Bio 215 GI Tract Lecture 2

Bio 215 GI Tract Lecture 2 - VERTEBRATE PHYSIOLOGY...

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Unformatted text preview: VERTEBRATE PHYSIOLOGY VERTEBRATE PHYSIOLOGY BIOL 215 Summer 2009 Amy I. Bentz, VMD, DACVIM University of Pennsylvania GASTROINTESTINAL SYSTEM GASTROINTESTINAL SYSTEM (Guyton; Boron) GI Tract provides body with water, electrolytes and nutrients: Movement of food through the system Secretion of digestive juices, digestion of food Absorption of digestive products, water, elytes Circulation of blood through digestive organs to carry away absorbed substances Control by nervous and endocrine systems ANATOMY OF GI TRACT ANATOMY OF GI TRACT Mouth (parotid, salivary glands) Esophagus Stomach Duodenum (pancreas, liver, gallbladder) Jejunum Ileum Colon­tract.gif GI TRACT GI TRACT Starting from GI wall: Serosa (outermost) Longitudinal muscle layer Circular muscle layer Submucosa Mucosa (lumen) on6/6ch1/6ch1img/page4.jpg GI SMOOTH MUSCLE GI SMOOTH MUSCLE Smooth muscle in GI tract is electrically connected by gap junctions – so ions can travel from one cell to the next Travels rapidly along length of muscle bundle Each muscle bundle fuses with other bundles at many points – so functions as a syncytium AP initiated anywhere within the muscle mass will travel in all directions in the muscle GI SMOOTH MUSCLE GI SMOOTH MUSCLE Smooth muscle is excited by almost continual, slow intrinsic electrical activity along the membrane of the muscle fibers 2 basic types of electrical waves Slow waves (resting) Spikes (stimulated by stretch, Ach) SLOW WAVES SLOW WAVES Most GI contractions occur rhythmically Determined by frequency of slow waves of the smooth muscle membrane potential Slow undulating changes in the resting membrane potential, intensity varies between 5­15 mV Frequency ranges: 3 in stomach, 12 in duodenum, 8­9 in terminal ileum Cause of slow waves is not known SLOW WAVES SLOW WAVES Don’t cause muscle contraction by themselves But mainly control appearance of intermittent spike potentials The spike potentials excite most of the muscle contraction SPIKE POTENTIALS SPIKE POTENTIALS True AP Occur automatically when resting membrane potential is more + than ­40mV Normal resting potential in smooth muscle fibers of GI tract is bet. ­50 to ­60 mV Spike potentials last as long as 10­20 millisecond (10­40x longer than large nerve fibers) ch3/6ch3img/page5.jpg ACTION POTENTIALS ACTION POTENTIALS In nerve fibers, AP due to rapid entry of Na+ through sodium channels to fiber interior In GI smooth muscle, Ca+ enters with smaller number of Na+ (Ca+ Na+ channels) Much slower to open and close than rapid Na+ channels Longer duration of AP GI TRACT GI TRACT Enteric Nervous System Lies in wall of gut (esophagus to anus) Controls GI movement and secretion 2 plexuses: Myenteric plexus (GI movements) Submucosal plexus (GI secretion and local blood flow) Sympathetic and parasympathetic innervation­ GI TRACT GI TRACT Myenteric plexus (GI movements) Increased tone of gut wall Increased intensity of rhythmic contractions Increased velocity of excitatory wave conduction along gut wall – more rapid movement of peristaltic waves GI TRACT GI TRACT Submucosal plexus concerned with: Local intestinal secretion Local absorption Local contraction of submucosal muscle NEUROTRANSMITTERS NEUROTRANSMITTERS Acetylcholine most often excites GI activity Norepinephrine and epinephrine mostly inhibits GI activity­ sympathetic branch INNERVATION INNERVATION Parasympathetic Vagus nerves Portions closest to oral and anal areas Sympathetic Originate in spinal cord (T5­L2) Innervates all portions of the GI tract REFLEXES REFLEXES From gut to prevertebral sympathetic ganglia to gut Gastrocolic reflex Enterogastric reflex Signal from stomach to cause evacuation of colon Signal from colon and SI to inhibit stomach motility and secretion­ (when full, tells GI tract to slow down emptying) Colonoileal reflex Reflex from colon to inhibit emptying of ileal contents into colon CHOLECYSTOKININ CHOLECYSTOKININ Secreted by I cells in mucosa of duodenum and jejunum due to presence of breakdown products of fat, fatty acids and monoglycerides in intestinal contents Increases gallbladder contractility to expel bile into SI (bile helps emulsify fatty substances to be digested/absorbed) Moderately inhibits stomach motility (to allow fat digestion in upper intestinal tract SECRETIN SECRETIN Secreted by “S” cells in duodenal mucosa in response to acidic gastric juice from stomach emptying into duodenum GASTRIC INHIBITORY PEPTIDE GASTRIC INHIBITORY PEPTIDE Secreted by mucosa of upper intestinal tract mainly in response to fatty acids and aa Mildly decreases motor activity of stomach to slow emptying of gastric contents into duodenum when the upper SI is already overloaded with