Digestive system

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Digestive system

  1. 1. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 21 Digestive System Villi of the Small Intestine
  2. 2. Function of the Digestive System <ul><li>Ingestion </li></ul><ul><ul><li>Taking food and water into the mouth </li></ul></ul><ul><li>Break down the food </li></ul><ul><ul><li>Mechanical digestion: chewing, mixing, and churning food </li></ul></ul><ul><ul><li>Chemical digestion: digestive enzymes breakdown food </li></ul></ul><ul><li>Absorb nutrients </li></ul><ul><ul><li>Movement of nutrients from the GI tract to the blood or lymph </li></ul></ul><ul><li>Release of waste </li></ul><ul><ul><li>Elimination of indigestible solid wastes </li></ul></ul>
  3. 3. Histology of the Digestive Tract <ul><li>Two groups of organs within the digestive system </li></ul><ul><ul><li>The digestive tract or gastrointestinal (GI) tract is a muscular tube that winds through the body and is responsible for the digestion and absorption of food </li></ul></ul><ul><ul><ul><li>Oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, and anus </li></ul></ul></ul><ul><ul><li>The accessory digestive organs aid in the breakdown of foodstuffs </li></ul></ul><ul><ul><ul><li>Teeth, tongue, gallbladder, salivary glands, liver, and pancreas </li></ul></ul></ul>
  4. 4. Fig. 21.1
  5. 6. Histology of the Digestive Tract <ul><li>Tunics </li></ul><ul><ul><li>Three major functions: </li></ul></ul><ul><ul><ul><li>Secretion of mucus </li></ul></ul></ul><ul><ul><ul><li>Absorption of end products of digestion </li></ul></ul></ul><ul><ul><ul><li>Protection against infectious disease </li></ul></ul></ul><ul><ul><li>Mucus secretions: </li></ul></ul><ul><ul><ul><li>Protect digestive organs from digesting themselves </li></ul></ul></ul><ul><ul><ul><li>Ease food along the tract </li></ul></ul></ul><ul><ul><li>Stomach and small intestine mucosa contain: </li></ul></ul><ul><ul><ul><li>Enzyme-secreting cells </li></ul></ul></ul><ul><ul><ul><li>Hormone-secreting cells (making them endocrine and digestive organs) </li></ul></ul></ul>
  6. 7. Histology of the Digestive Tract <ul><li>The digestive tract is composed of four tunics: mucosa, submucosa, muscularis, and serosa or adventitia. </li></ul><ul><ul><li>Mucosa </li></ul></ul><ul><ul><ul><li>Innermost layer that lines the lumen of the alimentary canal </li></ul></ul></ul><ul><ul><ul><li>Consists of a mucous epithelium, a lamina propria, and a muscularis mucosae </li></ul></ul></ul><ul><ul><li>Submucosa </li></ul></ul><ul><ul><ul><li>Connective tissue layer containing the submucosal plexus (part of the enteric plexus), blood vessels, and small glands </li></ul></ul></ul><ul><ul><li>Muscularis </li></ul></ul><ul><ul><ul><li>Consists of an inner layer of circular smooth muscle and an outer layer of longitudinal smooth muscle </li></ul></ul></ul><ul><ul><ul><li>The myenteric plexus is between the two muscle layers </li></ul></ul></ul><ul><ul><li>Serosa or adventitia </li></ul></ul><ul><ul><ul><li>Forms the outermost layer of the digestive tract </li></ul></ul></ul>
  7. 8. Fig. 21.2
  8. 9. Histology of the Digestive Tract <ul><li>Enteric Nervous System </li></ul><ul><ul><li>Consists of nerve plexuses within the wall of the digestive tract </li></ul></ul><ul><ul><li>Serves the digestive tract and regulates digestive activity. It is composed of two major intrinsic nerve plexuses: </li></ul></ul><ul><ul><ul><li>Submucosal nerve plexus: regulates glands and smooth muscle in the mucosa </li></ul></ul></ul><ul><ul><ul><li>Myenteric nerve plexus: major nerve supply that controls GI tract mobility </li></ul></ul></ul>
  9. 10. Histology of the Digestive Tract <ul><li>Enteric Nervous System </li></ul><ul><ul><li>Three major types of enteric neurons </li></ul></ul><ul><ul><ul><li>Sensory neurons detect changes in the chemical composition of the digestive tract </li></ul></ul></ul><ul><ul><ul><li>Motor neurons stimulate or inhibit smooth muscle contraction and glandular secretion </li></ul></ul></ul><ul><ul><ul><li>Interneurons connect sensory to motor neurons </li></ul></ul></ul><ul><ul><li>Nervous regulation involves local reflexes in the ENS and CNS reflexes </li></ul></ul>
  10. 11. Peritoneum <ul><li>The peritoneum is a serous membrane that lines the abdominopelvic cavity and organs </li></ul><ul><li>Mesenteries are peritoneum that extend from the body wall to many of the abdominopelvic organs </li></ul><ul><li>Retroperitoneal organs are located behind the peritoneum </li></ul>Fig. 21.3
  11. 12. Oral Cavity <ul><li>Oral cavity or mouth </li></ul><ul><ul><li>Is bounded by lips, cheeks, palate, and tongue </li></ul></ul><ul><ul><li>Has the oral orifice as its anterior opening </li></ul></ul><ul><ul><li>Is continuous with the oropharynx posteriorly </li></ul></ul><ul><li>Divided into two regions </li></ul><ul><ul><li>Vestibule </li></ul></ul><ul><ul><ul><li>Space between the lips or cheeks and the alveolar processes, which contain the teeth </li></ul></ul></ul><ul><ul><li>Oral Cavity Proper </li></ul></ul><ul><ul><ul><li>Lies medial to the alveolar processes </li></ul></ul></ul><ul><li>To withstand abrasions: </li></ul><ul><ul><li>The mouth is lined with stratified squamous epithelium </li></ul></ul>
  12. 13. Oral Cavity <ul><li>Lips and Cheeks </li></ul><ul><ul><li>Involved in facial expression, mastication, and speech </li></ul></ul><ul><li>Palate </li></ul><ul><ul><li>Forms the roof of the mouth. Consists of hard and soft areas </li></ul></ul><ul><ul><ul><li>Hard palate: assists the tongue in chewing </li></ul></ul></ul><ul><ul><ul><li>Soft palate: mobile fold formed mostly of skeletal muscle. Projecting from the soft palate is the uvula </li></ul></ul></ul><ul><ul><ul><ul><li>Closes off the nasopharynx during swallowing </li></ul></ul></ul></ul><ul><li>The mouth contains accessory glands (salivary glands) and accessory organs (teeth and tongue) which begin the digestive process </li></ul>
  13. 14. Fig. 21.4
  14. 15. Oral Cavity <ul><li>Tongue </li></ul><ul><ul><li>Occupies the floor of the mouth and fills the oral cavity when mouth is closed </li></ul></ul><ul><ul><li>Functions include: </li></ul></ul><ul><ul><ul><li>Gripping and repositioning food during chewing </li></ul></ul></ul><ul><ul><ul><li>Mixing food with saliva and forming the bolus </li></ul></ul></ul><ul><ul><ul><li>Initiation of swallowing, and speech </li></ul></ul></ul><ul><ul><li>Frenulum secures the tongue to the floor of the mouth </li></ul></ul>
  15. 16. Oral Cavity <ul><li>Teeth </li></ul><ul><ul><li>Tear and grind food </li></ul></ul><ul><ul><ul><li>Born with 20 deciduous teeth </li></ul></ul></ul><ul><ul><ul><ul><li>Erupt between about 6 months and 24 months of age </li></ul></ul></ul></ul><ul><ul><ul><li>Deciduous teeth are replaced by 32 permanent teeth </li></ul></ul></ul><ul><ul><ul><ul><li>Replacement starts at about 5 years and the process is complete by about 11 years </li></ul></ul></ul></ul><ul><ul><li>The types of teeth are </li></ul></ul><ul><ul><ul><li>Incisors </li></ul></ul></ul><ul><ul><ul><li>Canines </li></ul></ul></ul><ul><ul><ul><li>Premolars </li></ul></ul></ul><ul><ul><ul><li>Molars </li></ul></ul></ul>
  16. 17. Fig. 21.5
  17. 18. Oral Cavity <ul><li>A tooth consists of a crown, a neck, and a root </li></ul><ul><ul><li>Crown </li></ul></ul><ul><ul><ul><li>Exposed part of the tooth above the gingiva </li></ul></ul></ul><ul><ul><ul><li>Dentin covered by enamel </li></ul></ul></ul><ul><ul><ul><ul><li>Enamel: acellular material composed of calcium salts and hydroxyapatite crystals; the hardest substance in the body </li></ul></ul></ul></ul><ul><ul><li>Root </li></ul></ul><ul><ul><ul><li>Portion of the tooth embedded in the jawbone </li></ul></ul></ul><ul><ul><ul><li>Composed of dentin </li></ul></ul></ul><ul><ul><ul><ul><li>Within the dentin of the root is the pulp cavity, which is filled with pulp, blood vessels, and nerves </li></ul></ul></ul></ul><ul><ul><li>Periodontal ligaments hold the teeth in the alveoli </li></ul></ul>
  18. 19. Molar Tooth in Place in the Alveolar Bone Fig. 21.6
  19. 20. Oral Cavity <ul><li>Salivary Glands </li></ul><ul><ul><li>Produce and secrete saliva that: </li></ul></ul><ul><ul><ul><li>Cleanses the mouth </li></ul></ul></ul><ul><ul><ul><li>Moistens and dissolves food chemicals </li></ul></ul></ul><ul><ul><ul><li>Aids in bolus formation </li></ul></ul></ul><ul><ul><ul><li>Contains enzymes that break down starch </li></ul></ul></ul><ul><ul><li>Salivary glands produce serous and mucous secretions </li></ul></ul><ul><ul><li>The three pairs of large salivary glands are the parotid, submandibular, and sublingual </li></ul></ul>
  20. 21. Fig. 21.7
  21. 22. Oral Cavity <ul><li>Saliva </li></ul><ul><ul><li>Produce ~1 liter of saliva/day </li></ul></ul><ul><ul><li>Secreted from serous and mucous cells of salivary glands </li></ul></ul><ul><ul><ul><li>Serous: enzymes, ions, and mucin </li></ul></ul></ul><ul><ul><ul><li>Mucous: produce mucous </li></ul></ul></ul><ul><ul><li>97-99.5% water and slightly acidic solution containing </li></ul></ul><ul><ul><ul><li>Electrolytes </li></ul></ul></ul><ul><ul><ul><li>Digestive enzyme: salivary amylase </li></ul></ul></ul><ul><ul><ul><li>Proteins: mucin, lysozyme, defensins, and IgA </li></ul></ul></ul><ul><ul><ul><li>Metabolic wastes: urea and uric acid </li></ul></ul></ul>
  22. 23. Oral Cavity <ul><li>Control of Salivation </li></ul><ul><ul><li>Primarily by the parasympathetic division of the autonomic nervous system </li></ul></ul><ul><ul><li>Salivary glands secrete serous, enzyme-rich saliva in response to: </li></ul></ul><ul><ul><ul><li>Ingested food which stimulates chemoreceptors and pressoreceptors </li></ul></ul></ul><ul><ul><ul><li>The thought of food </li></ul></ul></ul><ul><ul><li>Strong sympathetic stimulation inhibits salivation and results in dry mouth </li></ul></ul>
  23. 24. Pharynx and Esophagus <ul><li>Pharynx </li></ul><ul><ul><li>Consists of the nasopharynx, oropharynx, and laryngopharynx </li></ul></ul><ul><ul><ul><li>Food and fluids to the esophagus </li></ul></ul></ul><ul><ul><ul><li>Air to the trachea </li></ul></ul></ul><ul><li>Esophagus </li></ul><ul><ul><li>Connects the pharynx to the stomach </li></ul></ul><ul><ul><ul><li>The upper and lower esophageal sphincters regulate movement </li></ul></ul></ul><ul><ul><li>Mucous glands produce a lubricating mucus </li></ul></ul>
  24. 25. Tab. 21.1
  25. 26. Swallowing <ul><li>During the voluntary phase of swallowing, a bolus of food is moved by the tongue from the oral cavity to the pharynx </li></ul>Fig. 21.8
  26. 27. Swallowing <ul><li>The pharyngeal phase is a reflex caused by the stimulation of stretch receptors in the pharynx </li></ul><ul><ul><li>The soft palate closes the nasopharynx, and the epiglottis, vestibular folds, and vocal folds close the opening into the larynx </li></ul></ul><ul><ul><li>Pharyngeal muscles move the bolus to the esophagus </li></ul></ul>Fig. 21.8
  27. 28. Swallowing <ul><li>The esophageal phase is a reflex initiated by the stimulation of stretch receptors in the esophagus. A wave of contraction (peristalsis) moves the food to the stomach </li></ul>Fig. 21.8
  28. 29. Peristalsis Fig. 21.9
  29. 30. Fig. 21.8
  30. 31. Anatomy and Histology of the Stomach <ul><li>Temporary “storage tank” where chemical breakdown of proteins begins and food is converted from a bolus to chyme </li></ul><ul><li>The openings of the stomach are the </li></ul><ul><ul><li>Gastroesophageal opening to the esophagus </li></ul></ul><ul><ul><li>Pyloric orifice to the duodenum </li></ul></ul><ul><li>The major regions are the </li></ul><ul><ul><li>Cardiac part </li></ul></ul><ul><ul><li>Fundus </li></ul></ul><ul><ul><li>Body </li></ul></ul><ul><ul><li>Pyloric part which is continuous with the duodenum through the pyloric sphincter </li></ul></ul><ul><li>The lateral sides of the stomach consist of greater and lesser curvatures </li></ul>
  31. 32. Anatomy and Histology of the Stomach <ul><li>The wall of the stomach consists of </li></ul><ul><ul><li>External serosa </li></ul></ul><ul><ul><li>Muscle layer (longitudinal, circular, and oblique) </li></ul></ul><ul><ul><li>Submucosa </li></ul></ul><ul><ul><li>Simple columnar epithelium </li></ul></ul><ul><ul><ul><li>Surface mucous cells: produce an alkaline mucous with bicarbonate, which coats and protects the stomach lining </li></ul></ul></ul><ul><li>An empty stomach has a volume of 50 mL but can hold about a gallon of food </li></ul><ul><li>When empty one can see the folds called rugae (submucosa and mucosa) </li></ul>
