The Digestive System General anatomy & digestive processes Mouth through esophagus Stomach Liver, gallbladder & pancreas Small intestine Chemical digestion & absorption Large intestine Subdivisions of the Digestive System Digestive system (GI tract) Alimentary Canal 30 foot long tube extending from mouth to anus oral cavity, pharynx, esophagus, stomach, small intestine, large intestine Accessory organs teeth, tongue, liver, gallbladder, pancreas, salivary glands Relationship to the Peritoneum Open to the environment Undigested food is technically never ?in? the body Most digestive organs are in the peritoneal cavity Only duodenum, pancreas and part of the large intestine are retroperitoneal Peritoneum - moist serous membrane Parietal peritoneum - outer membrane Visceral peritoneum ? inner membrane Dorsal mesentery - suspends GI tract and forms serosa (visceral peritoneum) of stomach and intestines Ventral mesentery - forms lesser and greater omentum lacy layer of connective tissue that contains lymph nodes, lymphatic vessels, blood vessels Lesser and Greater Omentum Lesser - attaches stomach to liver Greater - covers small intestines like an apron Mesentery and Mesocolon Mesentery of small intestines holds many blood vessels Mesocolon anchors colon to posterior body wall Digestive Functions & Processes Functions ingestion = selective intake of food digestion = breakdown of food into smaller molecules absorption = uptake of nutrients into blood or lymph defecation = elimination of undigested material 3 Processes motility = muscular contractions that break up food, mix it with enzymes & move it along secretion = digestive enzymes & hormones membrane transport = absorption of nutrients Stages of Digestion 2 stages of digestion Mechanical digestion - physical breakdown of food into smaller particles cutting & grinding of teeth churning action of stomach & intestines Chemical digestion - hydrolysis reactions enzymes from saliva, stomach, pancreas & intestines break macromolecules into their monomers polysaccharides into monosaccharides proteins into amino acids fats into glycerol and fatty acids nucleic acids into nucleotides The Mouth or Oral Cavity Features of the Oral Cavity Mouth, Oral Cavity or Buccal Cavity Anterior opening is oral orifice Posterior opening to throat is the fauces Cheeks and lips keep food between teeth for chewing essential for speech & suckling in infants Tongue is sensitive, muscular manipulator of food papillae & taste buds on dorsal surface lingual glands secrete saliva, tonsils in root Hard & soft palate allow breathing & chewing at same time Dentition Baby teeth (20) by 2 years; Adult (32) between 6 and 25 Permanent and Deciduous Teeth Periodontal ligament is modified periosteum anchors into bone Cementum and dentin are living tissue Enamel is noncellular secretion formed during development Root canal leads into pulp cavity nerves and blood vessels Gingiva - gums Tooth Structure Functions of the Mouth Ingestion Taste mastication - chewing breaks food into smaller pieces to be swallowed mixes more easily with digestive enzymes Begins chemical digestion of starch Deglutition - swallowing Speech Respiration Saliva Total of 1 to 1.5 L of saliva per day* Hypotonic solutions of 99.5% water and solutes: amylase = begins starch digestion Lingual lipase = digests fat after reaches the stomach must be activated by HCl in stomach mucus = aids swallowing by lubricating & binding food lysozyme = enzyme that kills bacteria IgA = antibodies to inhibit bacterial growth electrolytes = Na+, K+, Cl-, phosphate & bicarbonate pH of 6.8 to 7.0 poop = jason Functions of Saliva Moistens Mouth Partially digests starch - salivary amylase Inhibits bacterial growth Dissolves molecules to stimulate taste buds Moistens food Aids swallowing by binding food together into bolus Intrinsic Salivary Glands Indefinite number dispersed throughout oral tissue buccal - cheeks labial - lips lingual - tongue Secrete continuously at a constant rate Contains lysozyme & lingual lipase Salivary Glands 3 pairs extrinsic glands connected to oral cavity by ducts Parotid, submandibular and