The Digestive Tract

1. The Digestive Tract

2. The GI tract(gastrointestinal tract) The muscular alimentary canal • Mouth • Pharynx • Esophagus • Stomach • Small intestine • Large intestine • Anus • The accessory digestive organs Supply secretions contributing to the breakdown of food • Teeth & tongue • Salivary glands • Gallbladder • Liver • Pancreas

3. The Digestive Process • Ingestion • Taking in food through the mouth • Propulsion (movement of food) • Swallowing • Peristalsis – propulsion by alternate contraction &relaxation • Mechanical digestion • Chewing • Churning in stomach • Mixing by segmentation • Chemical digestion • By secreted enzymes: see later • Absorption • Transport of digested end products into blood and lymph in wall of canal • Defecation • Elimination of indigestible substances from body as feces

4. Chemical digestion • Complex food molecules (carbohydrates, proteins and lipids) broken down into chemical building blocks (simple sugars, amino acids, and fatty acids and glycerol) • Carried out by enzymes secreted by digestive glands into lumen of the alimentary canal

5. Ways to divide…. The more common Plus: epigastric periumbilical suprapubic flank

6. Histology of alimentary canal wallSame four layers from esophagus to anal canal • Mucosa • Submucosa • Muscularis externa • Serosa from lumen (inside) out

7. Inner layer: the mucosa*(mucous membrane) Three sub-layers • Lining epithelium • Lamina propria • Muscularis mucosae *

8. More about the mucosa • Epithelium: absorbs nutrients, secretes mucus • Continuous with ducts and secretory cells of intrinsic digestive glands (those within the wall) • Extrinsic (accessory) glands: the larger ones such as liver and pancreas • Lamina propria • Loose connective tissue with nourishing and absorbing capillaries • Contains most of mucosa-associated lymphoid tissue (MALT) • Muscularis mucosae • Thin layer of muscle producing only local movements

9. Second layer: the submucosa* • Connective tissue containing major blood and lymphatic vessels and nerves • Many elastic fibers so gut can regain shape after food passes *

10. Next in, the muscularis externa*(AKA just “muscularis”) Two layers of smooth muscle responsible for peristalsis and segmentation • Inner circular layer (circumferential) • Squeezes • In some places forms sphincters (act as valves) • Outer longitudinal layer: shortens gut *

11. Last (outer), the serosa*(the visceral peritoneum) • Simple squamous epithelium (mesothelium) • Thin layer of areolar connective tissue underneath • Exceptions: • Parts not in peritoneal cavity have adventitia, lack serosa • Some have both, e.g. retroperitoneal organs *

12. Smooth muscle Smooth muscle • Muscles are spindle-shaped cells • One central nucleus • Grouped into sheets: often running perpendicular to each other • Peristalsis • No striations (no sarcomeres) • Contractions are slow, sustained and resistant to fatigue • Does not always require a nervous signal: can be stimulated by stretching or hormones • 6 major locations: • inside the eye 2. walls of vessels 3. respiratory tubes • 4. digestive tubes 5. urinary organs 6. reproductive organs

13. Nerves • Enteric nervous system: the gut’s own • Visceral plexuses within gut wall controlling the muscles, glands and having sensory info • Myenteric: in muscularis • Submucosal • 100 million neurons! (as many as the spinal cord) • Autonomic input: speeds or slows the system • Parasympathetic • Stimulates digestive functions • Sympathetic • Inhibits digestion • Largely automatic

14. Review of some definitions…. • Peritoneum: serous membranes of the abdominopelvic cavity • Visceral peritoneum: covers external surfaces of most digestive organs • Parietal peritoneum: lines body wall • