The Digestive System

1. Chapter 24 The Digestive System

2. Structures in the digestive system Alimentary canal (GI tract) - continuous muscular tube mouth, pharynx, esophagus, stomach, sm int, & lg int accessory organs - teeth, tongue, gall bladder, salivary/liver/ pancreatic glands

3. 6 essential activities of digestive system 1. Ingestion - taking food into GI tract usually by mouth 2. Mechanical digestion - physically prepares food for chemical digestion by enzymes chewing, mixing w/saliva by tongue, churning of stomach, segmentation of intestines (food mixed over different parts of intestinal walls) 3. Chemical digestion - catabolism of foodstuffs into building blocks starts in mouth-->ends mostly in small intestine 4. Secretion – water, acids, enzymes, buffers, and salts 5. Absorption - passage of digested end products (+ vitamins, minerals, & H2O) from lumen of GI tract into blood or lymph mostly in small intestine 6. Excretion (defecation) - elimination of indigestible substances from the body via the anus in the form of feces

4. Stimuli of GI tract activity Both mechano & chemoreceptors are found in the walls of GI organs respond to stretching of organ by food, osmolarity (solute [ ]) & pH of food contents, & the presence of substrates or end products of digestion reflexes either (+) or (-) glands that secrete digestive juices into the lumen or Hs into blood or mix & move contents along the length of the tract by (+) smooth muscle

5. Digestive organs & peritoneum Info covered in lab peritoneum = serous membrane of abdominopelvic cavity visceral & parietal peritoneum separated by peritoneal cavity (fluid-filled) mesentary = double-walled peritoneum (4 layers); allow passage of BVs, Ns & lymph to organs retroperitoneal = some sm. int., some lg. int., most of pancreas, kidneys…all other abd organs are intraperitoneal or just peritoneal

6. Peritoneum and Peritoneal Cavity

7. Mesenteries of Digestive Organs

8. Blood supply to GI system Blood Supply: Splanchnic Circulation Celiac trunk (hepatic, splenic, L. gastric) to spleen, liver, & stomach sup & inf mesenteric to sm & lg intestines Celiac trunk & mesenterics receive 1/4 of C.O. percentage increases after ingestion of a meal hepatic portal system collects nutrient rich venous blood draining from digestive viscera & takes it to the liver liver collects nutrients for metabolism or storage b/f releasing them back to the bloodstream (via hepatic vein) for general cellular use

9. Histology of alimentary canal 4 layers (tunics) from esophagus to anal canal Mucosa Lining epithelium (mucus secreting simple columnar epi) Lamina propria (loose areolar CT, isolated lymph nodules-MALT) Muscularis mucosa (local movements of the mucosa to dislodge food particles that have adhered to the mucosa) Submucosa- dense CT containing blood & lymph vessels, lymph nodules, & nerve fibers; a lot of elastic fibers to help stretched organs regain their shape Muscularis externa – segmentation & peristalsis Usually an inner circular layer & outer longitudinal layer Forms sphincters at several spots to prevent backflow of contents Serosa- the visceral peritoneum (alveolar CT) Replaced by adventitia in the esophagus which is in the thoracic cavity, not the abdominopelvic cavity

10. Histology of the Alimentary Canal

11. Enteric nervous system Entero = gut Linked to CNS by PsNS (+) & SNS (-) Contains 100 million neurons (as many as the entire spinal cord!) Submucosal nerve plexus Regulates gland activity & smooth mm activity in the mucosa tunic Myenteric nerve plexus B/t circular & longitudinal smooth mm layers in muscularis externa Control GI tract mobility

12. Peristaltic Motion Circular muscles contract behind bolus: while circular muscles ahead of bolus relax Longitudinal muscles ahead of bolus contract: shortening adjacent segments Wave of contraction in circular muscles: forces bolus forward