  32. 33. Anatomy and Histology of the Stomach Fig. 21.10
  33. 34. Anatomy and Histology of the Stomach <ul><li>There are gastric pits with glands that produce juice </li></ul><ul><li>Glands include four secretory cells: </li></ul><ul><ul><li>Mucous neck: secrete acid mucus </li></ul></ul><ul><ul><li>Parietal cells: secrete HCl and intrinsic factor </li></ul></ul><ul><ul><li>Chief cells: produce pepsinogen </li></ul></ul><ul><ul><ul><li>Pepsinogen is activated to pepsin by: </li></ul></ul></ul><ul><ul><ul><ul><li>HCl in the stomach </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Pepsin itself via a positive feedback mechanism </li></ul></ul></ul></ul><ul><ul><li>Endocrine cells: produce regulatory hormones </li></ul></ul><ul><ul><ul><li>Gastrin, histamine, endorphins, serotonin, cholecystokinin (CCK), and somatostatin into the lamina propria </li></ul></ul></ul>
  34. 35. Anatomy and Histology of the Stomach
  35. 36. Fig. 21.10
  36. 37. Secretions of the Stomach <ul><li>Chyme is ingested food mixed with gastric juice </li></ul><ul><li>Gastric juice </li></ul><ul><ul><li>Mucus protects the stomach lining </li></ul></ul><ul><ul><li>Pepsinogen is converted to pepsin, which digests proteins </li></ul></ul><ul><ul><li>Hydrochloric acid promotes pepsin activity and kills microorganisms </li></ul></ul><ul><ul><li>Intrinsic factor is necessary for vitamin B12 absorption </li></ul></ul><ul><ul><li>Gastrin and histamine regulate stomach secretions </li></ul></ul><ul><li>A proton pump (H + -K + exchange pump) moves H + out of parietal cells </li></ul>
  37. 38. Fig. 21.11 HCl production by Parietal Cells in the Gastric Glands of the Stomach
  38. 40. Regulation of Stomach Secretion <ul><li>There are three phases of stomach secretion </li></ul><ul><ul><li>Cephalic phase </li></ul></ul><ul><ul><ul><li>Initiated by the sight, smell, taste, or thought of food </li></ul></ul></ul><ul><ul><ul><li>Nerve impulses from the medulla stimulate hydrochloric acid, pepsinogen, gastrin, and histamine secretion. </li></ul></ul></ul><ul><ul><li>Gastric phase </li></ul></ul><ul><ul><ul><li>Initiated by distention of the stomach, which stimulates gastrin secretion and activates CNS and local reflexes that promote secretion </li></ul></ul></ul><ul><ul><li>Gastrointestinal phase </li></ul></ul><ul><ul><ul><li>Acidic chyme, which enters the duodenum and stimulates neuronal reflexes and the secretion of hormones (secretin, cholecystokinin) that inhibit gastric secretions </li></ul></ul></ul>
  39. 41. Cephalic Phase
  40. 42. Gastric Phase
  41. 43. Gastrointestinal Phase
  42. 44. Fig. 21.12
  43. 46. Movements of the Stomach <ul><li>Waves mix the stomach contents with stomach secretions to form chyme </li></ul><ul><li>Peristaltic waves move the chyme into the duodenum </li></ul><ul><ul><li>Hunger contractions </li></ul></ul><ul><ul><li>Vomiting </li></ul></ul><ul><ul><ul><li>Reverse peristalsis </li></ul></ul></ul>
  44. 47. Fig. 21.13
  45. 48. Small Intestine <ul><li>The body’s major digestive organ </li></ul><ul><li>Digestion is completed and absorption occurs </li></ul><ul><li>Runs from pyloric sphincter to the ileocecal valve </li></ul><ul><li>Has three subdivisions: </li></ul><ul><ul><li>Duodenum - ~ 10” long. The bile and pancreatic ducts empty here </li></ul></ul><ul><ul><li>Jejunum - ~ 8’ long </li></ul></ul><ul><ul><li>Ileum - ~ 12’ long </li></ul></ul>
  46. 49. Fig. 21.14
  47. 50. Anatomy and Histology of the Small Intestines <ul><li>Structural modifications of the small intestine wall increase surface area about 600-fold </li></ul><ul><ul><li>Circular folds </li></ul></ul><ul><ul><ul><li>Deep folds of the mucosa and submucosa </li></ul></ul></ul><ul><ul><li>Villi </li></ul></ul><ul><ul><ul><li>Fingerlike extensions of the mucosa </li></ul></ul></ul><ul><ul><li>Microvilli </li></ul></ul><ul><ul><ul><li>Tiny projections of absorptive mucosal cells’ plasma membranes. This forms a brush border </li></ul></ul></ul><ul><li>The epithelium of the mucosa is simple columnar epithelium. Between the villi the mucosa contains pits called intestinal glands </li></ul>
  48. 51. Anatomy and Histology of the Small Intestines <ul><li>The epithelium of the mucosa is made up of: </li></ul><ul><ul><li>Absorptive cells </li></ul></ul><ul><ul><ul><li>Produce digestive enzymes </li></ul></ul></ul><ul><ul><li>Goblet cells </li></ul></ul><ul><ul><ul><li>Produce a protective mucus </li></ul></ul></ul><ul><ul><li>Granular cells (Paneth cells) </li></ul></ul><ul><ul><ul><li>Protect the intestinal epithelium form bacteria </li></ul></ul></ul><ul><ul><li>Endocrine cells </li></ul></ul><ul><ul><ul><li>Produce regulatory hormones </li></ul></ul></ul><ul><li>Peyer’s patches (aggregated lymphoid follicles) are found in the submucosa </li></ul>
  49. 52. Fig. 21.15
  50. 53. Secretions of the Small Intestine <ul><li>Mucus protects against digestive enzymes and gastric acids </li></ul><ul><li>Digestive enzymes (disaccharidases and peptidases) are bound to the intestinal wall </li></ul><ul><li>Chemical or tactile irritation, vagal stimulation, and secretin stimulate intestinal secretion </li></ul>
  51. 54. Movement of the Small Intestine <ul><li>Segmental contractions mix intestinal contents </li></ul><ul><li>Peristaltic contractions move materials distally </li></ul><ul><li>Distension of the intestinal wall, local reflexes, and the parasympathetic nervous system stimulate contractions </li></ul><ul><li>Distension of the cecum initiates a reflex that stimulates contraction of the ileocecal sphincter </li></ul>
  52. 55. Segmental Contractions Fig. 21.16
  53. 56. Anatomy and Histology of the Liver <ul><li>The liver has four external lobes: right, left, caudate, and quadrate </li></ul><ul><li>Internally, the liver is divided into eight segments </li></ul><ul><li>Liver segments are divided into lobules </li></ul><ul><ul><li>Hexagonal-shaped liver lobules are the structural and functional units of the liver </li></ul></ul><ul><ul><li>Composed of hepatocyte (liver cell) plates radiating outward from a central vein </li></ul></ul><ul><ul><li>Portal triads are found at each of the six corners of each liver lobule </li></ul></ul>
  54. 57. Fig. 21.17
  55. 58. Anatomy and Histology of the Liver <ul><li>Portal triads consist of </li></ul><ul><ul><li>Hepatic duct: conduct bile toward the duodenum </li></ul></ul><ul><ul><li>Hepatic artery: supplies oxygen-rich blood to the liver </li></ul></ul><ul><ul><li>Hepatic portal vein: carries venous blood with nutrients from digestive viscera </li></ul></ul><ul><li>The hepatic cords are composed of columns of hepatocytes separated by the bile canaliculi </li></ul><ul><li>Sinusoids are enlarged spaces filled with blood and lined with endothelium and hepatic phagocytic cells </li></ul><ul><ul><li>Kupffer cells: hepatic macrophages found in liver sinusoids </li></ul></ul>
  56. 59. Fig. 21.18
  57. 60. Functions of the Liver <ul><li>Produces bile, which contains bile salts that emulsify fats </li></ul><ul><li>Stores and processes nutrients, produces new molecules, and detoxifies molecules </li></ul><ul><li>Hepatic phagocytic cells phagocytize red blood cells, bacteria, and other debris </li></ul><ul><li>Produces blood components </li></ul>
  58. 61. Blood Flow Through the Liver <ul><li>Branches of the hepatic artery and the hepatic portal vein in the portal triads empty into hepatic sinusoids </li></ul><ul><li>Hepatic sinusoids empty into central veins, which join to form the hepatic veins, which leave the liver </li></ul>
  59. 62. Fig. 21.19 Blood and Bile Flow Through the Liver
  60. 63. Bile Transport <ul><li>Bile canaliculi collect bile from hepatocytes and join the small hepatic ducts in the portal triads </li></ul><ul><li>Small hepatic ducts converge to form the right and left hepatic ducts , which exit the liver </li></ul><ul><li>The left and right hepatic ducts join to form the common hepatic duct </li></ul><ul><li>The cystic duct from the gallbladder joins the common hepatic duct to form the common bile duct </li></ul><ul><li>The common bile duct and pancreatic duct join at the hepatopancreatic ampulla , which opens into the duodenum at the major duodenal papilla </li></ul>
  61. 64. Gallbladder and Bile <ul><li>Gallbladder </li></ul><ul><ul><li>A small sac on the inferior surface of the liver </li></ul></ul><ul><ul><li>Stores and concentrates bile </li></ul></ul><ul><li>Bile </li></ul><ul><ul><li>A yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes </li></ul></ul><ul><ul><li>Bile salts are cholesterol derivatives that: </li></ul></ul><ul><ul><ul><li>Emulsify fat </li></ul></ul></ul><ul><ul><ul><li>Facilitate fat and cholesterol absorption </li></ul></ul></ul><ul><ul><ul><li>Helps make cholesterol soluble </li></ul></ul></ul><ul><ul><li>Enterohepatic circulation recycles bile salts therefore they are never voided in the feces </li></ul></ul><ul><ul><li>The chief bile pigment is bilirubin, a waste product of heme. </li></ul></ul><ul><ul><ul><li>Bilirubin is metabolized by bacteria in the small intestines and urobilogen is produced, which gives feces its dark color </li></ul></ul></ul>
  62. 65. Liver, Gallbladder, Pancreas, and Duct System Fig. 21.20
  63. 66. Regulation of Bile Secretion <ul><li>Acidic, fatty chyme causes the duodenum to release: </li></ul><ul><ul><li>Cholecystokinin (CCK) and secretin into the bloodstream </li></ul></ul><ul><ul><ul><li>Cholecystokinin causes: </li></ul></ul></ul><ul><ul><ul><ul><li>The gallbladder to contract and releases bile </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Relaxation of the sphincters of the bile duct and hepatopancreatic ampulla </li></ul></ul></ul></ul><ul><ul><ul><li>Secretin increases bile secretion (water and bicarbonate ions) </li></ul></ul></ul><ul><ul><ul><li>As a result, bile enters the duodenum </li></ul></ul></ul><ul><li>Bile salts and secretin transported in blood stimulate the liver to produce bile </li></ul><ul><li>Vagal stimulation causes weak contractions of the gallbladder </li></ul>
  64. 67. Fig. 21.21 Control of Bile Secretion and Release
  65. 68. Anatomy and Histology of the Pancreas <ul><li>Location </li></ul><ul><ul><li>Lies deep to the greater curvature of the stomach </li></ul></ul><ul><ul><li>The head is encircled by the duodenum and the tail sits against the spleen </li></ul></ul><ul><li>Exocrine function </li></ul><ul><ul><li>Secretes pancreatic juice which breaks down all categories of foodstuff </li></ul></ul><ul><ul><li>The pancreas is divided into lobules that contain acini </li></ul></ul><ul><ul><ul><li>Clusters of secretory cells that contain zymogen granules with digestive enzymes </li></ul></ul></ul><ul><ul><ul><li>Connect to a duct system that eventually forms the pancreatic duct </li></ul></ul></ul><ul><ul><li>The pancreatic duct joins the hepatopancreatic ampulla. The accessory pancreatic duct empties into the duodenum at the minor duodenal papilla. </li></ul></ul><ul><li>The pancreas also has an endocrine function. The islets of Langerhans release of insulin and glucagon </li></ul>
  66. 69. Fig. 21.22
  67. 70. Pancreatic Secretions <ul><li>The aqueous component of pancreatic juice is produced by the small pancreatic ducts and contains bicarbonate ions </li></ul><ul><ul><li>Water solution of enzymes and electrolytes (primarily HCO 3 – ) </li></ul></ul><ul><ul><ul><li>Neutralizes acidic chyme </li></ul></ul></ul><ul><ul><ul><li>Provides optimal environment for pancreatic enzymes </li></ul></ul></ul><ul><li>The enzymatic component of pancreatic juice is produced by the acini and contains enzymes that digest carbohydrates, lipids, and proteins </li></ul><ul><ul><li>Enzymes are released in inactive form and activated in the duodenum </li></ul></ul>
  68. 71. Regulation of Bile Secretion and Release <ul><li>Secretin stimulates the release of the aqueous component, which neutralizes acidic chyme </li></ul><ul><li>Cholecystokinin stimulates the secretion of the enzymatic component and relaxation of the sphincters of the pancreatic duct and hepatopancreatic ampulla </li></ul><ul><li>Parasympathetic stimulation increases and sympathetic stimulation decreases secretion of enzymes </li></ul>
  69. 72. Fig. 21.23
  70. 73. Anatomy and Histology of the Large Intestine <ul><li>Extends from the ileocecal valve to the anus </li></ul><ul><li>Absorbs water and eliminate the waste via feces </li></ul><ul><li>Is subdivided into the cecum, appendix, colon, rectum, and anal canal </li></ul><ul><ul><li>The cecum forms a blind sac at the junction of the small and large intestines. </li></ul></ul><ul><ul><li>The vermiform appendix is a blind tube off the cecum. </li></ul></ul><ul><ul><li>The colon has distinct regions: ascending colon, transverse colon, descending colon, and sigmoid colon </li></ul></ul><ul><ul><li>The sigmoid colon joins the rectum </li></ul></ul><ul><ul><li>The anal canal, the last segment of the large intestine, opens to the exterior at the anus </li></ul></ul><ul><ul><ul><li>The anal canal contains two sphincters to allow the movement of feces </li></ul></ul></ul>
  71. 74. Anatomy and Histology of the Large Intestine <ul><li>Three unique features </li></ul><ul><ul><li>Teniae coli: three bands of longitudinal smooth muscle in its muscularis </li></ul></ul><ul><ul><li>Haustra: pocketlike sacs caused by the tone of the teniae coli </li></ul></ul><ul><ul><li>Epiploic appendages : fat-filled pouches of visceral peritoneum </li></ul></ul><ul><li>The mucosal lining of the large intestine is simple columnar epithelium with mucus-producing crypts </li></ul>
  72. 75. Large Intestine Fig. 21.24
  73. 76. Secretions of the Large Intestine <ul><li>Mucus protects the intestinal lining </li></ul><ul><li>The bacterial flora of the large intestine consist of: </li></ul><ul><ul><li>Bacteria surviving the small intestine that enter the cecum </li></ul></ul><ul><ul><li>Those entering via the anus </li></ul></ul><ul><li>These bacteria: </li></ul><ul><ul><li>Colonize the colon </li></ul></ul><ul><ul><li>Ferment indigestible carbohydrates </li></ul></ul><ul><ul><li>Release irritating acids and gases (flatus) </li></ul></ul><ul><ul><li>Synthesize B complex vitamins and vitamin K </li></ul></ul><ul><ul><li>Constitute about 30% of the dry weight of the feces </li></ul></ul>
  74. 77. Movement in the Large Intestine <ul><li>Haustra mix the contents and moves them slowly toward the anus </li></ul><ul><li>Mass movements are strong peristaltic contractions that occur three or four times a day </li></ul><ul><li>Defecation is the elimination of feces </li></ul><ul><ul><li>Distension of rectal walls caused by feces: </li></ul></ul><ul><ul><ul><li>Stimulates contraction of the rectal walls </li></ul></ul></ul><ul><ul><ul><li>Relaxes the internal anal sphincter </li></ul></ul></ul><ul><ul><li>Reflex activity moves feces through the internal anal sphincter </li></ul></ul><ul><ul><li>Voluntary activity regulates movement through the external anal sphincter </li></ul></ul>
  75. 78. Fig. 21.25
  76. 80. Digestion, Absorption, and Transport <ul><li>Digestion (mechanical and chemical) is the breakdown of organic molecules into their component parts </li></ul><ul><ul><li>Chemical digestion is the breaking of covalent chemical bonds in organic molecules by digestive enzymes </li></ul></ul><ul><li>Absorption is the uptake of digestive tract contents </li></ul><ul><li>Transport is the distribution of nutrients throughout the body </li></ul>
  77. 81. Chemical Digestion <ul><li>Carbohydrate digestion </li></ul><ul><ul><li>Begins in mouth with salivary amylase digesting starches </li></ul></ul><ul><ul><li>Pancreatic amylase finishes the process in the small intestine. Simple sugars are also broken down here </li></ul></ul><ul><li>Protein digestion </li></ul><ul><ul><li>Broken down into amino acids </li></ul></ul><ul><ul><li>Begins in the stomach with pepsin </li></ul></ul><ul><ul><li>Trypsin and chymotrypsin are also important protein digesting enzymes. </li></ul></ul>