sublingual mucous glands secrete mucous serous glands secrete amylase & electrolytes Pharynx Throat - skeletal muscle Longitudinal and circular sheaths Superior, middle & inferior constrictors Force food downward while swallowing Inferior constrictor remains constricted when food is NOT being swallowed Excludes air from the esophagus Tissue Layers of the GI Tract Mucosa epithelium lamina propria - loose connective tissue muscularis mucosae - thin layer of smooth muscle Submucosa Muscularis externa inner circular layer outer longitudinal layer Adventitia (above) or Serosa (below diaphragm) areolar tissue or mesothelium Tissue Layers of GI Tract The Esophagus Dorsal to trachea Straight muscular tube 25-30 cm long skeletal muscle in upper part & smooth in bottom Extends from pharynx to cardiac stomach Runs through the mediastinum of the thoracic cavity Passes through esophageal hiatus in the diaphragm Opening in the diaphragm Lower esophageal sphincter closes orifice to reflux Enters stomach at the cardiac orifice Swallowing Swallowing or Deglutition Series of muscular contractions coordinated by swallowing center in medulla & pons Buccal phase tongue collects food & pushes it back into oropharynx Pharyngeal-esophageal phase soft palate rises & blocks nasopharynx infrahyoid muscles lift larynx & epiglottis is folded back pharyngeal constrictors push bolus down esophagus liquids reach stomach in 2 seconds food bolus may take 8 seconds X ray of Swallowing in Esophagus Stomach Muscular sac in upper left abdominal cavity Below the diaphragm J - shaped organ with lesser & greater curvatures rugae - wrinkles 50 ml when empty 1 to 1.5 L after a typical meal Maximum of 4 L When extremely full, the stomach will extend into the pelvis Functions of the Stomach Mechanically breaks up food particles Liquifies the food resulting soupy mixture is called chyme Begins chemical digestion of protein & fat Stomach does not absorb any significant amount of nutrients does absorb aspirin & some lipid-soluble drugs All blood drained from stomach is filtered through the liver before returning to heart 4 Regions of the Stomach Cardiac region - just inside cardiac orifice Fundus - domed portion superior to esophageal opening Body - main portion of organ also called the gastric region Pyloric region - narrow inferior end antrum ? funnel pyloric canal ? terminates at pyloris Pylorus is opening to duodenum thick ring of smooth muscle forms a pyloric sphincter Gross Anatomy of Stomach Notice: bulge of fundus, narrowing of pyloric region, thickness of pyloric sphincter, greater & lesser curvatures Gross Anatomy of Stomach Unique Features of Stomach Wall Mucosa simple columnar glandular epithelium lamina propria is filled with tubular glands (gastric pits) Muscularis externa has 3 layers outer longitudinal, middle circular & inner oblique layers Cells of the Gastric Glands Mucous cells ? produce mucus Regenerative cells - divide rapidly to produce new cells that migrate upwards towards surface Parietal cells - secrete HCl acid & intrinsic factor Chief cells secrete chymosin (rennin) & lipase in infancy secrete pepsinogen throughout life Enteroendocrine (G) cells secrete hormones & paracrine messengers Opening of Gastric Pit Gastric Secretions 2 to 3 L of gastric juice per day Mostly H2O, HCl & pepsin Hydrochloric acid Stomach acid with pH as low as 0.8 Activates enzymes - pepsin & lingual lipase Breaks up connective tissues & plant cell walls Liquifies food to form chyme Converts iron to usable forms ingested ferric ions (Fe+3) to ferrous ions (Fe+2) that can be absorbed & utilized for hemoglobin synthesis Destroys ingested bacteria & pathogens Pepsin Partially digests protein Is first secreted by chief cells as pepsinogen (inactive) Pepsinogen is a zymogen zymogens are inactive proteins that must have some amino acids removed in order to convert to the active form Amino acids from pepsinogen are removed by HCl Pepsinogen + HCl ? pepsin Autocatalytic process ? since pepsin digests protein, once activated it can act upon pepsinogen to produce more pepsin (by removing amino acids) Gastric Enzymes & Intrinsic