The Digestive System Part II (4) PDF

The Digestive System Part II Chapter 23 p. 874 – 924 Section 23.1 – 23.11 1 Dr. Savory Université d’Ottawa | University of Ottawa...

The Digestive System Part II Chapter 23 p. 874 – 924 Section 23.1 – 23.11 1 Dr. Savory Université d’Ottawa | University of Ottawa Disclosure You may only access and use this PowerPoint presentation for educational purposes. You may not post this presentation or the associated videos online or distribute it without the permission of the author. uottawa.ca 2  SMALL INTESTINE & ITS ACCESSORY ORGANS 3 Accessory Digestive Organs of the Small Intestines Digestion in the small intestine depends on three accessory organs: Liver, gallbladder, and pancreas are accessory organs associated with small intestine ▪ Liver: digestive function is production of bile LIVER ▪ Gallbladder: chief function is storage of bile ▪ Pancreas: supplies most of enzymes needed to digest chyme, as well as PANCREAS bicarbonate to neutralize stomach acid DUODENUM Figure 23.28 Relationship of the liver, gallbladder, and pancreas to the duodenum.  The Liver Gross Anatomy ▪ Largest gland in body; ~3 lbs ▪ Consists of four primary lobes: ▪ anterior view: larger right & smaller left, caudate & quadrate ▪ Gallbladder rests in recess on inferior surface of right lobe Falciform ligament Anterior view ▪ Separates larger right and smaller left lobes ▪ Suspends liver from diaphragm and anterior abdominal wall Round ligament (ligamentum teres) o Remnant of fetal umbilical vein along free edge of falciform ligament Posterior view Lesser omentum anchors liver to stomach Figure 23.23  The Liver Gross Anatomy – Hepatic artery proper and hepatic portal vein enter liver at porta hepatis – Bile leaves liver via left and right hepatic ducts; fuse to form common hepatic duct ▪ Cystic duct fills/drains gallbladder ▪ Bile duct formed by union of common hepatic and cystic ducts Fig. 23.23 Illustration of the posterior view of the liver  The Liver Microscopic Anatomy Liver lobules ▪ Hexagonal sesame-seed-size structural and functional units ▪ plates of hepatocytes (liver cells) that filter and process nutrient-rich blood ▪ Central vein located in longitudinal axis Liver sinusoids ▪ leaky capillaries between hepatic plates ▪ lined with stellate (hepatic) macrophages that remove debris and old RBCs Portal triad in each corner of lobule contains: ▪ Branch of hepatic artery, which supplies O2 ▪ Branch of hepatic portal vein, which brings nutrient-rich blood from intestine ▪ Bile duct which receives bile from bile canaliculi (tiny canals between hepatocytes Figure 23.24c Microscopic anatomy of the liver Three-dimensional representation of a small portion of one liver lobule  The Liver Hepatocytes have large amounts of both rough and smooth ER, Golgi apparatus, peroxisomes and mitochondria allowing them to ▪ Produce 900 ml bile per day ▪ Process bloodborne nutrients – eg: store glucose as glycogen and make plasma proteins ▪ Store fat-soluble vitamins ▪ Perform detoxification – eg: converting ammonia to urea ▪ Excrete bilirubin to the bile ▪ Phagocytize (Stellate macrophages cell) old RBC, leukocytes and some bacteria ▪ Synthesize: - most of the clotting factors - lipoproteins The only digestive function of the liver is the production of bile  The Liver Bile: Composition and enterohepatic circulation Yellow- green alkaline solution containing ▪ Bile salts: cholesterol derivatives and phospholipids that function in fat emulsification & absorption ▪ Bilirubin: main bile pigment from heme of hemoglobin o Bacteria break down in intestine to stercobilin that gives brown color of feces ▪ Cholesterol, triglycerides, phospholipids, and electrolytes ▪ Enterohepatic circulation: recycling mechanism that conserves bile salts ▪ Reabsorbed in ileum (the last part of small intestine) and returned to liver via hepatic portal vein ▪ 95% of secreted bile salts recycled Recycling mechanism that conserves bile salts  The Gallbladder Thin-walled muscular sac on the inferior surface of the liver Stores and concentrates bile by absorbing its water and ions Contains many honeycomb folds that allow it to expand as it fills