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Digestive system

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Digestive system

  1. 1. Digestive system• Note: All the powerpoint presentations are meant for better understanding of text. These in no way replace the text books .All chapters in the text books must be thoroughly studied.
  2. 2. Digestive system25-2
  3. 3. Digestive System: Overview• The alimentary canal or gastrointestinal (GI) tract digests and absorbs food• Alimentary canal – mouth, esophagus, stomach, small intestine, and large intestine• Accessory digestive organs – teeth, tongue, gallbladder, salivary glands, liver, and pancreas
  4. 4. Digestive Process• There are six essential activities: –Ingestion –propulsion –mechanical digestion –Chemical digestion –Absorption –defecation
  5. 5. Gastrointestinal Tract Activities• Ingestion – taking food into the digestive tract• Propulsion – swallowing and peristalsis – Peristalsis – waves of contraction and relaxation of muscles in the organ walls• Mechanical digestion – chewing, mixing, and churning food• Chemical digestion – catabolic breakdown of food• Absorption – movement of nutrients from the GI tract to the blood or lymph• Defecation – elimination of indigestible solid wastes
  6. 6. Histology of the Alimentary Canal• From esophagus to the anal canal the walls of the GI tract have the same four tunics –From the lumen to outward they are the: – Mucosa – Submucosa – Muscularis externa – Serosa
  7. 7. Histology of the Alimentary Canal
  8. 8. Regulation of Digestive Tract• Motility and secretion of the digestive tract are controlled by neural, hormonal, and paracrine mechanisms• Neural control – Short (myenteric) reflexes – stretch or chemical stimulation acts through myenteric plexus • Stimulates parastaltic contractions of swallowing – Long (vagovagal) reflexes - parasympathetic stimulation of digestive motility and secretion• Hormones – Chemical messengers secreted into bloodstream, and stimulate distant parts of the digestive tract• Paracrine secretions – Chemical messengers that diffuse through the tissue fluids to stimulate nearby target cells25-8
  9. 9. Enteric Nervous System• Enteric nervous system – a nervous network in the esophagus, stomach, and intestines that regulated digestive tract motility, secretion, and blood flow – thought to have over 100 million neurons – more than the spinal cord – functions completely independently of the central nervous system • CNS exerts a significant influence on its action• composed of two networks of neurons – Submucosal (Meissner) plexus – in submucosa • controls glandular secretion of mucosa • controls movements of muscularis mucosae – Myenteric (Auerbach) plexus – parasympathetic ganglia and nerve fibers between the two layers of the muscularis interna • controls peristalsis and other contractions of muscularis externa• enteric nervous system contains sensory neurons that monitor 25-9 tension in gut wall and conditions in lumen
  10. 10. Mouth• Oral or buccal cavity: –Is bounded by lips, cheeks, palate, and tongue –Has the following organs: –Teeth –Salivary glands –tongue
  11. 11. Mouth or Oral Cavity Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Upper lipVestibule Superior labial frenulumPalatine raphePalatoglossal Hard palatearch and palatinePalatopharyngeal rugaearchPalatine tonsil Soft palateTongue UvulaLingual frenulumSalivary duct Lower lip Figure 25.4orifices: Sublingual Submandibular Inferior labial frenulum25-11
  12. 12. Tongue• Occupies the floor of the mouth and fills the oral cavity when mouth is closed• Functions include: – Gripping and repositioning food during chewing – Mixing food with saliva and forming the bolus – Initiation of swallowing, and speech Lingual Frenulum – median fold that attaches the body to the floor of the mouth• Sulcus terminalis – groove that separates the tongue into two areas: – Anterior 2/3 residing in the oral cavity – Posterior third residing in the oropharynx
  13. 13. Tongue• Superior surface bears four types of papillae – Filiform – give the tongue roughness and provide friction . Do not contain taste buds – Fungiform – scattered widely over the tongue and give it a reddish hue. Contains lots of taste buds. – Circumvallate – V-shaped row in back of tongue – Foliate –pleatlike shaped #Study:- Ankyloglossia
  14. 14. Tongue Figure 23.8
  15. 15. Salivary Glands• Three pairs of glands – parotid, submandibular, and sublingual – Parotid – lies anterior to the ear between the masseter muscle and skin. Parotid duct opens into the vestibule next to second upper molar• Submandibular – lies along the medial aspect of the mandibular body. Its ducts open at the base of the lingual frenulum• Sublingual – lies anterior to the submandibular gland under the tongue• # Study : Mumps
  16. 16. Salivary Glands Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parotid gland Parotid duct Tongue Sublingual ducts Figure 25.9Massetermuscle Submandibular duct Lingual Submandibular frenulum gland Sublingual Opening of gland Mandible submandibular duct 25-16
  17. 17. Salivary Glands Figure 23.9a
  18. 18. Saliva: Source and Composition• Secreted from serous and mucous cells of salivary glands• 97-99.5% water, hypo-osmotic, slightly acidic solution containing – Electrolytes – Na+, K+, Cl–, PO42–, HCO3– – Digestive enzyme – salivary amylase – Proteins – mucin, lysozyme, defensins, and IgA – Metabolic wastes – urea and uric acid
  19. 19. Teeth• Primary and permanent dentitions have formed by age 21• Primary – 20 deciduous teeth that erupt at intervals between 6 and 24 months• Permanent – enlarge and develop causing the root of deciduous teeth to be resorbed and fall out between the ages of 6 and 12 years – All but the third molars have erupted by the end of adolescence – Usually 32 permanent teeth
  20. 20. Classification of Teeth• Teeth are classified according to their shape and function – Incisors – chisel-shaped teeth for cutting or nipping – Canines – fanglike teeth that tear or pierce – Premolars (bicuspids) and molars – have broad crowns with rounded tips; best suited for grinding or crushing• During chewing, upper and lower molars lock together generating crushing force
  21. 21. Deciduous Teeth
  22. 22. Permanent Teeth Figure 23.10.2
  23. 23. Dental Formula: Permanent Teeth• A shorthand way of indicating the number and relative position of teeth – Written as ratio of upper to lower teeth for the mouth – Primary: 2I (incisors), 1C (canine), 2M (molars) – Permanent: 2I, 1C, 2PM (premolars), 3M 2I 1C 2PM 3M X 2 (32 teeth) 2I 1C 2PM 3M
  24. 24. Tooth Structure• Two main regions – crown and the root• Crown – exposed part of the tooth above the gingiva• Enamel – acellular, brittle material composed of calcium salts and hydroxyapatite crystals; the hardest substance in the body – Encapsules the crown of the tooth• Root – portion of the tooth embedded in the jawbone
  25. 25. Tooth Structure
  26. 26. Esophagus• – a straight muscular tube 25-30 cm long – begins at level between C6 and the cricoid cartilage – extends from pharynx to cardiac orifice of stomach passing through esophageal hiatus in diaphragm – lower esophageal sphincter – food pauses at this point because of this constriction • prevents stomach contents from regurgitating into the esophagus • protects esophageal mucosa from erosive effect of the stomach acid • heartburn – burning sensation produced by acid reflux into the esophagus • #Study: Deglutition
  27. 27. Esophagus
  28. 28. Stomach• Chemical breakdown of proteins begins and food is converted to chyme.it has the following parts:-• Cardiac region – surrounds the cardiac orifice• Fundus – dome-shaped region beneath the diaphragm• Body – midportion of the stomach• Pyloric region – made up of the antrum and canal which terminates at the pylorus• The pylorus is continuous with the duodenum through the pyloric sphincter
  29. 29. Gross Anatomy of Stomach Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Esophagus Fundic region Lesser curvature Cardiac orifice Cardiac region Duodenum Body Pyloric region: Gastric rugae Pylorus Pyloric sphincter Pyloric canal Antrum Greater curvature (b) © The McGraw-Hill Companies, Inc./Rebecca Gray, photographer/Don Kincaid, dissections Figure 25.12b longitudinal wrinkles called rugae can be seen in empty stomach wall.25-29
  30. 30. Microscopic Anatomy of the Stomach
  31. 31. Glands of the Stomach Fundus and Body• Gastric glands of the fundus and body have a variety of secretory cells – Mucous neck cells – secrete acid mucus – Parietal cells – secrete HCl and intrinsic factor – Chief cells – produce pepsinogen and prorenin – Enteroendocrine cells – secrete gastrin, histamine, endorphins, serotonin, cholecystokinin (CCK), and somatostatin into the lamina propria
  32. 32. Functions of Hydrochloric Acid• activates pepsin and lingual lipase• breaks up connective tissues and plant cell walls – helps liquefy food to form chyme• converts ingested ferric ions (Fe3+) to ferrous ions (Fe2+) – Fe2+ absorbed and used for hemoglobin synthesis• contributes to nonspecific disease resistance by destroying most ingested pathogens• #Study: Ulcer 25-33
  33. 33. Pepsin• zymogens – digestive enzymes secreted as inactive proteins – converted to active enzymes by removing some of their amino acids• pepsinogen – zymogen secreted by the chief cells – hydrochloric acid removes some of its amino acids and forms pepsin that digests proteins – autocatalytic effect – as some pepsin is formed, it converts more pepsinogen into more pepsin• pepsin digests dietary proteins into shorter peptide chains – protein digestion is completed in the small intestine25-34
