Digestive Physiology PDF

1 DIGESTIVE PHYSIOLOGY Dr. Samantha Solecki, DC, MS Instructor, Biology Thinker. Learner. Motivator. Lover of Anatomy & Physiology [email protected] © 2019 Pearson Education, Inc. ...

1 DIGESTIVE PHYSIOLOGY Dr. Samantha Solecki, DC, MS Instructor, Biology Thinker. Learner. Motivator. Lover of Anatomy & Physiology [email protected] © 2019 Pearson Education, Inc.  2 Learning Objectives *Acquired from the Human Anatomy and Physiology Society (HAPS) with personal additions Describe the major functions of the digestive system. List the structures involved in the process of deglutition and explain how they function, including the changes in position of the glottis and larynx that prevent aspiration. Define the terms peristalsis, segmentation, migrating myoelectric complex, and mass movement, and discuss the role that these activities play in the function of various regions of the alimentary canal. Explain how volume, chemical composition and osmolarity of the chyme affect motility in the stomach and in the duodenum. With respect to the processes of defecation: Describe the defecation reflex and the function of the internal and external anal sphincters. Explain the effect of rectal distension in the defecation reflex. Discuss the conscious control of the defecation reflex. Discuss the specific role of the sympathetic and parasympathetic nervous system in the reflex. With respect to mechanical digestion: Define mechanical digestion. List the organs and structures of the digestive system that function in mechanical digestion and explain the details of the process for each. With respect to enzymatic hydrolysis: Define enzymatic hydrolysis. List the organs and structures of the digestive system that function in enzymatic hydrolysis. List the enzymes used in enzymatic hydrolysis. Discuss the activation of specific enzymes, where applicable. List the substrates and products of enzymatic hydrolysis for each enzyme. Discuss the mechanisms used to regulate secretion and/or activation of each enzyme.  3 Learning Objectives *Acquired from the Human Anatomy and Physiology Society (HAPS) with personal additions Discuss the function, production and regulation of secretion of hydrochloric acid (HCl). With respect to the process of emulsification: Define emulsification and describe the process. List the organs and structures of the digestive system that function in the process of emulsification. With respect to monosaccharides, peptides and amino acids, and fatty acids and monoglycerides: List the organs and specific structures involved in the absorption of each of these types of nutrients. Discuss the absorption of fat-soluble and water-soluble vitamins and the absorption of vitamin B12. Discuss the enterohepatic circulation of bile salts. List the components of both a short reflex and a long reflex in the digestive system. Discuss regulation of reflexes by the enteric and parasympathetic nervous system. Explain the effect of the cephalic phase of regulation on the mucous glands. Explain the effect of the cephalic phase, gastric phase and intestinal phase on the functions of the stomach and give examples for each phase. Explain the effect of the cephalic phase, gastric phase and intestinal phase on the functions of the small intestine and give examples for each phase. With respect to the following hormones or paracrine agents – gastrin, cholecystokinin, secretin, glucose-dependent insulinotropic peptide, histamine, somatostatin, and motilin: State the organ or structure that produces each hormone or agent. State the target organ for each hormone or agent. Describe the action of each hormone or agent. Provide specific examples to demonstrate how the digestive system responds to maintain homeostasis in the body.  4 DIGESTION OVERVIEW  5 Overview Digestive system converts foods into materials for fuel and maintenance of the body Organs of digestion: Alimentary Canal Muscular tube extending from the mouth to the anus Functions in digesting food and absorbing nutrients Organs: mouth, pharynx, esophagus, stomach, small & large intestines Accessory Digestive Organs Lie outside of, but connects to the G.I.T Organs: teeth, tongue, gallbladder, liver, pancreas and salivary glands Figure 23.1 Alimentary canal and related accessory digestive organs. 