13. Segmentation Cycles of contraction: Churn and fragment bolus mix contents with intestinal secretions Does not follow a set pattern: does not push materials in any 1 direction

14. Control of Digestive Function Neural mechanisms Hormonal mechanisms Local mechanisms

15. Short reflexes: Mediated entirely by local (enteric) plexuses in response to GI tract stimuli the so-called “gut brain” stomach & small intestine also contain H-producing cells that release their product to the extracellular space to affect either intrinsic (same organ) or extrinsic (other organs) function Long Reflexes: Initiated by stimuli arising within or outside the GI tract & involve CNS centers & extrinsic autonomic nerves PsNS causes PRO-digestion SNS causes ANTI-digestion

16. Digestive Hormones At least 18 hormones that affect: most aspects of digestive function activities of other systems Are peptides Are produced by enteroendocrine cells in digestive tract Reach target organs after distribution in bloodstream

17. Local Mechanisms Coordinating response to changing conditions: e.g., variations in local pH, chemical, or physical stimuli Affect only a portion of tract

18. Mouth AKA oral cavity- stratified squamous epithelium Oral cavity proper w/in teeth Oral/buccal vestibule outside teeth Vermillion (red) zone on lips (labia) Lingual/labial frenulum Fauces (palatoglossal/palatopharyngeal arches) Contain palatine tonsils Tongue – intrinsic vs extrinsic mm Filiform, fungiform, circumvallate papillae Circumvallates (10-12) divide the tongue into ant 2/3 & post 1/3

19. Anatomy of the Oral Cavity: Mouth

20. Palate Hard palate – underlain by palatine bones and palatine processes of the maxillae Assists the tongue in chewing Slightly corrugated on either side of the raphe (midline ridge) Soft palate – mobile fold formed mostly of skeletal muscle Closes off the nasopharynx during swallowing Uvula projects downward from its free edge Palatoglossal and palatopharyngeal arches form the borders of the fauces

21. Tongue Intrinsic muscles change the shape of the tongue Extrinsic muscles alter the tongue’s position Lingual frenulum secures the tongue to the floor of the mouth Sublingual glands secrete lingual lipase to start lipid digestion

22. Tongue Superior surface bears three types of papillae Filiform – give the tongue roughness and provide friction Fungiform – scattered widely over the tongue and give it a reddish hue Circumvallate – V-shaped row in back of tongue Sulcus terminalis – groove that separates the tongue into two areas: Anterior 2/3 residing in the oral cavity Posterior third residing in the oropharynx

23. Salivary glands Parotid (near-ear) Overlies masseter; assoc w/ mumps; pierces buccinator & opens into vestibule by 2nd upper molar Submandibular Medial aspect of mandibular body; opens at base of lingual frenulum Sublingual Ant to submandibular under tongue; opens via 10-12 ducts into floor of mouth Salivary glands are composed of 2 types of cells 1. Serous (produce watery secretion containing enzymes) 2. Mucous (produce stringy, viscous solution) Parotids contain only serous; submandibular & buccals contain ~ half of each; sublingual contain mostly mucous

24. Salivary Glands

25. Composition of saliva: 1.0-1.5 L/day 99.4% water (hypo-osmotic) Slightly acidic pH 6.75-7.00 0.6% solutes include: Electrolytes(Na, K, Cl, Phosphate, bicarbonate ions), salivary amylase-a digestive enzyme, glycoprotein-mucin (when dissolved in H2O forms thick mucus to lubricate oral cavity) & hydrate foodstuffs, lysozyme & IgA (protection against microorganisms), metabolic wastes (urea & uric acid)

26. Control of salivation Intrinsic (buccal) glands keep mouth moist Food ingestion (+) extrinsic glands 1000-1500 ml/day production Chemoreceptors (sensitive to acidic substances) & pressoreceptors (any mechanical stimulus) PsNS (+) activity in CN VII & IX to increase output of saliva SNS (+) release of thick, mucin-rich saliva Very strong SNS (+) constricts BVs to salivary glands, causing dry mouth

