The Gastrointestinal System

Four Basic Digestive Processes Figure 20.1 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Major Structures of the GI System Figure 20.2 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Layers Figure 20.3 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Mouth, Pharynx, Esophagus, and Stomach Figure 20.4 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Esophagus Muscular tube from pharynx to stomach Upper 1/3—skeletal muscle Lower 2/3—smooth muscle Upper esophageal sphincter Skeletal muscle Between pharynx and esophagus Lower esophageal sphincter Smooth muscle Between esophagus and stomach Figure 20.4 (1 of 3) Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Gastric Pits Figure 20.4 (2–3 of 3) Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Acidic Environment of Stomach pH = 2 Necessary for activating pepsinogen Denatures proteins Kills bacteria Gastric mucosal barrier Protective layer of mucus and bicarbonate Secreted from neck cells and goblet cells Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Regulates passage of chyme between stomach and small intestine Pyloric Sphincter Regulates passage of chyme between stomach and small intestine Figure 20.4 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Small Intestine Figure 20.5 (1 of 3) Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Small Intestine Wall Villi increase surface area of epithelium Contain blood vessels and lacteal for absorption of nutrients Microvilli increase surface area of epithelial cells Form brush border Figure 20.5 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Crypts of Lieberkuhn Epithelial cells in crypts secrete bicarbonate-rich fluid Secreted in proximal small intestine Absorbed in distal small intestine Figure 20.5 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Circulatory Route: Absorbed Material Figure 20.6 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Features of the Colon Figure 20.7 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Accessory Structures Figure 20.2 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Structure of an Accessory Gland Figure 20.8 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Characteristics of Saliva Rich in bicarbonate ions Contains mucus Enzymes Salivary amylase Lysozyme Figure 20.9 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

The Pancreas Has exocrine and endocrine portions Exocrine portion produces pancreatic juice Rich in bicarbonate Enzymes Pancreatic amylase and lipases Proteases Nucleases Figure 20.10 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Liver Functions Secrete bile Processing of nutrients Glucose  glycogen Amino acids  fatty acids Triglycerides and cholesterol synthesis Lipoprotein synthesis Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Liver Functions Removal of old red blood cells Catabolize hemoglobin Generates bilirubin Elimination of wastes and toxins Bilirubin eliminated as bile pigments Synthesis of plasma proteins Secretion and modification of hormones Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Liver, gallbladder, and associated ducts Biliary System Liver, gallbladder, and associated ducts Liver synthesizes bile Gallbladder stores bile Common bile duct Transports bile from liver and/or gallbladder to duodenum Figure 20.11a Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Liver, gallbladder, and associated ducts Biliary System Liver, gallbladder, and associated ducts Joins with pancreatic duct to form Ampulla of Vater Sphincter of Oddi Regulates flow from pancreas and gallbladder to duodenum Figure 20.11a Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Liver Anatomy Materials to make bile taken up from blood in sinusoids into hepatocytes Hepatocytes secrete bile into bile canaliculi, on side opposite of sinusoids Figure 20.11b Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Liver Anatomy Canaliculi drain into bile ducts Bile ducts drain into common hepatic duct Figure 20.11b Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