6 Mouth (oral cavity) Parotid gland Tongue* Sublingual gland Salivary Submandibular gland glands* Esophagus Pharynx Stomach Pancreas* (Spleen) Liver* Gallbladder* Transverse colon Duodenum Descending colon Small Jejunum intestine Ileum Ascending colon Cecum Large intestine Sigmoid colon Rectum Appendix Anus Anal canal  7 Digestive Processes 6 Activities: 1. Ingestion 2. Propulsion 3. Mechanical Breakdown 4. Digestion 5. Absorption 6. Defecation Figure 23.2 Gastrointestinal tract activities. 8 Ingestion Food Mechanical breakdown Pharynx Chewing (mouth) Esophagus Churning (stomach) Propulsion Segmentation Swallowing (small intestine) (oropharynx) Peristalsis Digestion (esophagus, stomach, small intestine, large intestine) Stomach Absorption Lymph vessel Small intestine Large Blood intestine vessel Mainly H2O Feces Anus Defecation Figure 23.3 Peristalsis and segmentation. 9 From mouth Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, moving food along the tract distally. Segmentation: Nonadjacent segments of alimentary tract organs alternately contract and relax, moving food forward then backward. Food mixing and slow food propulsion occur.  10 Basic Concepts of Digestion Body constantly strives to maintain a constant internal environment All digestive tract regulatory mechanisms control luminal conditions  “Digestion” is provoked by mechanical and chemical stimuli Sensors throughout the G.I.T walls help control G.I.T activity Stimuli include: stretch, pH, osmolarity, presence of substances and end products w/stimulation  reflex  1. (+) or (-) glands for digestion 2. (+) smooth m. to mix contents and move it along  11 Digestive Processes Mechanics Mastication (cheeks, lips, teeth, mouth, tongue) Begins mechanical breakdown Partly voluntary, part reflexive (jaw and teeth stretch reflexes) Deglutition Involves 22 different muscle groups all in coordination 2 Phases: Buccal Phase: Occurs in the mouth Pharyngeal-esophageal Phase Figure 23.13 Deglutition (swallowing). 12 Slide 2 Bolus of food Tongue Pharynx Epiglottis Glottis Trachea 1 During the buccal phase, the upper esophageal sphincter is contracted. The tongue presses against the hard palate, forcing the food bolus into the oropharynx. Figure 23.13 Deglutition (swallowing). 13 Slide 3 Uvula Bolus Epiglottis Esophagus 2 The pharyngeal-esophageal phase begins as the uvula and larynx rise to prevent food from entering respiratory passageways. The tongue blocks off the mouth. The upper esophageal sphincter relaxes, allowing food to enter the esophagus. Figure 23.13 Deglutition (swallowing). 14 Slide 4 Upper esophageal Bolus sphincter 3 The constrictor muscles of the pharynx contract, forcing food into the esophagus inferiorly. The upper esophageal sphincter contracts (closes) after food enters.  15 Nerve Supply to the GI Tract = Enteric Nervous System Semiautonomous Enterics are linked to the CNS by afferent visceral fibers and autonomics Autonomics  (+)gut wall  synapse with enterics… Parasympathetics (+) digestion Sympathetics (-) digestion Located in the walls of the alimentary canal 1.Submucosal Plexus – lies within the submucosa 2.Myenteric Plexus – lies between the longitudinal and muscular walls of the muscularis externa Figure 23.4 Neural reflex pathways initiated by stimuli inside or outside the gastrointestinal tract. 