27. Teeth Function in mastication to form a food bolus in preparation for deglutition Deciduous (appear at intervals between 6 and 24 months) Fall out b/t age 6-12…AKA primary/milk/baby teeth 20 total (2 incisors (cut/nip food), 1 canine (tear/pierce), 2 molars (crush)…x2 (L/R) x 2 (top/bottom) Permanent (all but 3rd molars erupt by end of adolescence) 32 total (2 incisors, 1 canine, 2 premolars, 3 molars….x2 x2)

28. Classification of Teeth Teeth are classified according to their shape and function Incisors – chisel-shaped teeth adapted for cutting or nipping Canines (cuspids) – conical or fanglike teeth that tear or pierce Premolars (bicuspids) and molars – have broad crowns with rounded tips and are best suited for grinding or crushing

29. Tooth Structure Crown – exposed part of the tooth above the gingiva (gum) Enamel – acellular, brittle material composed of calcium salts and hydroxyapatite crystals is the hardest substance in the body Encapsules the crown of the tooth Root – portion of the tooth embedded in the jawbone Neck – constriction where the crown and root come together Cementum – calcified connective tissue Covers the root Attaches it to the periodontal ligament Periodontal ligament Anchors the tooth in the alveolus of the jaw Forms the fibrous joint called a gomphosis Gingival sulcus – depression where the gingiva borders the tooth

30. Tooth Structure Dentin – bonelike material deep to the enamel cap that forms the bulk of the tooth Pulp cavity – cavity surrounded by dentin that contains pulp Pulp – connective tissue, blood vessels, and nerves Root canal – portion of the pulp cavity that extends into the root

31. Pharynx Oro & laryngopharynx are common passageway for food Nasopharynx is for air only Histology resembles oral cavity - stratified squamous epithelium Two layers of skeletal muscle Inner layer is longitudinal Outer layer is pharyngeal constrictors (3) Both act to propel food into esophagus

32. Esophagus 10 inch long muscular tube that remains collapsed unless transporting a bolus Pierces diaphragm at esophageal hiatus Joins cardiac region of stomach at cardiac or gastroesophageal sphincter GERD w/loose sphincter or hiatal hernia Esophageal mucosa is non-keratinized strat. squamous; changes to simple columnar at jnx w/ stomach Submucosa has esophageal glands that secrete lubricating mucus when compressed Muscularis externa is skeletal mm at prox 1/3, mixture of skeletal & smooth in middle 1/3, & entirely smooth at distal 1/3

33. Upper Endoscopy

34. Deglutition (Swallowing) Buccal phase – voluntary & occurs in mouth; tip of tongue against hard palate & contract tongue to move bolus into oropharynx where it becomes involuntary Pharyngeal/esophageal phase – controlled by swallowing center in medulla & lower pons thru several CNs (esp CN X) to mm of pharynx & esophagus Tongue blocks off mouth; soft palate rises to close off nasopharynx; larynx rises to allow epiglottis to cover the opening into the trachea Solid food from oropharynx to stomach in 4-8 seconds Liquids in 1-2 seconds

35. Deglutition (Swallowing)

36. Stomach Storage tank where the chemical breakdown of proteins begin & food is converted into chyme – a creamy paste Empty – volume of ~50 ml & diameter slightly larger than large intestine Full – can hold ~4L (1 gallon) of food & extend all the way to the pelvis

37. Stomach gross anatomy 4 regions Cardiac, fundus, body, pylorus 2 curvatures Greater – convex; greater omentum drapes small intestines Lesser – concave; connects to liver via lesser omentum Rugae allow stomach to recoil as mucosa & submucosa recoil into longitudinal folds