GI Organs and Functions Table 20.1 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Carbohydrates Typical diet: 250–800 grams carbohydrates Most consumed as disaccharides or polysaccharides Sucrose Lactose Maltose Starch Glycogen Cellulose (fiber, cannot be digested) Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Carbohydrate Digestion Only monosaccharides absorbed Disaccharides and polysaccharides must be digested to monosaccharides Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Digestion of Starch Enzyme of digestion End product Salivary amylase Pancreatic amylase End product Disaccharides (maltose) Limit dextrins Figure 20.12 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Further Digestion Enzymes of digestion Dextrinase – limit dextrins  glucose Glucoamylase – polysaccharides  glucose Sucrase – sucrose  fructose + glucose Lactase – lactose  galactose + glucose Maltase – maltose  2 glucose Location of enzymes—brush border of small intestine “Brush border enzymes” Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Absorption of Monosaccharides Absorption = transport from lumen to blood Glucose and galactose absorbed by Secondary active transport across apical membrane Facilitated diffusion across basolateral membrane Fructose absorbed by Facilitated diffusion across both membranes Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Digestion / Absorption of Maltose Figure 20.13 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Proteins Typical diet: 125 grams/day protein Only require 40–50 grams Proteins to be digested and absorbed include Consumed in diet Secreted into lumen of intestinal tract Sloughed off with cells lining intestinal tract Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Protein Digestion in Stomach Protein digestion begins in stomach Enzyme = pepsin Inactive (secreted form) = pepsinogen Activated by acid Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Pepsin: Activation and Activity Chief cells secrete pepsinogen Pepsinogen is the inactive form HCl cleaves pepsinogen to pepsin Pepsin = active form Parietal cells secrete HCl Figure 20.14 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Protein Digestion in Small Intestine Pancreatic proteases Trypsin Chymotrypsin Carboxypeptidase Brush border proteases Aminopeptidase Enterokinase Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Lipids Typical diet: 25–160 grams lipids 90% triglycerides Lipids face special problem in digestion and absorption Not water soluble Do not mix with stomach, intestinal contents Form fat droplets Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Lipid Digestion Enzymes of digestion = lipases Secreted from pancreas Lipases can only act on molecules near edge of fat droplet Bile salts increase surface area of droplets by breaking large droplet into several small droplets = emulsification Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Bile Salts Synthesized in liver from cholesterol Secreted in bile to duodenum Amphipathic molecule Emulsify fat Figure 20.16 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Emulsification of a Fat Globule Before: little surface exposed to lipases After: much more surface exposed to lipases Figure 20.17 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Triglycerides  Monoglyceride + 2 Fatty acids Digestion by Lipases Some fatty acids and monoglycerides absorbed Others form micelles Equilibrium between micelles and fatty acids and monoglycerides formed by action of lipases Triglycerides  Monoglyceride + 2 Fatty acids Figure 2.4b Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Absorption of Monoglycerides and Fatty Acids Equilibrium between free fatty acids and monoglycerides and those in micelles Free form can be absorbed by simple diffusion across epithelium Inside epithelial cell Enter smooth ER and reform triglycerides and other lipids Lipids enter Golgi apparatus to be packaged into chylomicrons Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Lipase Activity Figure 20.18 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Enterohepatic Circulation Figure 20.19 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Absorption of Fat Figure 20.20 Lumen Monoglyceride Triglyceride Other lipids and proteins Golgi apparatus Fatty acid Smooth endoplasmic reticulum Enterocyte Blood Interstitial fluid Chylomicron Lacteal To general circulation Lymph Figure 20.20 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Absorption of Vitamins Fat-soluble vitamins (A, D, E, and K) Absorbed with lipids Dissolve in lipid droplets, micelles, chylomicrons Water-soluble vitamins Require special transport proteins Vitamin B12 Absorbed only when bound to intrinsic factor Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Absorption of Minerals: Na+, Cl-, and K+ Sodium Solvent drag with water reabsorption Actively absorbed in jejunum, ileum, and colon Chloride Passively follows sodium absorption Potassium Passively absorbed Secreted when lumenal concentrations very low Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Absorption of Minerals: Calcium Actively absorbed in duodenum and jejunum Binds to brush border protein = calcium-binding protein Transported into epithelial cell Transported out of cell across basolateral membrane by Ca2+ pump 1,25-(OH)2D3 increases calcium absorption by increasing concentration of calcium-binding protein Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Absorption of Minerals: Iron Transferrin secreted by enterocytes into lumen of small intestine Transferrin binds iron Transferrin-iron complex binds receptor Taken into cell by receptor-mediated endocytosis Some iron stored in enterocyte as ferritin Some iron transported into blood Transported in blood bound to transferrin Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Absorption and Secretion of Bicarbonate Jejunum Bicarbonate ions passively absorbed Ileum and colon Bicarbonate secreted in exchange for chloride ions Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Absorption of Water Water in GI tract Water absorption is passive 7 liters secretions 2 liters intake Water absorption is passive Water follows absorption of solutes by osmosis Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

GI Regulation Regulation of GI function is not based on the concept of homeostasis Regulate GI function to maximize absorption, regardless of whether nutrients are needed Regulate conditions in lumen of GI tract to maximize absorption Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

GI Hormones and Actions Table 20.2 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Neural and Endocrine Control Figure 20.21 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Phases of Gastrointestinal Control Cephalic phase—stimuli originate in head Thoughts, taste, and smell Requires input from CNS (long reflexes) Gastric phase—stimuli originate in stomach Long and short reflex and GI hormones Intestinal phase—stimuli originate in small intestine Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Short-Term Regulation of Food Intake Short-term regulation: hunger versus satiety Satiety factors Insulin CCK Neural input from mechano- and chemo-receptors Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Long-Term Regulation of Food Intake Leptin = hormone released from adipose cells when calories exceeds demands Leptin suppresses hunger and increases metabolism Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Saliva Secretion Taste and texture of food Mechanoreceptors and taste receptors in mouth Salivary center of medulla Autonomic nervous system Stimulate salivation Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Gastric Acid Secretion Parietal cells produce hydrochloric acid Carbonic anhydrase catelyzes production of bicarbonate and H+ H+ are actively secreted into lumen of stomach Bicarbonate is transported into intersitital fluid for Cl- Cl- diffuses into lumen of stomach through channels Figure 20.22 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Gastric Secretion: Cephalic Phase Figure 20.23a Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Gastric Secretion: Gastric Phase Figure 20.23b Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Pancreatic Juice Secretion Acinar cells Small volume of primary secretion Water, electrolytes, and digestive enzymes Duct cells Large volume Bicarbonate rich secretion Figure 20.8 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Bicarbonate-Rich Fluid Secretion Figure 20.24 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Regulation of Bile Entry Figure 20.25 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Rates of Fluid Movement Fluid entering lumen daily Ingested = 2000 mL Saliva = 1500 mL Gastric juice = 2000 mL Bile = 500 mL Pancreatic juice = 1500 mL Intestinal secretions = 1500 mL Total = 9000 mL Fluid leaving lumen daily Intestinal absorption = 8500 mL Colon absorption = 400 mL Total = 8900 mL Amount excreted in feces = 9000 mL – 8900 mL = 100 mL Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Fluid Flows in the GI System Figure 20.26 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

VII. Gastrointestinal Motility and Its Regulation Electrical activity in gastrointestinal smooth muscle Peristalsis and segmentation Chewing and swallowing Gastric motility Motility of the small intestine Motility of the colon Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

GI Motility GI motility Movements of the wall of the GI tract Due primarily to the contractions of the muscularis externa (outer muscle layers) Function: to mix and propel Figure 20.3 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

GI Smooth Muscle Activity Figure 20.27 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Peristalsis Figure 20.28a Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Segmentation Figure 20.28b Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Phases of Regulation Cephalic phase, excitations Anger, aggression Cephalic phase, inhibitions Pain, fear, depression Gastric phase, excitatory stimulus Distension of stomach Intestinal phase, inhibitory stimuli Distension of duodenum Contents: osmolarity, acidity, fat Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.