16 External stimuli (sight, smell, taste, Central nervous system thought of food) Long reflexes Visceral afferents Extrinsic visceral (autonomic) efferents Internal Chemoreceptors, Local (intrinsic) Effectors: (GI tract) osmoreceptors, or nerve plexus Smooth muscle stimuli mechanoreceptors ("gut brain") or glands Short reflexes Gastrointestinal Response: wall (site of short Change in reflexes) contractile or secretory activity Lumen of the alimentary canal  17 STOMACH Figure 23.14a Anatomy of the stomach. 18 Cardia Fundus Esophagus Muscularis Serosa externa Longitudinal layer Circular layer Oblique layer Body Lumen Lesser curvature Rugae of mucosa Greater curvature Pyloric sphincter Pyloric Pyloric Duodenum (valve) at pylorus canal antrum  19 Gastric Glands Located in the fundus and body and are considerably larger Glands are as follows: 1. Mucous neck: scattered throughout glands; produces thin, acidic mucus 2. Parietal cells: located in the apical region of glands intermixed with chief cells; secretes HCl (necessary for (+) pepsin) and intrinsic factor (for B12 absorption) 3. Chief cells: located in the basal regions of gastric glands; secretes pepsinogen (inactive pepsin) & lipases; In the presence of pepsinogen (released 1 st), activated by HCl in apical portion of gland Pepsinogen  pepsin (positive feedback loop) 4. Enteroendocrine cells: located deep in gastric pits; releases chemical messengers into interstitial fluid Histamine Serotonin Somatostatin Gastrin (regulates stomach secretion & motility) Figure 23.15b Microscopic anatomy of the stomach. 20 Gastric pits Surface epithelium (mucous cells) Gastric pit Mucous neck cells Parietal cell Gastric gland Chief cell Enteroendocrine cell Enlarged view of gastric pits and gastric glands Figure 23.15c Microscopic anatomy of the stomach. 21 Pepsinogen Pepsin HCI Mitochondria Parietal cell Chief cell Enteroendocrine cell Location of the HCl-producing parietal cells and pepsin-secreting chief cells in a gastric gland  22 Mucosal Barrier Gastric juices and digesting enzymes makes a mucosal barrier necessary 3 Factors create barrier: 1. Coating with bicarbonate rich mucus 2. Epithelial cells are joined by tight junctions to prevent spillage of gastric juice 3. Damaged epithelial mucosal cells are rid of quickly and replaced by stem cell (3-6D) EtOH and ASA easily pass through the stomach mucosa! *Read Gastric Ulcers  23 Digestion in the Stomach Stomach acts as a holding area for ingested food where further mechanical and chemical breakdown occurs Involves the delivery of chyme into the small intestine for further digestion and absorption of nutrients Stomach is the site of protein digestion and the continuation of lipolysis *Stomach produces rennin in infants for digestion of casein  24 Phases of Gastric Secretion Stimuli at head, stomach and small intestine can (+) or (-) The effector site is at the stomach 3 Gastric Secretion Phases: 1. Cephalic 2. Gastric 3. Intestinal  25 Phase 1: Cephalic Occurs before food enters the stomach! (+) by smell, taste, sight, thought of food Prepares the stomach for digestion *smell/taste* food  hypothalamus  (+) vagal nuclei  motor impulse (vagus n.)  (+) parasympathetic enteric ganglia  (+) glands of stomach ***sight/thought of food is a conditioned reflex***  26 Phase 2: Gastric Lasts 3-4hours Food in stomach  (+) gastric phase Stimulus 1. Distention Stretch R’s  (+) short (enteric) & long reflexes  Ach  (+) gastric juice secretion 2. Low Acidity 3. Peptides Increase pH (proteins bind H+)  (+) gastrin  parietal cell  HCl release  decreases pH  digestion of proteins protein, gastrin, HCl release Stress/Fear/Anxiety (neural stimuli)  (+) sympathetics  (-) gastrin release  27 Phase 3: Intestinal 2 components: stimulatory and inhibitory Stimulatory component: as food fills duodenum (from stomach)  (+) intestinal mucosal cells  release of intestinal gastrin (Brief) As chyme fills… duodenal lumen distends  (+) inhibitory component (enterogastric reflex) 1Enterogastric reflex  decrease secretory activity 1. (-) vagal nuclei 2. (-) local reflexes 3. (+) sympathetic to constrict the pyloric sphincter ((-) food entry) *If enterogastric reflex fails, nausea and eventual vomitting occurs 2 Enterogastrones: Secretin, Cholecystokinin & vasoactive Intestinal peptide  (-) gastric secretion Figure 23.17 Neural and hormonal mechanisms that regulate release of gastric juice. 