38. Common types of hernias

39. Histology of stomach Stomach wall contains all 4 tunics Muscularis externa has an additional oblique layer (w/longitudinal & circular) Simple columnar epithelium w/lots of goblets secreting protective, alkaline mucus Epithelium dotted w/millions of gastric pits leading into gastric glands which produce gastric juice, mucus, and gastrin

40. Gastric gland cell types Parietal cells – secrete HCL (to activate pepsin & kill bacteria) & intrinsic factor (needed for B12 absorption in small intestine) Chief cells – produce pepsinogen (inactive form of the protein-digesting pepsin) Enteroendocrine cells – release a variety of H’s or hormone like products directly into the lamina propria G cells – produce gastrin D cells - produce somatostatin (inhibits gastrin)

41. The Secretion of Hydrochloric Acid

42. Mucosal barrier Exposed to as much as 100,000 x H+ ion [ ] found in the blood Factors making barrier 1. Thick coat of bicarbonate rich mucus 2. Epithelial cells of mucosa joined together by tight junctions to prevent gastric juice from penetrating into underlying tissues 3. Deep in the glands (where mucus is absent) the external faces of the plasma membranes are impermeable to HCL 4. Damaged epithelial mucosal cells are quickly shed & replaced—entire epithelium is replaced every 3-6 days

43. Stomach digestive properties Only real digestion is protein by pepsin In children – stomach glands secrete rennin, an enzyme that acts on milk protein (casein) to convert it to a curdy looking substance Some lipid soluble substances pass thru it readily (especially aspirin & alcohol) Essential life function?secretion of intrinsic factor to allow absorption of B12 in small intestine?needed to produce mature erythrocytes Pernicious anemia…from lack of B12

44. Regulation of gastric secretion Vagus (PsNS) (+) secretion SNS (-) secretion Normal output from gastric mucosa is ~3L of gastric juice every day…potent enough to dissolve nails! Hormonal control mostly thru gastrin which is (+) for enzyme/HCL secretion and small intestinal H’s mostly antagonistic to gastrin

45. 3 phases of gastric secretion Phase 1. Cephalic reflex Before food even enters stomach; triggered by aroma, taste, sight, or thought of food Olfactory/taste bud receptors trigger hypothalamus…(+) vagal nuclei of M.O….causing motor impulses transmitted by CN X to PsNS enteric ganglia…to (+) stomach glands Thought or seeing food is a conditioned response to only foods we like (suppressed w/satiety or depression)

46. Phase 2. Gastric phase Provides ~2/3 of gastric juice released Most important stimuli are distension, peptides, & low acidity Distention initiates local (myenteric) reflexes & long vagovagal reflexes (stomach?CN X? M.O.?CN X?stomach) G-cells secrete gastrin; chemical stimuli such as partially digested proteins (peptides), caffeine, rise in pH directly (+) Proteins buffer H+ which raises pH…causes secretion of gastrin & HCL

47. Phase 3. Intestinal phase Excitatory – partially digested food enters small intestine…(+) intestinal mucosal cells release a H (called “intestinal gastrin” due to similar effects) encouraging the gastric glands to continue secreting juices Inhibitory (enterogastric) – from distention of duodenum w/chyme…inhibits vagal nuclei in M.O., inhibits local reflexes, activates SNS fibers causing pyloric sphincters to tighten, preventing more food from entering duodenum

48. Phases of Gastric Secretion

49. Gastric contractile activity Peristalsis begins near cardiac sphincter (where it is a gentle ripple) & becomes much more powerful near the pylorus Contents near fundus are relatively undisturbed while foodstuffs near pylorus are being readily churned & mixed Pyloric region acts as a “dynamic filter” that allows only liquids & small particles to pass thru the barely opened pyloric valve during the digestion period Pyloric region holds ~30 ml of chyme…~3 ml spill into duodenum w/each wave of peristalsis…the other 27 ml push backward for further mixing

50. Regulation of gastric emptying Usually empties completely w/in 4 hours after a meal The larger the meal (greater distention) & the more liquid the contents…the faster it empties A full duodenum will (-) stomach emptying Carb rich meals move thru duodenum rapidly; fats form an oily layer on top of chyme & are digested slower by intestinal enzymes…as fatty chyme enters duodenum, food may remain in stomach for 6 hours or more!