food MOVEMENT OF GI TRACT MOVEMENT OF GI TRACT 2 types of movement: Peristalsis – propels food through GI tract Mixing PERISTALSIS PERISTALSIS Stimulus is gut distension Stimulates enteric nervous system to contract gut wall Chemical or physical irritation of the epithelial lining of the gut is also a stimulus “Law of the Gut” Peristaltic reflex + anal direction of movement of peristalsis BLOOD VESSELS BLOOD VESSELS Splanchnic circulation Blood flowing through gut, spleen and pancreas flows nutrients into liver via portal vein screens for toxins etc. Decontaminated blood then enters hepatic veins to vena cava VILLUS VILLUS Countercurrent blood flow mechanism in villi (SI) Blood oxygen diffuses out of arterioles into adjacent venules without reaching tips of villi (as much as 80% oxygen) Diseases such as circulatory shock – oxygen deficit in tips of villi – villus tip or entire villus can disintegrate (blunted villi decreased absorption) /unit6/media/villus.structure.jpg BLOOD FLOW BLOOD FLOW Parasympathetic stimulation to stomach and lower colon increased blood flow (and also increases glandular secretion) Sympathetic stimulation leads to vasoconstriction of arterioles (e.g. exercise, fight or flight) temporarily Also constricts veins – can add 200­400 ml extra blood to circulation in a adult FOOD! FOOD! Mastication Cranial Nerve V(5) (muscles of mastication) Chew food to make it more easily digested and increase surface area available to enzymes) SWALLOWING SWALLOWING Voluntary phase: food at pharynx Initiates swallowing process Pharyngeal phase Involuntary, reflex Food passes from pharynx into esophagus Esophageal Involuntary Food passes from pharynx to stomach SWALLOWING Successive stages of swallowing are controlled by brainstem Medulla and lower pons: deglutition (swallowing center) Inhibits respiratory center of medulla at time of swallowing for swallowing to proceed ESOPHAGUS ESOPHAGUS Primary Peristalsis Continuation of peristaltic wave that begins in the pharynx and spreads to esophagus Food is transmitted to stomach in about 8­10 sec Secondary Peristalsis If primary peristalsis fails, secondary peristalsis takes over due to distension of esophagus MUSCLE MUSCLE Pharynx and upper 1/3 of esophagus is straited muscle So these areas are controlled by skeletal nerve impulses from glossopharyngeal and vagus nerves Lower 2/3 of esophagus is smooth muscle Also controlled by vagus nerve SPHINCTER SPHINCTER Lower end of esophagus is circular muscle Lower esophageal sphincter (gastroesophageal sphincter) Normally constricted to prevent reflux of gastric acid into esophageal lumen When peristaltic wave passes down esophagus, sphincter relaxes to allow propulsion of food into stomach STOMACH STOMACH Body (Guyton pg 731) Antrum Motor functions: 1. Storage of large amount of food 2. Mixing of food with gastric secretions to form chyme 3. Slow emptying of chyme into SI­ Dictionary/graphics/images/en/19223.jpg STOMACH STOMACH Digestive juices of the stomach are secreted by gastric glands (cover the body of the stomach) When stomach contains food, weak peristaltic waves occur every 15­20 sec Become stronger and push food towards pylorus (pyloric sphincter = distal opening of stomach) After food is mixed with gastric secretions, it is called chyme (semi­fluid) GASTRIC EMPTYING GASTRIC EMPTYING Stomach and duodenum control gastric emptying 1. Increased food volume in stomach increases emptying of stomach 2. Food entering duodenum slower stomach emptying 1. 2. If chyme entering duodenum has pH lower than 3.5 to 4, blocks more chyme from entering duodenum until chyme can be neutralized by pancreatic­ secretions Dictionary/graphics/images/en/19223.jpg Fatty substances lead to cholecystokinin release from jejunum to block stomach motility from gastrin SI MOVEMENTS SI MOVEMENTS Mixing contractions Propulsive contractions (peristaltic waves) Increases a lot after meal Chyme pushed to ileocecal valve; spread out along mucosa When portion of SI is distended with chyme, stretching of intestinal wall elicits localized contractions (segmentation of SI­ ex. Of foreign body with accordion like foldings) “Chop” chyme about 2­3x/min to mix it with SI secretions Foreign Body HORMONES HORMONES Gastrin, CCK, insulin, serotonin enhance intestinal motility Secretin, glucagon inhibit SI motility ILEOCECAL VALVE ILEOCECAL VALVE Prevents backflow of fecal contents from colon into SI Ileocecal sphincter – normally mildly constricted and slows emptying of ileal contents into cecum except immediately after a meal Chyme remains in ileum better absorption COLON COLON Primary Functions: Absorption of water and elytes from chyme to form solid stool Storage of fecal matter until it can be expelled Proximal ½ of colon: Absorption Distal ½ : Storage COLON MIXING MOVEMENTS COLON MIXING MOVEMENTS