28 Stimulatory events Inhibitory events Cephalic 1 Sight and thought Cerebral cortex Lack of Cerebral 1 Loss of phase of food Conditioned reflex stimulatory cortex appetite, impulses to depression 2 Stimulation of Hypothalamus Vagus parasym- taste and smell and medulla nerve pathetic receptors oblongata center 1 Stomach Vagovagal Medulla Vagus Gastrin G cells 1 Excessive distension reflexes nerve secretion acidity activates declines (pH < 2) stretch in stomach Gastric receptors Local Overrides Sympathetic 2 Emotional phase reflexes parasym- nervous stress pathetic system controls activation 2 Food chemicals G cells Gastrin (especially peptides and release caffeine) and rising pH to blood activate chemoreceptors Stomach secretory activity Entero- Local 1 Distension gastric reflexes of duodenum; reflex presence of 1 Presence of Intestinal fatty, acidic, or partially digested (enteric) Vagal hypertonic foods in duodenum gastrin nuclei chyme; and/or or distension of the release Brief Intestinal in medulla irritants in duodenum when to blood effect phase Pyloric the duodenum stomach begins to empty sphincter Release of 2 Distension; enterogastrones presence of (secretin, cholecystokinin, fatty, acidic, vasoactive intestinal partially peptide) digested food Stimulate in the Inhibit duodenum  29 SMALL INTESTINAL PHYSIOLOGY Figure 23.1 Alimentary canal and related accessory digestive organs. 30 Mouth (oral cavity) Parotid gland Tongue* Sublingual gland Salivary Submandibular gland glands* Esophagus Pharynx Stomach Pancreas* (Spleen) Liver* Gallbladder* Transverse colon Duodenum Descending colon Small Jejunum intestine Ileum Ascending colon Cecum Large intestine Sigmoid colon Rectum Appendix Anus Anal canal Figure 23.22a Structural modifications of the small intestine that increase its surface area for digestion 31 and absorption. Vein carrying blood to hepatic portal vessel Muscle layers Lumen Circular folds Villi Figure 23.22b Structural modifications of the small intestine that increase its surface area for digestion 32 and absorption. Microvilli (brush border) Absorptive cells Lacteal Villus Goblet cell Blood capillaries Mucosa- associated lymphoid tissue Intestinal Enteroendocrine crypt cells Muscularis Venule mucosae Lymphatic vessel Duodenal Submucosa gland  33 Digestive Processes of Small Intestine All nutrients and most water absorption occurs here and is accomplished by absorptive cells & microvilli Primary function is to break down chyme chemically  absorption Proper absorption requires a slow, controlled delivery of chyme from stomach  34 Migrating Motor Complex Series of movements where a pattern of peristaltic activity results from waves of smooth muscle contractions Coordinated by local enterics Peristalsis occurs late in intestinal phase once majority of the nutrients have been absorbed; segmenting movements decreases and mucosa of the duodenal wall releases: Motilin  (+) peristaltic waves  35 ACCESSORY ORGAN PHYSIOLOGY Figure 23.25a–b Microscopic anatomy of the liver. 36 Lobule Central Connective vein tissue septum  Figure 23.25c Microscopic anatomy of the liver. 37 Interlobular veins (to hepatic vein) Central vein Sinusoids Bile canaliculi Plates of hepatocytes Bile duct (receives bile from bile canaliculi) Fenestrated lining (endothelial cells) of sinusoids Bile duct Portal venule Portal triad Stellate macrophages Portal arteriole in sinusoid walls Portal vein  38 Bile Composition Alkaline solution that contains bile salts, pigments, chenodeoxcholic acids and cholesterol derivatives All work to emulsify fats and solubilizes cholesterol  39 Enterohepatic Circulation Conserves bile salts Reabsorbed into the blood by the ileum Returned to the liver via hepatic portal blood Resecreted in new bile Bilirubin is a waste product Figure 23.21 The duodenum of the small intestine, and related organs. 