51. Small intestine Responsible for completing digestion & most absorption extends from pyloric sphincter to ileocecal valve; b/t 8-13 feet long in vivo (20’ in cadaver due to lack of mm tone); diameter of ~ 1 inch 3 subdivisions: duodenum - ~10” long & curves around pancreas bile duct from liver & main pancreatic duct enter via the sphincter of Oddi (hepatopancreatic sphincter) jejunum - ~8’ long ileum - ~12’ long

52. Vascular/nervous supply PsNS via vagus nerve SNS via thoracic splanchnics PsNS & SNS are both relayed thru the superior mesenteric & celiac plexuses arterial supply mainly thru superior mesenteric a. veins parallel arteries & drain into the superior mesenteric v from there the nutrient-rich blood (from sm int) drains into hepatic portal v. which carries it to the liver

53. Modifications for absorption: Plicae circularis Deep, permanent folds of the mucosa & submucosa; approx. 1 cm tall force chyme to spiral thru the lumen, thereby slowing its transport & allowing time for full nutrient absorption

54. Modifications for absorption: Villi Fingerlike projections of the mucosa (~1 mm high) to increase surface area epithelial cells are mostly absorptive columnar cells called enterocytes villus core dense capillary bed & a wide lymph capillary called a lacteal nutrients are absorbed thru the epithelial cells into both the capillary blood & the lacteal

55. Modifications for absorption: Microvilli Tiny projections of the plasma membrane of the absorptive mucosal cells…give effect of “brush border” function to increase surface area for absorption & to... secrete enzymes referred to collectively as brush border enzymes to complete the final stages of digestion of carbs & proteins Carb digestion - dextrinase, glucoamylase, lactase, maltase, sucrase Protein digestion - aminopeptidase, carboxypeptidase, & dipeptidases Nucleic acid digestion - nucleosidases & phosphatases

56. Small intestine histology Externally not much difference along its length internally changes by location/function mucosa & submucosa are modified especially epithelium is mostly absorptive simple columnar cells connected by tight junctions w/lots of microvilli lots of goblet cells for mucus production interspersed T-cells (intraepithelial lymphocytes) - immunological fnx Do not need priming by thymus…release cytokines that kill infected target cells scattered enteroendocrine cells (source of enterogastrones - ie.gastrin, secretin & cholecystokinin)

57. Small intestine histology, cont. Crypts of Lieberkuhn - intestinal glands found in openings b/t villi epithelial cells secrete intestinal juice (watery mixture containing mucus that serves as a carrier fluid for the absorption of nutrients from chyme) crypts decrease in # as you move distally along sm. intest., while goblets increase in # stem cells at base of crypts rise & differentiate into daughter cells…villus epithelium is renewed every 3-6 days

58. Sm. Intest. Histology, cont. Submucosa typically loose, areolar CT Brunner’s glands - found only in the duodenum secrete a bicarbonate-rich (alkaline) mucus to help neutralize the acid-rich chyme from the stomach inadequate amounts may lead to duodenal ulcers Peyer’s patches – clusters of lymph nodes increase in # distally in sm. int. b/c the lg. int. contains a relatively high # of bacteria that must be prevented from entering the bloodstream

59. Small Intestine: Microscopic Anatomy

60. Components of intestinal juice Intestinal glands usually secrete 1-2 L/day major (+) is distension or irritation of mucosa by hypertonic or acidic chyme pH is normally slightly alkaline (7.4-7.8) normally isotonic w/ blood plasma mostly water w/some mucus enzyme poor b/c intestinal enzymes are normally limited to the bound enzymes of the brush border