Muscular contraction → releases bile via the cystic duct, which flows into the common bile duct What are gallstones? http://clinicalgate.com/wp-content/uploads/2015/03/B9780443066849500779_gr21.jpg 12  The Pancreas ▪ Location: mostly retroperitoneal, head encircled by duodenum; tail abuts spleen ▪ Contains endocrine and exocrine parts – Endocrine function: secretion of insulin and glucagon by pancreatic islet cells – Exocrine function: produce pancreatic juice Acini: clusters of secretory cells that produce zymogen granules containing proenzymes Ducts: secrete to duodenum via main pancreatic duct; smaller duct cells produce water and bicarbonate (HCO3-) Figure 23.26  The Pancreas Composition of pancreatic juice 1200–1500 ml/day is produced containing: ▪ Watery, alkaline solution (pH 8) to neutralize acidic chyme coming from stomach ▪ Electrolytes, primarily HCO3− ▪ Digestive enzymes ▪ Proteases (for proteins): secreted in inactive form & require activation. Why? ▪ Amylase (for carbohydrates) ▪ Lipases (for lipids) ▪ Nucleases (for nucleic acids) – Proteases activated in duodenum, where they work ▪ Enteropeptidase (formerly called enterokinase): enzyme bound to apical membrane of duodenal epithelial cells, activates trypsinogen to trypsin ▪ Trypsin in turn can then activate: – More trypsinogen to trypsin – Procarboxypeptidase to carboxypeptidase – Chymotrypsinogen to chymotrypsin  The Pancreas 15 Figure 23.27 Activation of pancreatic proteases in the small intestine.  Bile and Pancreatic Secretion Into the Small Intestine Bile & major pancreatic duct unite in wall of duodenum → hepatopancreatic ampulla and sphincter ▪ which opens into duodenum via major duodenal papilla ▪ Hepatopancreatic sphincter controls entry of bile & pancreatic juice into duodenum ▪ Note: smaller accessory pancreatic duct empties directly into duodenum ▪ Hepatopancreatic sphincter is closed, unless digestion is active ▪ Bile is stored in gallbladder and released to small intestine only with contraction  17 Figure 23.28 Relationship of the liver, gallbladder, and pancreas to the duodenum.  Bile & Pancreatic Secretion into the Small Intestine Regulation of bile and pancreatic secretion – Both regulated by neural and hormonal controls Hormonal include two enterogastrones, cholecystokinin (CCK) and secretin – CCK secretion which is stimulated by protein-rich, fatty chyme: » stimulates acini to secrete enzyme-rich pancreatic juice » contracts gallbladder and relaxes hepatopancreatic sphincter – Secretin secretion stimulated by acidic chyme: » stimulates duct cells to secrete bicarbonate-rich pancreatic juice » weakly stimulates bile secretion Neural include long (vagus nerve) reflexes that weakly stimulate gallbladder contraction and pancreatic secretion during cephalic and gastric phases – Unless digestion occurring, hepatopancreatic sphincter is closed, bile backs up into gallbladder via cystic duct, where it is stored and concentrated until needed  Mechanisms Promoting Secretion and Release of Bile & Pancreatic Juice Chyme entering duodenum causes release of cholecystokinin (CCK – red dots) & secretin (yellow dots) from duodenal enteroendocrine cells. ▪ CCK release stimulated by proteins & fat in chyme ▪ Secretin release stimulated by the acidic chyme Pancreas secretion ▪ CCK induces acinar cells → enzyme rich pancreatic juice CCK & secretin enter ▪ Secretin causes the blood stream secretion by duct cells of HCO3- rich pancreatic juice ▪ Weakly stimulated by the vagus nerve Mechanisms Promoting Secretion and Release of Bile & Pancreatic Juice Bile secretion by the liver ▪ Bile salts returning from the enterohepatic circulation are the most important stimulus ▪ Secretin is a minor stimulus Gallbladder contraction ▪ CCK causes the GB to contract ▪ Vagus nerve→ weak GB contraction during cephalic & gastric phase Hepatopancreatic sphincter relaxation ▪ CCK → HP sphincter to relax ▪ Bile and pancreatic juice enter duodenum 21  Small Intestine The major organ of digestion and absorption ▪ Digestion is completed (with the help of bile and pancreatic enzymes) and virtually all absorption ▪ Extends from pyloric sphincter → ileocecal valve ▪ 7-13 