Factor Intrinsic factor secreted by parietal cells (less with aging) essential for absorption of B12 by small intestine necessary for RBC production deficiency results in pernicious anemia Gastric lipase lipase digests butterfat of milk in infant Chymosin (Rennin) curdles milk by coagulating its proteins Chemical Messengers 20+ Gastrin stimulates HCl and enzyme secretions stimulates intestinal motility relaxes iliocecal valve (in btwn small & large intestine) Seratonin ? stimulates gastric motility Histamine ? stimulates HCl secretion Somatostatin inhibits gastric secretions and motility inhibits pancreatic and gall bladder secretions released in between meals (paracrine secretion) Gastric Motility Swallow Swallowing center sends impulses to stomach Stomach relaxes Food arrives Activates a receptive-relaxation response resists stretching briefly, but relaxes to hold more food Peristaltic contractions begin (in esophagus) controlled by pacemaker cells in longitudinal muscle layer of muscularis externa gentle ripple of contraction every 20 seconds churns & mixes food with gastric juice stronger as reaches pyloric region squirting out 3 mL Gastric Excretion 3 ml of chyme enters the duodenum of the small intestine at one time Typical meal is emptied from stomach in 4 hours A liquid meal is emptied much sooner A meal high in fat leaves the stomach in ~6 hours Vomiting Induced by excessive stretching of stomach psychological stimuli chemical irritants (bacterial toxins) Emetic center in medulla causes lower esophageal sphincter to relax as diaphragm & abdominal muscles contract contents forced up the esophagus may even expel contents of small intestine Protection of the Stomach Mucous coat ? alkaline Epithelial cell replacement Cells live just 3 to 6 days (fastest replaced cells of the body) Tight junctions Prevent gastric juices from seeping between epithelial cells Peptic Ulcers HCl and pepsin erode stomach wall Can occur in duodenum and esophagus as well Sometimes caused by - smoking, aspirin Treatment ? reduce acidity Now know that many ulcers are caused by bacteria Helicobacter pylori Acid-resistant bacterium that invades the mucosa Treatment: antibiotics Acidophile Linked to stomach cancer Regulation of Gastric Secretion Cephalic Phase Stomach is being controlled by the brain Sight, smell, taste or thought of food Hypothalamus sends signal to medulla Medulla signals vagus nerves Vagus nerves stimulates enteric (digestive) nervous system of the stomach Enteric nerves stimulate gastric secretions prior to swallowing Regulation of Gastric Function (Phase 1) Regulation of Gastric Function (Phase 2) Gastric phase stomach controls itself stretches as food arrives activated by presence of food or semidigested protein secretion stimulated by acetylcholine, histamine & gastrin Regulation of Gastric Function (Phase 3) Intestinal phase duodenum of small intestine regulates gastric activity through hormones & nervous reflexes gastric activity increases if duodenum is stretched or amino acids in chyme cause gastrin release Enterogastric reflex = duodenum inhibits stomach Chyme stimulates duodenal cells to release Secretin Cholecystokinin (CCK) gastric inhibitory peptide all 3 suppress gastric secretion & motility Gross Anatomy of Liver 3 lb. organ located inferior to the diaphragm 4 lobes ? right, left, quadrate & cuadate falciform ligament separates left and right round ligament is remnant of umbilical vein Gall bladder adheres to ventral surface between right and quadrate lobes Inferior Surface of Liver Microscopic Anatomy of Liver Tiny cylinders called hepatic lobules (2mm by 1mm) Central vein surrounded by sheets of hepatocyte cells separated by sinusoids lined with fenestrated epithelium Blood filtered by hepatocytes on way to central vein nutrients, toxins, bile pigments, drugs, bacteria & debris filtered Histology of Liver -- Hepatic Triad 3 structures found in corner between lobules - hepatic triad hepatic portal vein and hepatic artery bring blood to the liver Bile ductile collects bile from bile canaliculi between sheets of hepatocytes to be secreted from liver in