  78. 83. Chemical Digestion <ul><li>Lipid digestion </li></ul><ul><ul><li>Small intestine is the sole source of lipid digestion by lipases </li></ul></ul><ul><ul><li>Enter lacteals and are transported to systemic circulation via lymph </li></ul></ul><ul><li>Nucleic acids </li></ul><ul><ul><li>Broken down by pancreatic nucleases in the small intestine </li></ul></ul>
  79. 84. Tab. 21.2
  80. 85. Carbohydrates <ul><li>Include starches, glycogen, sucrose, lactose, glucose, and fructose </li></ul><ul><li>Polysaccharides are broken down into monosaccharides by a number of different enzymes </li></ul><ul><li>Monosaccharides </li></ul><ul><ul><li>Taken up by intestinal epithelial cells by symport that is powered by a Na + gradient or by facilitated diffusion </li></ul></ul><ul><ul><li>Carried to the liver, where the non-glucose sugars are converted to glucose </li></ul></ul><ul><ul><li>Glucose is transported to the cells that require energy </li></ul></ul><ul><ul><li>Glucose enters the cells through facilitated diffusion </li></ul></ul><ul><li>Insulin influences the rate of glucose transport </li></ul>
  81. 86. Transport of Monosaccharides Across the Intestinal Epithelium Fig. 21.26
  82. 87. Lipids <ul><li>Include triglycerides, phospholipids, steroids, and fat-soluble vitamins </li></ul><ul><li>Emulsification is the transformation of large lipid droplets into smaller droplets and is accomplished by bile salts </li></ul><ul><li>Lipase digests lipid molecules to form fatty acids and a monoglyceride </li></ul><ul><li>Micelles form around lipid digestion products and move to epithelial cells of the small intestine, where the products pass into the cells by simple diffusion </li></ul>
  83. 88. Lipids <ul><li>Within the epithelial cells, free fatty acids are combined with monoglycerides to form triglycerides </li></ul><ul><li>Proteins coat triglycerides, phospholipids, and cholesterol to form chylomicrons </li></ul><ul><li>Chylomicrons enter lacteals within intestinal villi and are carried through the lymphatic system to the bloodstream </li></ul><ul><li>Triglycerides are stored in adipose tissue, converted into other molecules, or used as energy </li></ul>
  84. 89. Transport of Lipids Across the Intestinal Epithelium Fig. 21.27
  85. 90. Lipoproteins <ul><li>Lipoproteins include chylomicrons, VLDL, LDL, and HDL </li></ul><ul><li>LDL transports cholesterol to cells, and HDL transports it from cells to the liver </li></ul><ul><li>LDL are taken into cells by receptor-mediated endocytosis, which is controlled by a negative-feedback mechanism </li></ul>Fig. 21.28
  86. 91. Proteins <ul><li>Pepsin in the stomach breaks proteins into smaller polypeptide chains </li></ul><ul><li>Proteolytic enzymes from the pancreas produce small peptide chains </li></ul><ul><li>Peptidases, bound to the microvilli of the small intestine, break down peptides </li></ul><ul><li>Tripeptides, dipeptides, and amino acids are absorbed by symport that is powered by a Na gradient </li></ul><ul><li>Amino acids are transported to the liver, where the amino acids can be modified or released into the bloodstream </li></ul><ul><li>Amino acids are actively transported into cells under the stimulation of growth hormone and insulin </li></ul><ul><li>Amino acids are used as building blocks or for energy </li></ul>
  87. 92. Amino Acid Transport Across the Intestinal Epithelium Fig. 21.29
  88. 93. Water and Ions Fig. 21.30 <ul><li>About 9 liters of water enters the digestive tract each day </li></ul><ul><ul><li>Can move in either direction across the wall of the small intestine, depending on the osmotic gradients across the epithelium </li></ul></ul><ul><ul><li>Epithelial cells actively transport Na, K, Ca +2 , and Mg +2 from the intestine </li></ul></ul><ul><ul><li>Chloride ions move passively through the wall of the duodenum and jejunum but are actively transported from the ileum </li></ul></ul>
  89. 94. Effects of Aging on the Digestive System <ul><li>The mucous layer, the connective tissue, the muscles, and the secretions all tend to decrease as a person ages </li></ul><ul><li>These changes make an older person more open to infections and toxic agents </li></ul>
  90. 95. Page 678

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