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The Digestive system • The alimentary canal or gastrointestinal (GI) tract digests and absorbs food • Alimentary canal – mouth, pharynx, esophagus, stomach, small intestine, and large intestine • Accessory digestive organs – teeth, tongue, gallbladder, salivary glands, liver, and pancreas Organization of the Digestive tract • Mucosa lines digestive tract (mucous epithelium) – Moistened by glandular secretions – Lamina propria and epithelium form mucosa • Submucosa - layer of dense irregular connective tissue • Muscularis externa - smooth muscle arranged in circular and longitudinal layers • Serosa - serous membrane covering most of the muscularis externa Digestive System Anatomy Figure 21-2c Digestive System Anatomy Digestive System Activities • The GI tract is a “disassembly” line • Nutrients become more available to the body in each step – Ingestion – taking food into the digestive tract – Mechanical digestion – chewing, churning food, segmentation – Propulsion – swallowing and peristalsis – Chemical digestion – catabolic breakdown of food – Absorption – movement of nutrients from the GI tract to the blood or lymph – Excretion – elimination of indigestible solid wastes Basic Processes of the Digestive System Figure 21-1 Motility • Movement of digestive materials • Visceral smooth muscle – Tonic contractions • Sustained • Smooth muscle sphincters and stomach – Phasic contractions • • • • Rhythmic cycles of activity - pacemaker cells Last a few seconds Peristalsis moves bolus forward Segmentation mixes Autonomous Smooth Muscle • Pacesetter cells • Slow wave potentials – digestive tracts’s basic electrical rhythm • Wavelike fluctuations in membrane potential • Transmitted throughout smooth muscle via gap junctions • Threshold is reached by the effect of various mechanical, neural and hormonal factors Motility • Peristalsis - waves that move a bolus Motility • Segmentation – a churn and fragment a bolus Ingestion and Mechanical Digestion • • • • Food is ingested Mechanical digestion begins (chewing) Propulsion is initiated by swallowing Salivary amylase begins chemical breakdown of starch • The pharynx and esophagus serve as conduits to pass food from the mouth to the stomach • Uvula guards opening to pharynx The Oral Cavity • Mechanical processing by the teeth, tongue, and palatal surfaces • Lubrication • Assistance in swallowing • Limited digestion Figure 24.6a, b Salivary glands (three pairs) • Parotid, sublingual, and submandibular glands produce saliva – Stimulated by thought of food or ingested food – Secreted from serous and mucous cells of salivary glands – Watery solution includes electrolytes, buffers, glycoproteins, antibodies, enzymes • Functions include: – Lubrication, moistening, and dissolving – Initiation of digestion of complex carbohydrates • Strong sympathetic stimulation inhibits salivation and results in dry mouth Swallowing (deglutition) Process • Involves the coordinated activity of the tongue, soft palate, pharynx, esophagus and 22 separate muscle groups • Three phases – Buccal phase – tongue pushes bolus against soft palate and forced into the oropharynx, triggering swallowing reflex – controlled by the medulla and lower pons – Pharyngeal – esophogeal sphincter relaxes while epiglottis closes – Esophageal – bolus moves into esophagus propelled by peristalsis and into stomach Figure 24.11a-h Functions of the stomach • Holds ingested food • Degrades this food both physically and chemically • Delivers chyme to the small intestine • Enzymatically digests proteins with pepsin • Secretes intrinsic factor required for absorption of vitamin B12 Stomach Lining • • The stomach is exposed to the harshest conditions in the digestive tract To keep from digesting itself, the stomach has a mucosal barrier with: – A thick coat of bicarbonate-rich mucus on the stomach wall – Epithelial cells that are joined by tight junctions – Gastric glands that have cells impermeable to HCl Glands of the Stomach • Gastric glands have a variety of secretory cells – Mucous neck cells – mucus – Parietal cells – HCl and intrinsic factor – Chief cells – pepsinogen • Pepsinogen is activated to pepsin by HCl in the stomach • Pepsin itself via a positive feedback mechanism – Enteroendocrine cells – gastrin, histamine, cholecystokinin (CCK Regulation of Gastric Secretion • Neural and hormonal mechanisms regulate the release of gastric juice • Stimulatory and inhibitory events occur in three phases – Cephalic (reflex) phase: prior to food entry – Gastric phase: once food enters the stomach – Intestinal phase: as partially digested food enters the duodenum Cephalic Phase • Cephalic phase prepares stomach to receive ingested material • Directed by CNS via the vagus nerve • Stimulated by sight, smell, taste, or thought of food • Accelerates gastric juices • Inhibited by loss of appetite, depression Gastric Phase • Enhanced secretion of gastric