietary iron not needed immediately 4 – stored in enterocytes (in crypts of Lieberkühn) by ferritin (not absorbed into blood). 5 Excess iron in blood → dumped in the ferritin pool. 6 Unused iron → lost in feces – ferritin-containing epithelial cells sloughed off and replaced. 7 Dietary iron that is not absorbed is also lost in feces. © L. Al-Nakkash 2024 7. Examine the processes involved in the absorption of iron 19  Ileocecal Juncture Ileocecal sphincter relaxes more, when food in stomach & gastrin is released. Ileocecal sphincter – Pressure on cecal side Distension on ileum side prevents bacteria–laden contents of LI moving back wards into SI. © L. Al-Nakkash 2024 8. Describe the role of the Ileocecal juncture. 20  Diarrhea X’s loss of intestinal contents: dehydration, metabolic acidosis (loss of HCO3-) HCO3-? HCO3- comes from secretions into GIT lumen AND…..we lose K+ due to ↑ K+ secretion in colon (results in hypokalemia) Causes of diarrhea: X’s intestinal motility – due to irritation of gut wall from bacterial/viral infection - due to stress. X’s intestinal secretion - due to cholera toxin Leading cause of death in infants in developing nations. Why? lack of clean drinking water What kind of treatment is standard? oral rehydration therapy © L. Al-Nakkash 2024 9. Discuss the pathophysiology: diarrhea 21  Secretion of chloride occurs in crypts Absorption of nutrients occurs in villi CFTR Cl Cl Permanently activated Eschericia Coli H2O 22 © L. Al-Nakkash 2024 9. Discuss the pathophysiology: diarrhea  PNS & SNS stimulation both increase contraction of the SI True / False The rate of duodenum & ileum segmentation is the same True / False The ileocecal sphincter contracts more with pressure on the cecal side True/False Dietary disaccharides are broken down at the BBM True/False Salivary amylase is the only enzyme to digest polysaccs True/False Single AA absorption requires Na-dependent active transport True / False Within SI epithelial cells, MG’s and FFA’s re-esterify to form a TG True / False 23 © L. Al-Nakkash 2024  PNS & SNS stimulation both increase contraction of the SI True / False PNS - ↑contraction SNS – ↓contraction The rate of duodenum & ileum segmentation is the same True / False Duodenum = 12/min Ileum = 9/min The ileocecal sphincter contracts more with pressure on the cecal side True/False this prevents bacteria-laden LI contents going into SI Dietary disaccharides are broken down at the BBM True/False by BBE’s Salivary amylase is the only enzyme to digest polysaccs True/False Pancreatic amylase also helps Single AA absorption requires Na-dependent active transport True / False Single AA =Na, Di and tri peptides =H Within SI epithelial cells, MG’s and FFA’s re-esterify to form a TG True / False 24 © L. Al-Nakkash 2024