40 Right and left hepatic ducts of liver Cystic duct Common hepatic duct Bile duct and sphincter Accessory pancreatic duct Mucosa with folds Tail of pancreas Pancreas Gallbladder Jejunum Major duodenal papilla Main pancreatic duct and sphincter Hepatopancreatic ampulla and sphincter Duodenum Head of pancreas  41 Microscopic Anatomy Acini: clusters of secretory cells surrounding ducts; contains zymogen granules (proenzymes) Acinar cells Pancreatic duct  42 Pancreatic Juice Composition Produced daily Composed of water, enzymes and electrolytes Bicarbonate-rich to help buffer acidity from the stomach Amylase, lipase & nuclease & Pancreatic proteases released, inactive Reaches duodenum & gets activated Trypsinogen: inactive form of trypsin Trypsin: Positive feedback to (+) more trypsinogen & stimulate carboxytrypsin & Chymotrypsin ProCarboxypeptidase Chymotrypsinogen Enteropeptidase: enzyme catalyst that activates trypsinogen  trypsin. NOT a pancreatic protease, but an enzyme that originates from the duodenal wall Figure 23.27 Activation of pancreatic proteases in the small intestine. 43 Stomach Pancreas Epithelial cells Membrane-bound enteropeptidase Trypsinogen Trypsin (inactive) Chymotrypsinogen Chymotrypsin (inactive) Procarboxypeptidase Carboxypeptidase (inactive)  Figure 23.28 Mechanisms promoting secretion and release of bile and pancreatic juice. 44 Slide 1 1 Chyme enter 4 Bile salts -ing duodenum and, to a lesser causes duodenal extent, secretin enteroendocrine transported via cells to release bloodstream cholecystokinin stimulate Liver to produce bile (CCK) and more rapidly. secretin. 5 CCK (via 2 CCK (red blood stream) causes gallbladder dots) and to contract and secretin (yellow Hepatopancreatic dots) enter the Sphincter to bloodstream. relax. Bile Enters duodenum. 3 CCK induces 6 During cephalic secretion of and gastric phases, enzyme-rich vagal Nerve stimu- pancreatic juice. lates gallbladder to Secretin causes contract weakly. secretion of CCK secretion HCO3− -rich Secretin secretion pancreatic juice.  45 LARGE INTESTINAL PHYSIOLOGY  Figure 23.29a Gross anatomy of the large intestine. 46 Left colic (splenic) flexure Right colic Transverse (hepatic) flexure mesocolon Transverse colon Epiploic appendages Superior mesenteric artery Descending colon Haustrum Ascending colon IIeum Cut edge of mesentery IIeocecal valve Tenia coli Sigmoid colon Cecum Appendix Rectum Anal canal External anal sphincter  47 Motility of Large Intestine Musculature Haustral contractions Mass movements Gastrocolic reflex Segmenting movements  48 Defecation Mass movements, forces feces into the rectal walls (+) stretch  sigmoid colon and rectum contract & internal anal sphincter relaxes As feces are forced into the anal canal, messages reach the brain to allow us to decide when the external anal sphincter should relax Abdominal wall muscles & diaphragm aid this process to increase intra-abdominal pressure (Valsalva’s maneuver) Read Diarrhea & constipation Figure 23.31 Defecation reflex. 49 Slide 4 Impulses from cerebral cortex (conscious control) Sensory nerve fibers 1 Feces move into and Voluntary motor distend the rectum, nerve to external stimulating stretch receptors anal sphincter there. The receptors transmit Sigmoid signals along afferent fibers colon to spinal cord neurons. Stretch receptors in wall 2 A spinal reflex is initiated in which parasympathetic motor (efferent) fibers stimulate contraction of the rectum and sigmoid colon, and relaxation of the Rectum internal anal sphincter. External anal sphincter Involuntary motor nerve (skeletal muscle) (parasympathetic division) Internal anal sphincter (smooth muscle) 3 If it is convenient to defecate, voluntary motor neurons are inhibited, allowing the external anal sphincter to relax so feces may pass.  50 DIGESTION  