juices due to the arrival of food in the stomach – Homogenize and acidify chyme – Production of pepsinogen - digestion of proteins • Stimulated by • Stomach distension - activation of stretch receptors • Chemoreceptors detects - peptides, caffeine, and rising pH – Neural - plexuses – Hormonal - secretion of gastrin • Inhibitory events include: – A pH lower than 2 – Emotional upset that overrides the parasympathetic division Intestinal phase • Intestinal phase – release of hormones controls the rate of gastric emptying • Excitatory phase – distension of duodenum, presence of partially digested foods • Releases enterogastrones that inhibit gastric secretion: CCK, GIP, Secretin • Inhibited by low pH, presence of fatty, acidic, or hypertonic chyme, and/or irritants in the duodenum Release of Gastric Juice Figure 23.16 Regulation of HCl Secretion • HCl secretion is stimulated by ACh, histamine, and gastrin through second-messenger systems • Release of hydrochloric acid: – Is low if only one ligand binds to parietal cells – Is high if all three ligands bind to parietal cells • Antihistamines block H2 receptors and decrease HCl release Gastric Contractile Activity • Peristaltic waves move toward the pylorus at the rate of 3 per minute • This basic electrical rhythm (BER) is initiated by pacemaker cells • Most vigorous peristalsis and mixing occurs near the pylorus • Chyme is either: – Delivered in small amounts to the duodenum or – Forced backward into the stomach for further mixing Regulation of Gastric Emptying • Gastric emptying is regulated by: – The neural enterogastric reflex – Hormonal (enterogastrone) mechanisms • These mechanisms inhibit gastric secretion and duodenal filling • Carbohydrate-rich chyme quickly moves through the duodenum • Fat-laden chyme is digested more slowly causing food to remain in the stomach longer Gastrointestinal Hormones • Gastrin – Release is stimulated by presence of protein in stomach – Secretion inhibited by accumulation of acid in stomach • Acts in several ways to increase secretion of HCl and pepsinogen • Enhances gastric motility, stimulates ileal motility, relaxes ileocecal sphincter, induces mass movements in colon • Helps maintain well-developed, functionally viable digestive tract lining Gastrointestinal Hormones • Secretin – Presence of acid in duodenum stimulates release – Inhibits gastric emptying in order to prevent further acid from entering duodenum until acid already present is neutralized – Inhibits gastric secretion to reduce amount of acid being produced – Stimulates pancreatic duct cells to produce large volume of aqueous NaHCO3 secretion – Stimulates liver to secrete NaCO3 rich bile which assists in neutralization process – Along with CCK, is trophic to exocrine pancreas Gastrointestinal Hormones • CCK – Inhibits gastric motility and secretion – Stimulates pancreatic acinar cells to increase secretion of pancreatic enzymes – Causes contraction of gallbladder – Along with secretin, is trophic to exocrine pancreas – Implicated in long-term adaptive changes in proportion of pancreatic enzymes in response to prolonged diet changes – Important regulator of food intake • GIP – Glucose-dependent insulinotrophic peptide – Stimulates insulin release by pancreas Regulation of Gastric Emptying Figure 23.19 Digestion And Absorption In The Stomach • Preliminary digestion of proteins - pepsin • Permits digestion of carbohydrates • Very little absorption of nutrients – Some drugs, however, are absorbed Small intestine • Important digestive and absorptive functions – Secretions and buffers provided by pancreas, liver, gall bladder • Three subdivisions: – Duodenum – Jejunum – Ileum • Ileocecal sphincter transition between small and large intestine Small Intestine • Structural modifications of the small intestine wall increase surface area – Plicae circulares: deep circular folds of the mucosa and submucosa – Villi – fingerlike extensions of the mucosa – Microvilli – tiny projections of absorptive mucosal cells’ plasma membranes Small Intestine • The epithelium of the mucosa is made up of: – Absorptive cells and goblet cells – Enteroendocrine cells – Interspersed T cells called intraepithelial lymphocytes (IELs) • Cells of intestinal crypts secrete intestinal juice – Secreted in response to distension or irritation of the mucosa – Slightly alkaline and isotonic with blood plasma – Largely water, enzyme-poor, but contains mucus Small Intestine • Glands of the duodenum – Moisten chyme – Help buffer acids – Maintain digestive material in solution • Hormones – Secretin - produces alkaline buffers, increase bile by liver and pancreas – Cholecystokinin – increase pancreatic enzymes, stimulates contraction of gall bladder, reduces hunger sensation – GIP – stimulates release of insulin, inhibits gastric secretion and motility Activities of Major Digestive Tract Hormones Figure 24.22 The liver • The largest gland in the body • Performs metabolic