Longitudinal muscle of the colon is aggregated into 3 longitudinal strips (teniae coli) which contract Along with circular contractions – cause unstimulated portions of LI to bulge outward into baglike sacs (haustrations) So fecal matter in colon is mixed gradually to be exposed to LI surfaces for absorption REFLEXES REFLEXES Gastrocolic and duodenocolic reflexes from distended stomach and duodenum facilitate evacuation of colon Internal and external anal sphincters are constricted unless conscious signals inhibit the constriction THE LIVER THE LIVER (Guyton pg. 797­802) Many functions: Filters and stores blood Metabolize carbs, proteins, fats, hormones, foreign substances Formation of bile Stores vitamins and iron Formation of coagulation factors THE LIVER THE LIVER Largest organ in body 1.5 kg in average adult human Basic functional unit (liver lobule) Contains 50,000­100,000 individual lobules Does regenerate (organ donation) Constructed around central vein Empties into hepatic veins, then vena cava THE LIVER THE LIVER Lobule is made of many hepatic cellular plates (2 cells thick) Between adjacent cells lie small bile canaliculi Empty into bile ducts Present are also: Small portal venules (blood from GI tract via portal vein) Blood flows into hepatic sinusoids Hepatic arterioles­content/uploads/2007/06/villi.jpg THE LIVER (Guyton pg 798) THE LIVER (Guyton pg 798) Venous sinusoids are lined by: Hepatic cells Endothelial cells Beneath this lining, narrow tissue spaces called spaces of Disse Connect with lymphatic vessels to remove excess fluid Kupffer cells (reticuloendothelial cells) Macrophages able to phagocytize bacteria and other foreign material THE LIVER THE LIVER High blood flow and low vascular resistance (~27% of resting CO) Low pressure in portal vein leading into liver (9 mmHg) Low pressure in hepatic vein leading from liver into vena cava (~0 mmHg) CLINICALLY… CLINICALLY… Cirrhosis Fibrous tissue replaces normal parenchyma Fibrous tissue contracts around blood vessels Impedes portal blood flow Alcohol, toxins, viral disease Portal system can also be blocked by large clot THE LIVER THE LIVER Blood reservoir Liver is an expandable organ and can store a lot of blood (10% of body’s total volume) High pressure with right­sided heart failure liver expansionlarge abdomen (+ fluid leaking into abdomen: ascites) THE LIVER: METABOLISM THE LIVER: METABOLISM Carbohydrate Fat Protein THE LIVER: METABOLISM THE LIVER: METABOLISM Carbohydrate metabolism: Important in maintaining normal blood glucose concentration Storage of large amts of glycogen Conversion of galactose and fructose to glucose Gluconeogenesis Formation of many chemical cpds from intermediate products of metabolism THE LIVER: METABOLISM THE LIVER: METABOLISM Fat metabolism Most cells in body metabolize fat, but some parts are only performed in liver Oxidation of fatty acids to supply Energy for other body functions Synthesis of large quantities of cholesterol, phospholipids and most lipoproteins Most cholesterol (80%) is converted to bile salts, secreted in bile, rest is transported in lipoproteins Synthesis of fat from proteins and carbs THE LIVER: METABOLISM THE LIVER: METABOLISM Protein metabolism Deamination of aa Formation of urea for removal of ammonia from body fluids Formation of plasma proteins (e.g. albumin) Interconversions of some aa and synthesis of other compounds from aa THE LIVER THE LIVER Storage site for vitamins (A, D, B12) Storage site of iron (stored as ferritin) Forms many coagulation factors Removes toxins, meds, other compounds CLINICALLY… CLINICALLY… Biilirubin is end product of Hb degradation Red blood cells lysed (~120 days), Hb is phagocytized into globulin and heme biliverdin free bilirubin (gradually released from macrophages into plasma), combines with plasma albumin, transported though blood and interstitial fluids Free bilirubin is absorbed through hepatic cell membrane, converted then excreted into bile calaliculiintestines Bilirubin glucuronide Bilirubin sulfate­1.jpg CLINICALLY… CLINICALLY… Once in the intestines, ½ of conjugated bilirubin is converted by bacteria into urobilinogen Some is excreted (stercobilin)­ gives fecal matter brown color Some is resorbed back into blood, re­excreted by hepatocytes, to enter intestines again ~5% is excreted into urine (urobilinogen) JAUNDICE (ICTERUS) JAUNDICE (ICTERUS) Large amt of bilirubin in extracellular fluid Due to: Increased rbc destruction (hemolysis)­ sometimes from parasites, etc. destroyed acutely Bilirubin is in “free form” Obstruction of: Bile ducts Damage to hepatocytes­ live itself Bilirubin is in “conjugated” form vs. “free” No stercobilin (clay­colored stool), urobilinogen Next week… Next week… Endocrine and Renal Systems Guyton; Boron ...
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