ft during life; ~20 ft in a cadaver ▪ Small diameter of 2.5 - 4 cm (1.0 -1.6 inches) Subdivisions ▪ Duodenum (retroperitoneal) ▪ ~ 25.0 cm (10.0 in) long; curves around head of pancreas ▪ Has the most features ▪ Jejunum ▪ ~ 2.5 m (8 ft); attached posteriorly by mesentery ▪ Ileum ▪ ~3.6 m (12 ft) attached posteriorly by mesentery); joins large intestine at ileocecal valve  Small Intestine Blood supply – Superior mesenteric artery brings oxygenated blood supply – Veins (carrying nutrient-rich blood) drain into superior mesenteric veins, then into hepatic portal vein, and finally into liver Nerve supply – Parasympathetic innervation via vagus nerve, and sympathetic innervation from thoracic splanchnic nerves 23  Small Intestine-Microscopic Anatomy Modifications of small intestine for ABSORPTION ▪ length & other structural modifications → huge surface area ▪ modifications – circular folds, villi, and microvilli - increase surface area 600 to ~200 m2 (size of a tennis court) ▪ Circular folds ▪ Permanent folds of mucosa and submucosa (~1 cm deep) that force chyme to slowly spiral through lumen, allowing more time for nutrient absorption ▪ Villi ▪ Fingerlike projections of mucosa (~1 mm high) with a core that contains dense capillary bed and lymphatic capillary called a lacteal (lymphatic capillary for lipid absorption) for absorption ▪ Microvilli ▪ Cytoplasmic extensions of apical surface of enterocytes that create fuzzy appearance called the brush border ▪ contains membrane-bound enzymes brush border enzymes used for final carbohydrate and protein digestion Figure 23.31 Structural modifications of the small intestine that increase its surface area for digestion and absorption.  Small Intestine Histology of the SI wall ▪ Four tunics: mucosa & submucosa modified for digestion ▪ Epithelium of villi and the tubular intestinal crypts (intestinal glands) between villi consists of five main cell types ▪ ↓ in number along the length of the SI. ▪ Produce intestinal juice - watery mixture of mucus that acts as carrier fluid for chyme  Small Intestine Histology of the SI wall Five major types of cells are found in the villi & crypts ▪ Enterocytes: make up bulk of epithelium ▪ Simple columnar absorptive cells bound by tight junctions with many microvilli ▪ Function Villi: absorb nutrients and electrolytes Crypts: produce intestinal juice, watery mixture of mucus that acts as carrier fluid for chyme ▪ Goblet cells: mucus-secreting cells in epithelia of villi & crypts ▪ Enteroendocrine cells: source of enterogastrones (CCK /secretin) & GIP ▪ Found scattered in villi but some in crypts ▪ Paneth cells: deep in crypts, specialized secretory cells that fortify SI’s defenses. ▪ Secrete antimicrobial agents (defensins & lysozyme) that can destroy bacteria ▪ Stem cells that continuously divide to produce other cell types ▪ Villus epithelium renewed every 2–4 days 27  Small Intestine Histology of the SI wall ▪ Mucosa ▪ MALT protects intestine against microorganisms and includes: ▪ Individual lymphoid follicles & Peyer’s patches (aggregated lymphoid nodules) in lamina propria ▪  numbers in distal part of small intestine. Why? ▪ Lamina propria: large numbers of plasma cells that secrete IgA ▪ Submucosa ▪ areolar tissue ▪ duodenal glands secrete alkaline mucus to neutralize acidic chyme ▪ Muscularis ▪ circular & longitudinal muscle ▪ Most of the duodenum (retroperitoneal) → adventitia ▪ Visceral peritoneum (serosa) covers the external intestinal surface Fig. 20.5  Small Intestine Intestinal juice ▪ 1–2 L secreted daily by the intestinal glands in response to distension or irritation of mucosa ▪ hypertonic or acidic chyme is the major stimulus ▪ Slightly alkaline and isotonic with blood plasma ▪ Consists largely of water but also contains mucus secreted by duodenal glands and goblet cells of mucosa ▪ Absorptive cells synthesize digestive (brush border) enzymes Chyme entering the small intestine contains partially digested carbohydrates and proteins, and largely undigested fats ~3-6 hours to pass through small intestine as nutrients and most of the water is absorbed  Digestive Processes in the Small Intestine Sources of enzymes for digestion ▪ Substances such as bile, bicarbonate, digestive enzymes (not brush border enzymes) are from liver and pancreas ▪ Brush border enzymes bound to plasma membrane perform final digestion of chyme Regulating chyme entry Chyme from stomach contains ▪ partially digested CHO, PRO & undigested fats ▪ usually hypertonic; therefore, delivery must be slow to prevent osmotic loss of H2O from blood ▪ low pH must be adjusted upward ▪ must be mixed with bile & pancreatic juice to continue digestion ▪ Enterogastric reflex & enterogastrones control movement of food into duodenum to regulate duodenal filling  Digestive Processes in the Small Intestine Motility of the Small Intestine Two motility patterns: After a meal: segmentation ▪ Initiated by intrinsic pacemaker cells ▪ alternately contracting/relaxing rings of smooth muscle move chyme back-and-forth (also slowly moving it toward cecum) ▪ Mixes/moves contents toward ileocecal valve ▪ Intensity is altered by long /short reflexes & hormones » Parasympathetic  motility; sympathetic  Between meals: peristalsis ▪ initiated by  in hormone motilin in late intestinal phase ▪ Wave every 90–120 minutes ▪ Each wave starts distal to previous; called migrating motor complex (MMC) ▪ Meal remnants, bacteria & debris are moved toward large intestine ▪ Complete trip from duodenum to ileum: ~2 h https://encrypted- tbn2.gstatic.com/images?q=tbn:ANd9GcSlD5RBdWi4DVQewaFNqVV2Fwr_v5vgszakmv4 VoWtXkEkbHTYW  Ileocecal valve control Ileocecal valve control ▪ ileocecal valve relaxes and admits chyme into large intestine when: Gastroileal reflex enhances force of segmentation in ileum Gastrin increases motility of ileum ▪ Ileocecal valve flaps close when chyme exerts backward pressure Prevents regurgitation into ileum 33  The Large Intestine Absorbs Water and Eliminates Feces Large intestine frames small intestine on three sides, extends from ileocecal valve to anus – Much shorter than small intestine (1.5 m v s 6 m), but ersu about twice the diameter (7 cm) – Major digestive functions: ▪ Absorb most of the remaining water from indigestible food residues – Also absorb metabolites produced by resident bacteria ▪ Store residues temporarily, then eliminate them as semisolid feces (or stool) 34  The Large Intestine (Colon) Gross Anatomy ▪ Walls contain typical 4 layers found in the rest of the GI tract ▪ Has 3 unique features Teniae coli Haustra SMA Epiploic appendages Ileocecal valve Subdivisions of the colon 1. Cecum: first part of large intestine 2. Appendix: masses of lymphoid tissue Appendicitis: acute inflammation of appendix; usually results from a blockage by feces that traps infectious bacteria  The Large Intestine (Colon) Gross Anatomy cont. left colic (splenic) flexure 3. Colon: has 4 regions, ▪ Ascending colon: travels up right side of abdominal cavity to level of right kidney – Ends in right-angle turn - right colic (hepatic) flexure Transverse Descending ▪ Transverse colon: travels across abdominal cavity SMA – Ends in another right-angle turn, left colic (splenic) flexure ▪ Descending colon: travels down left side of abdominal cavity Sigmoid Rectum ▪ Sigmoid colon: S-shaped portion that travels through pelvis Anal Canal External Anal Sphincter 4. Rectum: three transverse folds (rectal valves) stop feces from being passed with gas (flatus)  The Large Intestine (Colon) Gross Anatomy cont. 5. Anal canal: last segment of large intestine external to the abdominopelvic cavity that opens to body exterior at anus Has two sphincters that close the anys except during defection ▪ Internal anal sphincter: Anal Canal smooth muscle ▪ External anal sphincter: skeletal muscle Figure 23.31  The Large Intestine (Colon) Relationship to the peritoneum ▪ Colon is also retroperitoneal, except for its transverse and sigmoid parts ▪ Intraperitoneal regions are anchored to posterior abdominal wall by mesentery sheets called mesocolons ▪ Rectum (sometimes cecum) also retroperitoneal Figure 23.32  The Large Intestine (Colon) Microscopic Anatomy ▪ Like the small intestine, the large is lined with simple columnar epithelium (except anal canal has stratified squamous to withstand abrasion) ▪ Does not