and hematological regulation and produces bile • Histological organization – Lobules containing single-cell thick plates of hepatocytes – Lobules unite to form common hepatic duct – Duct meets cystic duct to form common bile duct The Liver • Hexagonal-shaped liver lobules are the structural and functional units of the liver • Composed of hepatocytes • Hepatocytes’ functions include: – Production of bile – Processing bloodborne nutrients – Storage of fat-soluble vitamins – Detoxification • Secreted bile flows between hepatocytes toward the bile ducts in the portal triads Composition of Bile • A yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes • Bile salts are cholesterol derivatives that: – Emulsify fat – Facilitate fat and cholesterol absorption – Help solubilize cholesterol • Enterohepatic circulation recycles bile salts • The chief bile pigment is bilirubin, a waste product of heme The Gallbladder • Thin-walled, green muscular sac on the ventral surface of the liver • Stores and concentrates bile by absorbing its water and ions • Releases bile via the cystic duct, which flows into the bile duct Regulation of Bile Release • Acidic, fatty chyme causes the duodenum to release: – Cholecystokinin (CCK) and secretin into the bloodstream • Bile salts and secretin transported in blood stimulate the liver to produce bile • Vagal stimulation causes weak contractions of the gallbladder • Cholecystokinin causes: – The gallbladder to contract – The hepatopancreatic sphincter to relax • As a result, bile enters the duodenum Regulation of Bile Release Figure 23.25 The Pancreas • • • Pancreatic duct penetrates duodenal wall Endocrine functions - insulin and glucagons Exocrine functions – Secretes pancreatic juice secreted into small intestine which breaks down all categories of foodstuff – Acini (clusters of secretory cells) contain zymogen granules with digestive enzymes Composition of Pancreatic Juice • Water solution of enzymes and electrolytes (primarily HCO3–) – Neutralizes acid chyme – Provides optimal environment for pancreatic enzymes • Enzymes are released in inactive form and activated in the duodenum • Examples include – Trypsinogen is activated to trypsin – Procarboxypeptidase is activated to carboxypeptidase • Active enzymes secreted – Amylase, lipases, and nucleases – These enzymes require ions or bile for optimal activity Regulation of Pancreatic Secretion • Secretin and CCK are released when fatty or acidic chyme enters the duodenum • CCK and secretin enter the bloodstream • Upon reaching the pancreas: – CCK induces the secretion of enzyme-rich pancreatic juice – Secretin causes secretion of bicarbonate-rich pancreatic juice • Vagal stimulation also causes release of pancreatic juice Regulation of Pancreatic Secretion Figure 23.28 Digestion in the Small Intestine • As chyme enters the duodenum: – Carbohydrates and proteins are only partially digested – No fat digestion has taken place • Digestion continues in the small intestine – Chyme is released slowly into the duodenum – Because it is hypertonic and has low pH, mixing is required for proper digestion – Required substances needed are supplied by the liver – Virtually all nutrient absorption takes place in the small intestine Motility in the Small Intestine • The most common motion of the small intestine is segmentation – It is initiated by intrinsic pacemaker cells (Cajal cells) – Moves contents steadily toward the ileocecal valve • After nutrients have been absorbed: – Peristalsis begins with each wave starting distal to the previous – Meal remnants, bacteria, mucosal cells, and debris are moved into the large intestine Control of Motility • Local enteric neurons of the GI tract coordinate intestinal motility • Cholinergic neurons cause: – Contraction and shortening of the circular muscle layer – Shortening of longitudinal muscle – Distension of the intestine • Other impulses relax the circular muscle • The gastroileal reflex and gastrin: – Relax the ileocecal sphincter – Allow chyme to pass into the large intestine Functions of the large intestine • Reabsorb water and compact material into feces • Absorb vitamins produced by bacteria • Store fecal matter prior to defecation Functions of the Large Intestine • Other than digestion of enteric bacteria, no further digestion takes place • Vitamins, water, and electrolytes are reclaimed • Its major function is propulsion of fecal material toward the anus • Though essential for comfort, the colon is not essential for life Motility of the Large Intestine • Haustral contractions – Slow segmenting movements that move the contents of the colon – Haustra sequentially contract as they are stimulated by distension • Presence of food in the stomach: – Activates the gastrocolic reflex – Initiates peristalsis that forces contents toward the rectum The Rectum • Last portion of the digestive tract • Terminates at the anal canal • Internal and external anal sphincters • Defecation