  34. 34. Intrinsic Factor • intrinsic factor – a glycoprotein secreted by parietal cells • essential to absorption of vitamin B12 by the small intestine – binds vitamin B12 and intestinal cells absorb this complex by receptor-mediated endocytosis • vitamin B12 is needed to synthesize hemoglobin – prevents pernicious anemia • secretion of intrinsic factor is the only indispensable function of the stomach – digestion can continue if stomach is removed25-35 (gastrectomy), but B12 supplements will be needed
  35. 35. Small Intestine• Runs from pyloric sphincter to the ileocecal valve• Has three subdivisions:• Duodenum, Jejunum, And Ileum• 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
  36. 36. Small Intestine
  37. 37. Duodenum and Related Organs
  38. 38. Liver• The largest gland in the body• Superficially has four lobes – right, left, caudate, and quadrate• The gallbladder rests in a recess on the inferior surface of the right lobe
  39. 39. Liver:functions• Hepatocytes’ functions include: – Production of bile – Processing bloodborne nutrients – Storage of fat-soluble vitamins – Detoxification – Deamination – Gluconeogenesis, glucogenesis etc.
  40. 40. Liver: Associated Structures• Bile produced by the liver leaves it via: –Bile ducts, which is formed by fusion of cystic duct of gall bladder and hepatic duct of liver –This bile duct then fuses with the pancreatic duct to form hepatopancreatic duct/ampulla(sphincter of Oddi)
  41. 41. Liver: Microscopic Anatomy• Hexagonal-shaped liver lobules are the structural and functional units of the liver – Composed of hepatocyte (liver cell) plates radiating outward from a central vein Figure 23.24c
  42. 42. Microscopic Anatomy of Liver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Hepatocytes Bile canaliculiStromaCentral veinHepatic triad: Hepatic Branch of sinusoid hepatic portal vein Branch of proper hepatic Stroma arteryBile ductule (a) Figure 25.20a 25-43
  43. 43. Liver: Microscopic Anatomy• Liver sinusoids – enlarged, leaky capillaries located between hepatic plates• Kupffer cells – hepatic macrophages found in liver sinusoids
  44. 44. Histology of Liver - Hepatic Triad Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Stroma Central vein Hepatic lobule Branch of hepatic portal vein Bile ductule Lymphatic vessel Branch of proper hepatic artery(b) 0.5 mm © The McGraw-Hill Companies, Inc./Dennis Strete, photographer Figure 25.20b 25-45
  45. 45. Gross Anatomy of theGallbladder, Pancreas, and Bile Passages Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gallbladder: Hepatic ducts Neck Body Head Common hepatic duct Cystic duct Bile duct Accessory Pancreatic duct pancreatic duct Duodenum Minor duodenal Pancreas: papilla Tail Body Circular folds Head Hepatopancreatic Duodenojejunal sphincter flexure Major duodenal papilla Jejunum Hepatopancreatic ampullaFigure 25.21 25-46
  46. 46. 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• The chief bile pigment is bilirubin, a waste product of heme
  47. 47. 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• #Study: Gallstones and Lithotripsy
  48. 48. Pancreas• Location – Lies deep to the greater curvature of the stomach – The head is encircled by the duodenum and the tail abuts the spleen. – Pancreas is a heterocrine gland that has both endocrine and exocrine functions:-• Exocrine function – Secretes pancreatic juice which breaks down all categories of foodstuff. This pancreatic juice is secreted in acini. – Acini (clusters of secretory cells) contain zymogen granules with digestive enzymes• Endocrine function – release of insulin and glucagon
  49. 49. Pancreatic Acinar Cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Acinar cells Zymogen granules (a) Stroma Ducts Figure 25.22 a-b Exocrine acinar cells Vein (b) 50 µm 25-50 © The McGraw-Hill Companies, Inc./Dennis Strete, photographer
  50. 50. Large Intestine• Is subdivided into the cecum, appendix, colon, rectum, and anal canal• The saclike cecum: – Lies below the ileocecal valve in the right iliac fossa – Contains a wormlike vermiform appendix
  51. 51. Anatomy of Large Intestine Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. GreaterRight colic omentumflexure (retracted) Left colic flexureTransversecolonSuperior Taenia colimesentericartery MesocolonHaustrum Figure 25.31aAscending Descendingcolon colonIleocecal Omentalvalve appendagesIleumCecumAppendix Sigmoid colonRectumAnal canal External anal sphincter 25-53(a) Gross anatomy
  52. 52. Colon• Has distinct regions:• ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, and sigmoid colon• The sigmoid colon joins the rectum• The anal canal, the last segment of the large intestine, opens to the exterior at the anus
  53. 53. Large intestine• Has three unique features: – Teniae coli – three bands of longitudinal smooth muscle in its muscularis – Haustra – pocketlike sacs caused by the tone of the teniae coli – Epiploic appendages – fat-filled pouches of visceral peritoneum
  54. 54. 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• #Study :- Diarrhea and Constipation.