hepatic ducts Liver Functions Filters blood ? removes glucose Amino acids Iron Vitamins Hormones Toxins Bile pigments Drugs Secretes into blood albumin, lipoproteins, clotting factors, angiotensinogen, other proteins Breaks down stored glycogen Produces and secretes bile Ducts of Gallbladder, Liver and Pancreas Pathway of Bile bile secreted into bile canaliculi Bile ductules right & left hepatic ducts join outside the liver form common hepatic duct joins cystic duct from gallbladder forms common bile duct joins pancreatic duct forms hepatopancreatic ampulla empties into duodenum at major duodenal papilla hepatopancreatic sphincter - regulates release of bile & pancreatic juice The Gallbladder Sac on underside of liver ~10 cm long Stores bile releases bile in response to lipids in the duodenum Concentrates bile bile backs up into gallbladder from a filled bile duct between meals, bile is concentrated by factor of 20 absorbs water & electrolytes Bile Yellow-green fluid secreted by liver 0.5 - 1L bile secreted daily Serves in the digestion of fats Contains minerals, bile acids, cholesterol, bile pigments & phospholipids Bilirubin - main pigment from hemoglobin breakdown metabolized by intestinal bacteria ? urobilinogen urobilinogen = brown color of feces bile acids (salts) emulsify fats Bile Acids Synthesized from cholesterol Most reabsorbed in the small intestine & recycled Some modified in the large intestine Become promoters of colon cancer Less fat in the diet = less bile release Typical Western Diet High fat & low fiber content Associated with colon cancer High incidence: US, Germany, Austria, Sweden Countries with low fat and high fiber diets have less incidence of colon cancer Studies show that Asian men in US eating high fat/low fiber diet have colon cancer incidence comparable to Caucasian men Fiber binds bile acids! Less fat absorbed Bile acids in large intestine are unavailable for modification Low colon cancer rate in Finland despite a high fat diet Finish eat lots of brown bread Gall Stones (Biliary calculi) Composed of cholesterol, calcium carbonate, bilirubin Up to 1 cm in diameter Cause great pain Block bile flow jaundice Poor fat digestion Impaired fat-soluble vitamin absorption (ie Vitamin E) Treatments Surgical removal, stone-dissolving drugs, lithotripsy Stent ? tube in bile duct that increases diameter thus allowing small stones to pass Gross Anatomy of Pancreas Retroperitoneal gland posterior to stomach Head, body, and tail Endocrine and exocrine gland secretes insulin & glucagon into the blood Secretes 1500 mL pancreatic juice/day into duodenum water, enzymes, zymogens, electrolytes and sodium bicarbonate zymogens are inactive until converted by other enzymes other pancreatic enzymes are activated by exposure to bile and ions in the intestine Pancreatic duct runs length of gland to open at hepatopancreatic sphincter Accessory duct opens independently on duodenum Pancreatic Acinar Cells Zymogens = proteases (digest proteins) Trypsinogen ? trypsin Chymotrypsinogen ? chymotrypsin Procarboxypeptidase ? carboxypeptidase Other enzymes Pancreatic amylase - digests starch Pancreatic lipase - digests fats Ribonuclease - digests RNA Deoxyribonuclease Digests DNA Activation of Zymogens Trypsinogen converted to trypsin by intestinal epithelium Trypsin converts other 2 as well as digests dietary protein Hormonal Control of Secretion Cholecystokinin (CCK) - released from duodenum in response to arrival of acid and fat causes contraction of gallbladder (bile ? fat) secretion of pancreatic enzymes relaxes hepatopancreatic sphincter Secretin - released from duodenum in response to acidic chyme stimulates all ducts to secrete sodium bicarbonate* (neutralizes the stomach) Gastrin ? secreted from stomach & duodenum weakly stimulates gallbladder contraction & pancreatic enzyme secretion Small Intestine Small Intestine Most chemical digestion and nutrient absorption 6-7 m long in cadaver 2 m long in live person due to muscle tone Highly folded Villi & microvilli increase surface area Gross Anatomy of Small Intestine Duodenum curves around head of pancreas retroperitoneal