61. Pancreas Secretes pancreatic juice via the main pancreatic duct which fuses w/the bile duct (from liver & gall bladder) to enter the duodenum at the hepatopancreatic ampulla (ampulla=flask) Acinar cells surround ducts & are responsible for secreting digestive enzymes Islets of Langerhans are endocrine cells

62. Composition of pancreatic juice 1000 ml/day produced; pH of ~8.0 Consists mostly of water, with some enzymes (produced by acinar cells) & electrolytes (primarily bicarbonate ions) Pancreatic proteases are released in inactive form & are activated in the duodenum (to prevent self-digestion of the pancreas) Amylase, lipases, & nucleases are released in their active form but require the presence of certain ions or bile be in the intestinal lumen for optimal activity

63. Regulation of pancreatic secretion Local H regulation Secretin – released in response to HCL in the duodenum; targets duct cells to release bicarb-rich pancreatic juice Cholecystokinin – released in response to entry of proteins & fats into the duodenum; (+) acinar cells to release enzyme rich pancreatic juice Vagal (+) release of pancreatic juice primarily during cephalic & gastric phases of gastric secretion

64. Liver Largest gland in body ~3 pounds Very multifunctional organ digestive role - to produce bile for export to the duodenum bile is a fat emulsifier…breaks up fats into tiny particles so they are more accessible to digestive enzymes bile is stored in the gall bladder

65. Gross anatomy of liver- for lab 4 primary lobes: right, left, caudate, quadrate (inf to left lobe) falciform ligament - a mesentary, separates rt & lt lobes anteriorly & suspends liver from diaphragm round ligament (ligamentum teres) -along free, inferior edge of falciform lig; remnant of the fetal umbilical vein Connected to lesser curvature of the stomach by lesser omentum gall bladder rests on inferior aspect of right lobe bile leaves liver thru common hepatic duct (fused from several bile ducts) merges along its way to the cystic duct, becoming the bile duct porta hepatis (doorway to the liver) - contains hepatic artery & hepatic portal vein

66. Microscopic anatomy of the liver Functional & structural units called liver lobules compose the bulk of the liver roughly six-sided structures contain layers of hepatocytes radiating outward from a central vein running longitudinally portal triad found at the junction of the 6 corners to the next lobule branch of the hepatic artery, branch of the hepatic portal vein, a bile duct liver sinusoids are leaky capillaries found b/t hepatocytes

67. Microscopic anatomy of the liver, cont. Sinusoids empty blood from portal triad into central vein…into the hepatic veins which drain the liver & empty into the IVC Kupfer cells are star-shaped hepatic macrophages found w/in the sinusoids that remove debris such as bacteria & worn out RBCs from the blood bile flows opposite direction from blood thru bile canaliculi runs towards the bile duct branches in the portal triad to leave the liver via the common hepatic duct towards the duodenum

68. Microscopic Anatomy of the Liver

69. Metabolic Activities of the Liver Carbohydrate metabolism Lipid metabolism Amino acid metabolism Waste product removal Vitamin storage (A, D, E, K) Mineral storage Drug inactivation

70. The Liver and Hematological Regulation Phagocytosis and antigen presentation Synthesis of plasma proteins Removal of circulating hormones Removal of antibodies Removal or storage of toxins Synthesis and secretion of bile

71. Composition of bile A yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes Bile salts are cholesterol derivatives that: Emulsify fat Facilitate fat and cholesterol absorption Help solubilize cholesterol Enterohepatic circulation recycles bile salts The chief bile pigment is bilirubin, a waste product of heme

72. Bilirubin - Chief bile pigment a waste product of the heme of hemoglobin formed during the breakdown of worn-out RBCs globin & iron parts of hemoglobin are saved & recycled bilirubin is absorbed from blood by liver cells & is actively secreted into the bile most bilirubin in bile is metabolized in small intestine by normal flora…byproduct is urobilirubin--gives feces its brown color feces are grey-white color w/o bile & have fatty streaks called steatorrhea