  55. 55. Physiology of digestion. 1.Digestion of carbohydrates. 2.Digestion of proteins. 3.Digestion of fats.
  56. 56. Digestion of carbohydrates• Digestion of carbohydrates starts in the oral cavity.• Salivary amylase present in the saliva causes the breakdown of complex carbohydrates into smaller parts.• Polysaccharides salivary amylase oligosaccharides, disaccharides, monosaccharides.
  57. 57. Carbohydrate digestion in stomach• Stomach does not have any enzymes for digestion of carbohydrates.
  58. 58. Carbohydrate digestion in small intestine• Small intestine receives two juices:- intestinal juice and the pancreatic juice• Pancreatic juice have one enzymes for carbohydrate digestion:- pancreatic amylase• Pancreatic amylase will act upon complex carbohydrates and break them into small carbohydrates Eg. Starch can be broken down into maltose, isomaltose, etc.
  59. 59. Carbohydrate digestion in small intestine• Intestine contains many enzymes for carbohydrate digestion:- intestinal amylase, maltase, isomaltase, lactase, s ucrase.• Thus all these enzymes will ultimately convert complex carbohydrates into monosaccharide like glucose, fructose and galactose
  60. 60. Digestion of proteins• The final product of protein digestion are the amino acids.• There are no protein digesting enzymes in the mouth and thus Protein digestion starts in the stomach . Enzyme in stomach is pepsinogen – Chief cells of stomach – produce pepsinogen • Pepsinogen is activated to pepsin by: – HCl in the stomach – Pepsin itself via a positive feedback mechanism –Pepsin will then act on large protein and will break them into small peptides and amino acids
  61. 61. Digestion of proteins in small intestine• Enzymes acting in the small intestine –Pancreatic enzymes – trypsinogen, chymotrypsinogen, and procarboxypeptidase Pancreatic enzymes are in inactive form and they need to be activated to become functional - Intestinal enzymes – aminopeptidases, carboxypeptidases, and dipeptidases
  62. 62. Activation of Pancreatic Enzymes in the Small Intestine Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chymotrypsinogen Chymotrypsin Procarboxypeptidase Carboxypeptidase Trypsinogen Trypsin Enterokinase Figure 25.23 25-64
  63. 63. Digestion of fats• Fat (triglycerides) digestion starts in the stomach and is completed in the intestine. Saliva do not have any fat digesting enzyme.• Enzymes for fat digestion are :-• Lingual lipase• Gastric lipase• Pancreatic lipase• Intestinal lipase• Bile plays a very important role of emulsifying the fats.• All these enzymes will break fats into fatty acids and glycerol.
  64. 64. Action of bile on fat
  65. 65. Absorption• Absorption refers to transport of digested products of food from the intestinal lumen into the blood circulation.• Carbohydrate absorption: glucose and galactose are carried by active transport from the intestinal lumen into the epithelial cells of intestine. Then they move out of these cells into the blood capillaries by simple diffusion• Protein absorption: it is very similar to absorption of carbohydrates. amino acids are carried from lumen to epithelial cells by active transport and then they leave these cells by diffusion.