along with pancreas (~10 in.) receives stomach contents, pancreatic juice & bile neutralizes stomach acids emulsifies fats inactivates pepsin by pH increase pancreatic enzymes present Jejunum is next 8 ft. (in upper abdomen) absorption of nutrients Ileum is last 12 ft. (in lower abdomen) ends at ileocecal junction with large intestine Circular folds - in mucosa and submucosa chyme flows in spiral path causing more contact slows chyme for maximum digestion and absorption Large Surface Area of Small Intestine Villi - fingerlike projections 1 mm tall contain blood vessels & lymphatics (lacteal) nutrient absorption Microvilli - 1 micron tall brush border on cells brush border enzymes for final stages of digestion Pores opening between villi lead to intestinal crypts absorptive cells, goblet cells & at base, rapidly dividing cells life span of 3-6 days as migrate up to surface & get sloughed off & digested paneth cells with unknown function Brunner?s glands in submucosa secrete bicarbonate mucus Peyer patches are populations of lymphocytes to fight pathogens Secrete 1-2 L of intestinal juice/day in small intestine water & mucus, pH 7.4-7.8 Intestinal Crypts Intestinal Villi Villi of Jejunum Histology of duodenum Intestinal Motility Mixes chyme with intestinal juice, bile & pancreatic juice Churns chyme to increase contact with mucosa for absorption & digestion Moves residue towards large intestine segmentation random ringlike constrictions mix & churn contents 12 times per minute in duodenum Peristaltic waves begin in duodenum but each one moves further down (?the wave?) suppressed by refilling of stomach Food in stomach causes gastroileal reflex relaxing of iliocecal valve & filling of cecum Segmentation in the Small Intestine Purpose of segmentation is to mix & churn not to move material along as in peristalsis Peristalsis Gradual movement of contents towards the colon Migrating motor complex controls waves of contraction second wave begins distal to where first wave began Carbohydrate Digestion in Small Intestine act upon oligosaccharides Maltose Sucrose Lactose Fructose lactose indigestible after age 4 in most humans due to a lack of lactase Salivary amylase stops working in 4.5 pH acidic stomach 50% of dietary starch digested before it reaches small intestine Pancreatic amylase completes first step in 10 minutes Brush border enzymes Carbohydrate Absorption Sodium-glucose transport proteins (SGLT) in membrane help absorb glucose and galactose Fructose absorbed by facilitated diffusion then converted to glucose inside the cell Protein Digestion and Absorption Pepsin has optimal pH of 1.5 to 3.5 inactivated when passes into duodenum and mixes with alkaline pancreatic juice (pH 8) Pancreatic enzymes take over protein digestion hydrolyze polypeptides into shorter oligopeptides Protein Digestion and Absorption Brush border enzymes finish task, producing amino acids that are absorbed into intestinal epithelial cells amino acid cotransporters move into epithelial cells facilitated diffusion moves amino acids out into blood stream Infants absorb proteins by pinocytosis (maternal IgA enters blood) Fat Digestion and Absorption Fat Digestion and Absorption Fat Digestion and Absorption Nucleic Acids, Vitamins, and Minerals Nucleases hydrolyze DNA & RNA to nucleotides brush border enzymes split them into phosphate ions, ribose or deoxyribose sugar & nitrogenous bases Vitamins are absorbed unchanged A, D, E & K with other lipids B complex & C by simple diffusion B12 if bound to intrinsic factor Minerals are absorbed all along small intestine Na+ cotransported with sugars & amino acids Cl- exchanged for bicarbonate reversing stomach Iron & calcium absorbed as needed Water Balance Digestive tract receives about 9 L of water/day .7 L in food, 1.6 L in drink, 6.7 L in secretions 8 L is absorbed by the small intestine & 0.8 L by the large intestine Water is absorbed by osmosis following the absorption of salts & organic nutrients Diarrhea occurs when too little water is absorbed feces pass through too quickly if irritated (by chemical or bacteria) feces contains high concentrations of a solute (lactose) Anatomy of Large Intestine Gross Anatomy of Large Intestine 5 feet long and 2.5 inches in diameter in cadaver Called ?large? because of diameter, not length Haustra (pleural)/ Haustrum (singular) pouches due to strips of longitudinal muscle Begins as cecum & appendix Ascending, transverse and descending colon Sigmoid colon leading down into pelvis Rectum ? 