73. Bile production Liver produces 500-1000 ml/day Amt increased when GI tract contains fatty chyme Major (+) for bile secretion is bile salts themselves If more bile salts are returned to the liver via enterohepatic circulation, its output of bile increases dramatically Secretin from intestinal cells also (+) bile secretion w/ exposure of intestinal cells to fatty chyme

74. Gallbladder Located on the ventral surface of the liver Functions to store bile not immediately needed for digestion & concentrates it by absorbing some of its water & ions The bile exiting may be 10x more [ ] than the bile entering it Bile is expelled into the cystic duct & on to the bile duct

75. Cholecystokinin (CCK) An intestinal H that’s the major (+) for gall bladder contraction CCK is released into the blood when acidic, fatty chyme enters the duodenum 2 other functions of CCK 1. (+) secretion of pancreatic juice 2. Relaxes the hepatopancreatic sphincter so bile & pancreatic juice can enter the duodenum When sphincter is closed bile backs up the cystic duct into the gall bladder for storage

76. Cholelithiasis

77. Hormones of Duodenal Enteroendocrine Cells Coordinate digestive functions: Secretin Cholecystokinin (CCK) Gastric inhibitory peptide (GIP) secreted when fats and carbohydrates enter small intestine Vasoactive intestinal peptide (VIP) Stimulates secretion of intestinal glands, dilates regional capillaries, & inhibits acid production in stomach Gastrin-promotes increased stomach motility Enterocrinin-released when chyme enters small intestine & stimulates mucin production by submucosal glands of duodenum

78. Digestive processes in sm intest Chyme takes ~3-6 hours to travel thru the small intestine Remember…although most digestion & absorption takes place in the small intest, most substances needed for chemical digestion are imported from the liver & pancreas…any dis-ease process affecting either organ or the transport of products from those organs will affect the small intestines ability to effectively perform its function of absorption

79. Modification of chyme for absorption Chyme entering the duodenum is hypertonic If too much chyme were to enter the duodenum water would osmotically rush from the BVs into the intestinal lumen leading to a dangerously low blood volume Acidic chyme must also be mixed well w/ bile & pancreatic juice for digestion to continue Remember…chyme entering the duodenum is controlled by the pylorus at 3 ml/ contraction

80. Motility of small intestine Segmentation As w/the stomach, strength of contractions vary but rate remains fairly constant Duodenum~12-14 contractions/min Ileum~8-9 contractions/min Segmentation massages food back & forth a few centimeters at a time to allow intestinal contents to move towards the ileum at a rate to allow for complete digestion & absorption

81. Motility of small intestine, cont. Peristalsis Occurs after most nutrients have been absorbed Migrating motility complex – peristaltic waves starting in the duodenum travel ~ 10-70 cm b/f dying out…then the next wave begins slightly more distally than the first A ‘complete trip’ takes ~ 2 hours & then the process starts all over again Most of the time the ileocecal valve is closed except for two main exceptions Enhanced activity of the stomach initiates the gastroileal reflex which enhances force of segmentation on ileum Gastrin released by stomach increases ileum motility & relaxes ileocecal sphincter

82. Large intestine 1.5 meters in length (6 m long small intestine) Major function to absorb water from indigestible food residues & then eliminate them thru feces 3 unique features: Teniae coli – longitudinal mm layer reduced to 3 bands of smooth mm Haustra – pocket-like sacs formed by tone of teniae coli Epiploic appendages – small fat-filled pouches of visceral peritoneum hanging from the surface Significance is unknown

83. Subdivisions of the large intestine Cecum – below ileocecal valve Appendix – contains masses of lymph tissue Twisted structure allows easy entry of enteric bacteria Colon – ascending, right colic (hepatic) flexure, transverse, left colic (splenic) flexure, descending, sigmoid Rectum - ~level of S3; has 3 lateral curves/bends (transverse folds internally) that separate feces from flatus Anal canal - ~3cm long, begins where rectum penetrates levator ani mm; has 2 sphincters: internal anal sphincter – smooth mm (part of muscularis) & external anal sphincter – distinct skeletal mm