  66. 66. Figure 23.34
  67. 67. Fatty Acid Absorption• Fatty acids and monoglycerides combine with bile salt and cholesterol to form micelles• Micelles act as ‘ferry’ and help fatty acids to enter the intestinal cells by diffusion• Once inside the intestinal cells the free fatty acids and glycerol are resynthesized into trigylceride. Triglycerides combine with protein to form chylomicron. Resulting chylomicrons are extruded and they enter lacteals and are transported to the circulation via lymph
  68. 68. Fatty Acid Absorption Fatty acids and Lumen of monoglycerides intestine associated with micelles in lumen of intestine 1 Fatty acids and monoglycerides resulting from fat Absorptive digestion leave epithelial cell micelles and enter cytoplasm epithelial cell by diffusion. 2 Fatty acids are ER used to synthesize triglycerides in smooth endo- plasmic reticulum. Golgi apparatus 3 Fatty globules are combined with proteins to form chylomicrons (within Golgi apparatus). 4 Vesicles containing chylomicrons migrate to the basal membrane, are extruded from the epithelial cell, and enter a lacteal (lymphatic capillary). 5 Lymph in the Chylomicron lacteal transports Lacteal chylomicrons away from intestine. Figure 23.36
  69. 69. Fatty Acid Absorption Fatty acids and Lumen of monoglycerides intestine associated with micelles in lumen of intestine Absorptive epithelial cell cytoplasm ER Golgi apparatus Lacteal Figure 23.36
  70. 70. Fatty Acid Absorption Fatty acids and Lumen of monoglycerides intestine associated with micelles in lumen of intestine 1 Fatty acids and monoglycerides resulting from fat Absorptive digestion leave epithelial cell micelles and enter cytoplasm epithelial cell by diffusion. ER Golgi apparatus Lacteal Figure 23.36
  71. 71. Fatty Acid Absorption Fatty acids and Lumen of monoglycerides intestine associated with micelles in lumen of intestine 1 Fatty acids and monoglycerides resulting from fat Absorptive digestion leave epithelial cell micelles and enter cytoplasm epithelial cell by diffusion. 2 Fatty acids are ER used to synthesize triglycerides in smooth endo- plasmic reticulum. Golgi apparatus Lacteal Figure 23.36
  72. 72. Fatty Acid Absorption Fatty acids and Lumen of monoglycerides intestine associated with micelles in lumen of intestine 1 Fatty acids and monoglycerides resulting from fat Absorptive digestion leave epithelial cell micelles and enter cytoplasm epithelial cell by diffusion. 2 Fatty acids are ER used to synthesize triglycerides in smooth endo- plasmic reticulum. Golgi apparatus 3 Fatty globules are combined with proteins to form chylomicrons (within Golgi apparatus). Lacteal Figure 23.36
  73. 73. Fatty Acid Absorption Fatty acids and Lumen of monoglycerides intestine associated with micelles in lumen of intestine 1 Fatty acids and monoglycerides resulting from fat Absorptive digestion leave epithelial cell micelles and enter cytoplasm epithelial cell by diffusion. 2 Fatty acids are ER used to synthesize triglycerides in smooth endo- plasmic reticulum. Golgi apparatus 3 Fatty globules are combined with proteins to form chylomicrons (within Golgi apparatus). 4 Vesicles containing chylomicrons migrate to the basal membrane, are extruded from the epithelial cell, and enter a lacteal (lymphatic capillary). Chylomicron Lacteal Figure 23.36
  74. 74. Fatty Acid Absorption Fatty acids and Lumen of monoglycerides intestine associated with micelles in lumen of intestine 1 Fatty acids and monoglycerides resulting from fat Absorptive digestion leave epithelial cell micelles and enter cytoplasm epithelial cell by diffusion. 2 Fatty acids are ER used to synthesize triglycerides in smooth endo- plasmic reticulum. Golgi apparatus 3 Fatty globules are combined with proteins to form chylomicrons (within Golgi apparatus). 4 Vesicles containing chylomicrons migrate to the basal membrane, are extruded from the epithelial cell, and enter a lacteal (lymphatic capillary). 5 Lymph in the Chylomicron lacteal transports Lacteal chylomicrons away from intestine. Figure 23.36
  75. 75. Hormonal regulation of digestion• The release of digestive secretions is under control of various hormones:-• Gastrin: stimulate secretion of gastric juice• Enterogastrone:stops secretion of gastric juice• Cholecystokinin:release of bile from gall bladder and release of enzymes of pancreatic juice.
  76. 76. Hormonal regulation of digestion• Secretin: speeds up release of bile and cause release of sodium bicarbonate in pancreatic juice• Villikinin: accelerates the movement of villi• Duocrinin: release of mucus from brunner’s glands• Enterocrinin :release of enzymes from crypts of leiberkuhn

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