contain circular folds, villi, or digestive secretions ▪ contains thicker mucosa made up of simple columnar epithelium except in anal canal, where it becomes stratified squamous epithelium to withstand abrasion ▪ Contains abundant deep crypts with many mucus-producing goblet cells  The Large Intestine (Colon) Microscopic Anatomy cont. ▪ Mucosa of anal canal hangs in long ridges or folds referred to as anal columns ▪ Anal sinuses: recesses located between anal columns; secrete mucus to aid in emptying ▪ Pectinate line: horizontal line that parallels the wavy inferior margins of the anal sinuses – Visceral sensory fibers innervate area superior to this line; relatively insensitive to pain – Somatic nerves innervate inferior to this line, sensitive to pain Figure 23.33 Two superficial venous plexuses of anal canal form hemorrhoids if dilated and inflamed  The Large Intestine (Colon) Bacterial Microbiota (Bacterial flora) ▪ consist of 1000+ different types of bacteria (outnumber our cells 10:1) ▪ enter from small intestine or anus to colonize colon Metabolic functions ▪ Fermentation Ferment indigestible carbohydrates and mucin releasing short-chain fatty acids that can be absorbed Release irritating acids and gases (~500 ml/day) ▪ Vitamin synthesis Synthesize B complex & some vitamin K needed by liver to produce clotting factors ▪ Keep pathogenic bacteria in check – Beneficial bacteria outcompete and suppress (and so outnumber) them – Immune system prevents harmful bacteris from crossing gut epithelium Dendritic cells sample microbial antigens in lumen, then migrate to nearby lymphatic follicles (MALT) to trigger I g A antibody–mediated response  The Large Intestine (Colon) Gut bacteria and health – Mounting evidence supports findings that the types & proportions of gut bacteria can influence: Body weight Susceptibility to various diseases (including diabetes, atherosclerosis, fatty liver disease) Response to various drugs Our moods – Manipulating gut bacteria may become a routine health-care strategy in future https://www.frontiersin.org/files/Articles/336645/fmicb-09-01510-HTML/image_m/fmicb-09-01510-g001.jpg  Digestive Processes of the Large Intestine ▪ Residue remains in large intestine 12–24 hours ▪ No food breakdown occurs except what enteric bacteria digest ▪ Vitamins (made by bacterial flora), H2O & electrolytes (especially Na+ and Cl−) are reclaimed ▪ Major functions of large intestine: propulsion of feces to anus & defecation Motility of the large intestine ▪ Haustral contractions: most contractions of colon, haustra sequentially contract in response to distension Slow segmenting movements, mostly in ascending and transverse colon ▪ Mass movements: long, slow-moving, powerful contractile waves that move feces toward rectum; occur about 3-4X per day Descending colon & sigmoid colon act as storage reservoir Usually occur during/after eating (via gastrocolic reflex) https://classconnection.s3.amazonaws.com/544/flashca rds/666544/png/colon_reflexes1322630368379.png Digestive processes of the Large Intestine Figure 23.35 Defecation reflex  Digestive processes of the Large Intestine Defecation ▪ Mass movements force feces toward rectum ▪ Distension initiates defecation reflex (parasympathetic spinal reflex) Stimulate contraction of sigmoid colon and rectum Relax internal anal sphincter Voluntary control allows contraction/relaxation of external anal sphincter ▪ If defecation delayed, contractions end after few seconds (until reflex triggered again) ▪ During reflex, rectal muscles contract to expel feces, which is assisted by: ▪ Valsalva’s maneuver - Closing of glottis, contraction of diaphragm and abdominal wall muscles cause increased intra-abdominal pressure ▪ Levator ani muscle contracts, causing anal canal to be lifted superiorly and allowing feces to leave body What is diarrhea? Constipation? Physiology of Digestion and Absorption https://cnx.org/resources/a8fd4c3ee096f086722dbad7b844e1d6f ef62b68/2405_Digestive_Process.jpg 46  Physiology of Digestion & Absorption Mechanism of Digestion: Enzymatic Hydrolysis ▪ Digestion: catabolic process that breaks down of ingested macromolecules (polymers) down into → monomers (chemical building blocks) small enough for absorption across the small intestine wall ▪ via enzymes secreted into GI tract from intrinsic and accessory glands, and intestinal brush border enzymes ▪ Enzymes carry out hydrolysis -water is added to break chemical bonds http://images.slideplayer.com/26/8497716/slides/slide_9.jpg ▪ In general, pancreatic (also salivary and gastric) enzymes break down larger polymers into smaller polymers that are eventually broken down into absorbable monomers by (intestinal) brush border enzymes  Physiology of Digestion & Absorption Mechanisms of Absorption ▪ Absorption: moving substances from lumen of gut into body ▪ Tight junctions: molecules pass through rather than between cells Materials enter cell through apical membrane (lumen side) and exit through basolateral membrane (interstitial side) Once in IF, substances (except lipids) diffuse into capillaries (lipids enter lacteals) ▪ Lipid molecules can be absorbed passively through membrane, but other polar molecules are absorbed by active transport ▪ Most nutrients are absorbed before chyme reaches ileum  Processing of Nutrients Carbohydrates Three classes ▪ Mono-, di- and polysaccharides (only monosaccharides can be absorbed) ▪ the principal polysaccharide in human body is glycogen Digestion begins in mouth via salivary amylase (splits starch into oligosaccharides) – Continues working until low stomach pH denatures it ▪ Starch and disaccharides → oligo- and disaccharides – Begins in mouth with salivary amylase – SI: further broken down by pancreatic amylase → lactose, maltose & sucrose ▪ Final breakdown into monosaccharides (glucose, fructose, galactose) by brush border enzymes (glucoamylase, dextrinase, maltase, sucrose) Figure 23.36 Flowchart of digestion and absorption of foodstuffs.  Processing of Nutrients Carbohydrates ▪ Monosaccharides are co- transported across apical membrane of absorptive epithelial cell, mostly by secondary active transport with Na+ ▪ Monosaccharides exit across the basolateral membrane by facilitated diffusion Figure 23.37 Lactose intolerance People with lactose intolerance have deficient amounts of lactase and cannot consume lactose Any lactose eaten remains undigested and creates an osmotic gradient in intestine that prevents water from being absorbed, resulting in diarrhea – can also can pull water from interstitial space into intestinal lumen Bacterial metabolism of undigested solutes produces large amounts of gas, resulting in bloating, flatulence, and cramping pain Treatment: add lactase enzyme “drops” to milk or take a lactase tablet before consuming milk products Intranet.tdmu.edu.ua  Processing of Nutrients Proteins ▪ 3D coiled up chain of amino acids (aa) arranged in a precise sequence ▪ Source of protein: diet, digestive enzymes & from breakdown of mucosal cells ▪ Digestion begins in stomach as pepsinogen is converted to pepsin at p H 1.5–2.5 ▪ Digests proteins into polypeptides (and some amino acid monomers); becomes inactive in high pH of duodenum ▪ Proteins are broken into: ▪ Large polypeptides ▪ Small polypeptides and small peptides ▪ Finally, into amino acid monomers, with some dipeptides and tripeptides http://2012books.lardbucket.org/books/introduction-to-chemistry-general-organic- and-biological/section_21/6d9b4af09c98e66f071eb9560fd984ef.jpg  Processing of Nutrients Proteins Figure 23.36 Flowchart of digestion and absorption of foodstuffs. Pancreatic proteases break down proteins and protein fragments into smaller pieces and some individual amino acids ▪ trypsin & chymotrypsin cleave protein into smaller peptides while carboxypeptidase takes off one aa at a time from end Brush border enzymes ▪ aminopeptidases, carboxypeptidases & dipeptidases break oligopeptides and dipeptides into amino acids  Processing of Nutrients Proteins ▪ AAs are co-transported across apical membrane of absorptive epithelial cell via secondary active transport carriers (Na+ or H+) ▪ Amino acids exit across basolateral membrane via facilitated diffusion  Processing of Nutrients Lipids ▪ Triglycerides: most abundant fats in diet ▪ Need pre-treatment with bile salts that break large fat