51 Digestion Digestion Catabolic; macromolecules  monomers small enough for absorption Enzymes Intrinsic and accessory gland enzymes break down food Hydrolysis Water is added to break bonds  52 Digestion of Carbohydrates Only monosaccharides can be absorbed Monosaccharides absorbed as ingested Glucose, fructose, galactose Digestive enzymes Salivary amylase, pancreatic amylase, and brush border enzymes (dextrinase, glucoamylase, lactase, maltase, and sucrase) Break down disaccharides sucrose, lactose, maltose; polysaccharides glycogen and starch  53 Digestion of Carbohydrates Starch digestion Salivary amylase (saliva)  oligosaccharides at pH 6.75 – 7.00 Pancreatic amylase (small intestine)  breaks down any that escaped salivary amylase  oligosaccharides Brush border enzymes (dextrinase, glucoamylase, lactase, maltase, sucrase)  oligosaccharides  monosaccharides  Figure 23.32 Flowchart of digestion and absorption of foodstuffs. (1 of 4) 54 Foodstuff Enzyme(s) and source Site of action Path of absorption Starch and disaccharides Glucose and galactose are absorbed via cotransport with Salivary amylase Mouth sodium ions. Fructose passes via facilitated diffusion. Pancreatic Small All monosaccharides leave the amylase intestine epithelial cells via facilitated Oligosaccharides diffusion, enter the capillary blood and disaccharides in the villi, and are transported to Carbohydrate digestion the liver via the hepatic portal vein. Brush border Small Lactose Maltose Sucrose enzymes in intestine small intestine (dextrinase, gluco- amylase, lactase, Galactose Glucose Fructose maltase, and sucrase)  55 Digestion of Proteins Source is dietary, digestive enzymes, mucosal cells; digested to amino acid monomers Begins with pepsin in stomach at pH 1.5 – 2.5 Inactive in high pH of duodenum Pancreatic proteases Trypsin, chymotrypsin, and carboxypeptidase Brush border enzymes Aminopeptidases, carboxypeptidases, and dipeptidases  Figure 23.32 Flowchart of digestion and absorption of foodstuffs. (2 of 4) 56 Foodstuff Enzyme(s) and source Site of action Path of absorption Proteins Amino acids are absorbed via Pepsin Stomach cotransport with sodium ions. (stomach glands) Some dipeptides and tripeptides in presence are absorbed via cotransport with Large polypeptides of HCl H+ and hydrolyzed to amino acids within the cells. Pancreatic Small Infrequently, transcytosis of small enzymes (trypsin, intestine Protein peptides occurs. Small polypeptides, chymotrypsin, Amino acids leave the epithelial digestion small peptides carboxypeptidase) cells by facilitated diffusion, enter the capillary blood in the villi, and Brush border Small are transported to the liver via the enzymes intestine hepatic portal vein. Amino acids (aminopeptidase, (some dipeptides carboxypeptidase, and tripeptides) and dipeptidase)  57 Digestion of Lipids Pre-treatment—emulsification by bile salts Does not break bonds Enzymes—pancreatic lipases  Fatty acids and monoglycerides Figure 23.34 Emulsification, digestion, and absorption of fats. 58 Slide 1 Fat globule 1 Bile salts in the duodenum emulsify Bile salts large fat globules (physically break them up into smaller fat droplets). 2 Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate Fat droplets with bile salts to form micelles which coated with “ferry” them to the intestinal mucosa. bile salts Micelles made up of fatty acids, monoglycerides, and bile salts 3 Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other fatty substances and proteins to form chylomicrons. 