3 folds/valves allow retention of feces while passing gas Anal canal (3 cm) ? Anus (opening) Cecum Cecum ? large pouch located inferior to iliocecal junction Appendix? located at the lower end of the cecum rich in lymphocytes to collect pathogens may become infected if perforates (bursts) can cause life-threatening infection in the peritoneal cavity Cecum Bacterial Flora & Intestinal Gas Bacterial flora populate large intestine diet dictates what type of bacteria thrive ferment cellulose & other undigested carbohydrates synthesize vitamins B and K humans don?t get enough from diet alone! Flatus/Flatulence (gas) average person produces 500 mL per day most is swallowed air can contain methane, hydrogen sulfide, indole & skatole that produce the odor (produced by bacteria living in intestines) Absorption and Motility Transit time is 12 to 24 hours reabsorbs water and electrolytes Feces consist of water & solids bacteria, mucus, undigested fiber, fat & sloughed epithelial cells Haustral contractions occur every 30 minutes distension of a haustrum stimulates it to contract Mass movements occur 1 to 3 times a day triggered by gastrocolic and duodenocolic reflexes filling of the stomach & duodenum stimulates motility moves residue for several centimeters with each contraction Anatomy of Anal Canal Anal canal is 3 cm total length Anal columns are longitudinal ridges separated by mucus secreting anal sinuses Hemorrhoids are permanently distended veins Defecation Stretching of the rectum stimulates defecation intrinsic defecation reflex via the myenteric plexus causes muscularis to contract & internal sphincter to relax relatively weak contractions defecation occurs only if external anal sphincter is voluntarily relaxed parasympathetic defecation reflex involves spinal cord stretching of rectum sends sensory signals to spinal cord splanchnic nerves return signals intensifying peristalsis Abdominal contractions increase abdominal pressure as levator ani lifts anal canal upwards feces will fall away Neural Control of Defecation 1. Filling of the rectum 2. Reflex contraction of rectum & relaxation of internal anal sphincter 3. Voluntary relaxation of external sphincter Colon Cancer 3rd leading cause of cancer deaths in US Always begins as a benign polyp 1st colonoscopy recommended at 50 years of age earlier if there is a family history Some are genetically predisposed to certain types of colon cancer Majority of cases are caused by environmental factors CPS Which of the following would be considered a ?periodontal? tissue? a) dentin b) cementum c) pulp cavity d) gingiva (peri - around) e) root canal Fat digestion begins in the mouth. a) true b) false If you have your gall bladder removed, you will no longer be able to digest fats? a) true b) false Which of the following cells secretes digestive enzymes? a) mucous b) chief c) parietal (Hcl, not enzyme) d) enteroendocrine e) goblet Which of the following is NOT an enzyme? a) amylase (carbs) b) pepsin (protein) c) chymosin (milk) d) gastric lipase (milk) e) secretin Which of the following would NOT be a complication of hepatic cirrhosis? a) jaundice b) low blood osmolarity c) impaired protein digestion d) impaired protein synthesis e) impaired blood clotting The small intestine has an alkaline pH because a) bicarbonate ions are secreted from the pancreatic and bile ducts b) Cl- ions are released from the Hcl of the stomach. c) the duodenum secreted H+ d) parietal cells secrete mucus e) of the high concentration of CCK Removal of which of the following would have the most severe effect on the body? a) gall bladder b) stomach c) teeth d) jejunum (without this, wouldn't be able to absorb nutrients) e) appendix
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