84. Large Intestine

85. Colonoscopy

86. Colostomy

87. Microscopic anatomy Colon mucosa is simple columnar epithelium except for the anus No villi & almost no digestive secretory cells present as most absorption of nutrients has already occurred Mucosa is thicker w/very large amounts of goblets in deep crypts Mucus eases the passage of feces & protects intestinal wall from acids & gases released by resident bacteria in the colon Anal canal – mucosa merges w/skin surrounding the anus (stratified squamous) Mucosa hangs in long folds called anal columns Anal sinuses b/t columns secrete mucus when compressed for ease of defecation No teniae coli or haustra present Muscularis mm layer well developed to generate strong contractions for defecation

88. Bacterial flora Most bacteria entering the cecum from the ileum are dead (lysozyme,HCl, enzymes) The remaining live bacteria along w/ those entering via the anus constitute the bacterial flora These bacteria synthesize B complex vitamins & most of the vitamin K the liver needs to synthesize some of the clotting proteins They also colonize the colon & ferment some of the indigestible carbs (ie. Cellulose) releasing irritating acids & a mixture of gases-flatus (dimethyl sulfide, H2, N2, CH4, & CO2) ~500 ml of flatus produced daily…more w/carbo-rich foods (ie. Beans)

89. Absorption in the Large Intestine Reabsorption of water Reabsorption of bile salts in the cecum transported in blood to liver Absorption of vitamins produced by bacteria Vitamin K: synthesizing 4 clotting factor Biotin: important in glucose metabolism Pantothenic acid: required in manufacture of steroid hormones and some neurotransmitters Absorption of organic wastes Colon is not essential for life (ileostomy)

90. Lg intestine motility Haustral contractions Slow, segmental movements Occur every 30 minutes or so Distension of 1 haustra causes contraction into the next one… Mass movements Long, slow, powerful movements moving over large areas of the colon 3-4 times/day (often after a meal—gastric distension?gastroileal reflex &?gastroenteric reflex…) forcing contents towards the rectum Bulk (fiber) increases strength of contractions & softens stool

91. Diverticulosis

92. Defecation reflex Of the 500 ml of food residue entering the colon only 150 ml becomes feces Stretching of the rectum wall by feces leads to the defecation reflex A spinal cord-mediated PsNS reflex causing the walls of the sigmoid colon & rectum to contract & the anal sphincters to relax W/feces entering canal, messages to brain allow us to decide whether to constrict or keep relaxed the external anal sphincter If defecation is delayed the reflex contractions end w/in a few seconds until the next mass movement occurs

93. Defecation Reflex

94. Absorption Up to 10 L of food, drink, & GI secretions enter the GI tract daily but only 1 L or less actually reaches the large intestine Remember…epithelial cells of intestinal mucosa are connected by tight jnxs…substances cannot move b/t cells but must move thru them…transepithelial transport Most nutrients are absorbed thru the intestinal villi by active transport processes driven directly or secondarily by ATP They then enter the capillary blood in the villus to be transported in the hepatic portal vein to the liver The exception is some lipids…they absorb passively by diffusion & enter the lacteal in the villus to be carried to the blood via lymphatic fluid

95. Effects of aging on the digestive system Division of epithelial stem cells declines: digestive epithelium becomes more susceptible to damage by abrasion, acids, or enzymes Smooth muscle tone and general motility decreases: peristaltic contractions become weaker Cumulative damage from toxins (alcohol, other chemicals) absorbed by digestive tract and transported to liver for processing Rates of colon cancer and stomach cancer rise with age Decline in olfactory and gustatory sensitivities: lead to dietary changes that affect entire body