4 Chylomicrons are extruded from the epithelial cells by exocytosis. The Epithelial chylomicrons enter lacteals and are cells of carried away from the intestine in small lymph. intestine Lacteal  Figure 23.32 Flowchart of digestion and absorption of foodstuffs. (3 of 4) 59 Foodstuff Enzyme(s) and source Site of action Path of absorption Unemulsified triglycerides Fatty acids and monoglycerides enter the intestinal cells via Lingual lipase Mouth diffusion. Fatty acids and monoglycerides are recombined to form triglycerides and then combined with other lipids and proteins within the cells. The Gastric lipase Stomach resulting chylomicrons are extruded by exocytosis. The chylomicrons enter the lacteals of the villi and are transported to Emulsification by Small the systemic circulation via the the detergent intestine lymph in the thoracic duct. Fat action of bile Some short-chain fatty acids are digestion salts ducted absorbed, move into the capillary in from the liver blood in the villi by diffusion, and are transported to the liver via the Pancreatic Small hepatic portal vein. lipases intestine Monoglycerides (or diglycerides with gastric lipase) and fatty acids  60 Digestion of Nucleic Acids Enzymes Pancreatic ribonuclease and deoxyribonuclease  nucleotide monomers Brush border enzyme nucleosidases and phosphatases  free bases, pentose sugars, phosphate ions  Figure 23.32 Flowchart of digestion and absorption of foodstuffs. (4 of 4) 61 Foodstuff Enzyme(s) and source Site of action Path of absorption Nucleic acids Units enter intestinal cells by active Pancreatic ribo- Small transport via membrane carriers. nuclease and intestine Units are absorbed into capillary deoxyribonuclease blood in the villi and transported to Nucleic acid the liver via the hepatic portal vein. digestion Brush border Small enzymes intestine Pentose sugars, (nucleosidases N-containing bases, and phosphatases) phosphate ions  62 ABSORPTION  63 Absorption ~ All food; 80% electrolytes; most water absorbed in small intestine Most prior to ileum Ileum reclaims bile salts Most absorbed by active transport  blood Exception - lipids  64 Absorption of Carbohydrates Glucose and galactose Secondary active transport (cotransport) with Na+  epithelial cells Move out of epithelial cells by facilitated diffusion  capillary beds in villi Fructose Facilitated diffusion to enter and exit cells  65 Absorption of Protein Amino acids transported by several types of carriers Most coupled to active transport of Na+ Dipeptides and tripeptides actively absorbed by H+-dependent cotransport; digested to amino acids within epithelial cells Enter capillary blood by diffusion  66 Homeostatic Imbalance Whole proteins not usually absorbed Can be taken up by endocytosis/exocytosis Most common in newborns  food allergies Usually disappear with mucosa maturation Allows IgA antibodies in breast milk to reach infant's bloodstream  passive immunity  67 Absorption of Lipids Absorption of monoglycerides and fatty acids Cluster with bile salts and lecithin to form micelles Released by micelles to diffuse into epithelial cells Combined with lecithin, phospholipids, cholesterol, & coated with proteins to form chylomicrons Enter lacteals; transported to systemic circulation Hydrolyzed to free fatty acids and glycerol by lipoprotein lipase of capillary endothelium Cells can use for energy or stored fat Absorption of short chain fatty acids Diffuse into portal blood for distribution  68 Absorption of Nucleic Acids Absorption Active transport across epithelium  bloodstream  69 Absorption of Vitamins In small intestine Fat-soluble vitamins (A, D, E, and K) carried by micelles; diffuse into absorptive cells Water-soluble vitamins (vitamin C and B vitamins) absorbed by diffusion or by passive or active transporters. Vitamin B12 (large, charged molecule) binds with intrinsic factor, and is absorbed by endocytosis ------------------------------------------------------------------------------------ In large intestine Vitamin K and B vitamins from bacterial metabolism are absorbed  70 Absorption of Water 9 L water, most from GI tract secretions, enter small intestine 95% absorbed in the small intestine by osmosis Most of rest absorbed in large intestine Net osmosis occurs if concentration gradient established by active transport of solutes Water uptake coupled with solute uptake