reflex triggered by distention of rectal walls Defecation • Distension of rectal walls caused by feces: – Stimulates contraction of the rectal walls – Relaxes the internal anal sphincter • Voluntary signals stimulate relaxation of the external anal sphincter and defecation occurs Regulation of digestion • Intrinsic control by local centers – Autonomous smooth muscle pacesetter cells – Intrinsic nerve plexuses and sensory receptors • Extrinsic control – ANS – GI hormones Regulation of digestion • Mechano- and chemoreceptors respond to: – Stretch, osmolarity, and pH – Presence of substrate, and end products of digestion • They initiate reflexes that: – Activate or inhibit digestive glands – Mix lumen contents and move them along Nervous Control of the GI Tract • Intrinsic controls – Nerve plexuses near the GI tract initiate short reflexes – Short reflexes are mediated by local enteric plexuses (gut brain) • Extrinsic controls – Long reflexes arising within or outside the GI tract – Involve CNS centers and extrinsic autonomic nerves – Parasympathetic reflexes Enteric Nervous System • Composed of two major intrinsic nerve plexuses – Submucosal nerve plexus – regulates glands and smooth muscle in the mucosa – Myenteric nerve plexus – Major nerve supply that controls GI tract mobility • Segmentation and peristalsis are largely automatic involving local reflex arcs • Linked to the CNS via long autonomic reflex arc Control of the digestive system • Neural and hormonal mechanisms coordinate glands • Hormonal mechanisms enhance or inhibit smooth muscle contraction • Local mechanisms coordinate response to changes in pH or chemical stimuli Digestion And Absorption Of Nutrients • Disassembling organic food into smaller fragments • Hydrolyzing carbohydrates, proteins, lipids and nucleic acids for absorption Chemical Digestion: Carbohydrates • Begins in the mouth – Salivary and pancreatic enzymes catabolize into disaccharides and trisaccharides – Brush border enzymes catabolize into monosaccharides • Absorption of monosaccharides occurs across the intestinal epithelia • Absorption: via cotransport with Na+, and facilitated diffusion – Enter the capillary bed in the villi – Transported to the liver via the hepatic portal vein • Enzymes used: salivary amylase, pancreatic amylase, and brush border enzymes Chemical Digestion: Proteins • Low pH destroys tertiary and quaternary structure • Enzymes used include pepsin, trypsin, chymotrypsin, and elastase – Liberated amino acids are absorbed • Absorption: similar to carbohydrates • Enzymes used: pepsin in the stomach • Enzymes acting in the small intestine – Pancreatic enzymes – trypsin, chymotrypsin, and carboxypeptidase – Brush border enzymes – aminopeptidases, carboxypeptidases, and dipeptidases Lipid digestion and absorption • Lipid digestion utilizes lingual and pancreatic lipases – Bile salts improve chemical digestion by emulsifying lipid drops – Lipid-bile salt complexes called micelles are formed – Micelles diffuse into intestinal epithelia which release lipids into the blood as chylomicrons Chemical Digestion: Fats • Absorption: Diffusion into intestinal cells where they: – Combine with proteins and extrude chylomicrons – Enter lacteals and are transported to systemic circulation via lymph • Glycerol and short chain fatty acids are: – Absorbed into the capillary blood in villi – Transported via the hepatic portal vein • Enzymes/chemicals used: bile salts and pancreatic lipase Fatty Acid Absorption • Fatty acids and monoglycerides enter intestinal cells via diffusion • They are combined with proteins within the cells • Resulting chylomicrons are extruded • They enter lacteals and are transported to the circulation via lymph Chemical Digestion: Nucleic Acids • Absorption: active transport via membrane carriers • Absorbed in villi and transported to liver via hepatic portal vein • Enzymes used: pancreatic ribonucleases and deoxyribonuclease in the small intestines Absorption • Water - nearly all (95%) that is ingested is reabsorbed – Net osmosis occurs whenever a concentration gradient is established by active transport of solutes into the mucosal cells – Water uptake is coupled with solute uptake, and as water moves into mucosal cells, substances follow along their concentration gradients • Vitamins – Water soluble vitamins are absorbed by diffusion – Fat soluble vitamins are absorbed as part of micelles • Vitamin B12 requires intrinsic factor Electrolyte Absorption • Most ions are actively absorbed along the length of small intestine – Na+ is coupled with absorption of glucose and amino acids – Ionic iron is transported into mucosal cells where it binds to ferritin • Anions passively follow the electrical potential established by Na+ • K+ diffuses across the intestinal mucosa in response to osmotic gradients • Ca2+ absorption: – Is related to blood levels of ionic calcium – Is regulated by vitamin D and parathyroid hormone (PTH)