globules into smaller ones → emulsification ▪ lipid droplets are coated with bile salts & lecithin, forming emulsion droplets (larger surface area → digestive action of lipases) that are water soluble - micelles ▪ Digestion: pancreatic lipases break down fat into monoglyceride plus two free FAs (all are insoluble in H2O) ▪ Micelle formation: products from digestion become coated with bile salts and lecithin ▪ Diffusion: lipid products leave micelles https://abdominalkey.com/wp-content/uploads/2016/06/A322953_1_En_7_Fig3_HTML.gif and cross epithelial membrane via diffusion  Emulsification ▪ Bile salts have both hydrophilic & hydrophobic domains o the hydrophobic part → to the lipids o the hydrophilic part → the surface ▪ The molecules in micelles are clustered together exposing polar ends toward H2O making micelles soluble in water o in this form FA & monoglycerides reach the intestinal epithelial cells  Processing of Nutrients Lipids ▪ Chylomicron formation: lipid products are converted back into TGs and packaged phospholipids, cholesterol, proteins; forming lipoprotein called chylomicron ▪ Chylomicron transport: chylomicrons exit from basolateral membrane via exocytosis ▪ once in the IF they diffuse into lacteals (too large for capillary) ▪ Eventually emptied into venous blood at thoracic duct ▪ Once in blood, chylomicrons are broken into free fatty acids and glycerol by lipoprotein lipase so they can be used by https://ib.bioninja.com.au/_Media/lipid-metabolism_med.jpeg cells ▪ Short-chain fatty acids can diffuse directly into blood 57  Processing of Nutrients Lipids Figure 23.36 Flowchart of digestion and absorption of foodstuffs. 58  Processing of Nutrients Nucleic Acids ▪ coiled up chains of nucleotides linked together in a precise arrangement ▪ Nuclei of ingested cells in food contain DNA and RNA ▪ Pancreatic nucleases hydrolyze nucleic acid to nucleotide monomers ▪ Brush border enzymes, nucleosidases, and phosphatases break nucleotides down into free nitrogenous bases, pentose sugars & phosphate ions ▪ Breakdown products are actively transported by special carriers in epithelium of villi Figure 23.36 Flowchart of digestion and absorption of foodstuffs.  Absorption of Vitamins, Electrolytes, and Water 60  Absorption of Vitamins ▪ In small intestine Fat-soluble vitamins (A, D, E, and K) are carried by micelles; diffuse into absorptive cells Water-soluble vitamins (C and B) are absorbed by diffusion or by passive or active transporters Vitamin B12 (large, charged molecule) binds with IF (produced where?) and is absorbed by endocytosis ▪ In large intestine vitamin K and B vitamins from bacterial metabolism are absorbed  Absorption of Electrolytes ▪ Most ions are transported actively along length of GI Amount in intestine is amount absorbed regardless of nutritional state except for Fe and Ca ▪ Na+ absorption is coupled with active absorption of glucose and amino acids ▪ Cl− is transported actively ▪ K+ diffuses (facilitated) in response http://intranet.tdmu.edu.ua/data/kafedra/internal/normal_phiz/classes _stud/en/stomat/2%20course/5%20Cycle%20Physiology%20of%20d igestion%20system%20and%20kidneys/01%20Digestion%20in%20o to osmotic gradients; lost if water ral%20cavity.%20Digestion%20in%20stomach,%20intestine%20and %20colon.files/image052.jpg absorption is poor ▪ HCO3- actively secreted into the lumen  Absorption of Electrolytes ▪ Fe and Ca2+ : absorption in duodenum is related to need Ionic Fe actively transported into mucosal cells and stored bound to ferritin (local storehouse) Normal state: 10-20% pass into portal blood; most lost with epithelial cells Fe depleted: ↑ uptake and release; transported in blood by transferrin ▪ Ca+ absorption: regulated by vitamin D & parathyroid hormone (PTH) Vitamin D promotes absorption PTH releases Ca2+ from the bone matrix 63  Absorption of Water ▪ ~9 L water, most from GI tract secretions, enter small intestine 95% is absorbed in the small intestine by osmosis Most of rest is absorbed in large intestine leaving ~ 100 ml in feces ▪ Net osmosis occurs if concentration gradient is established by active transport of solutes ▪ Water uptake is coupled with solute uptake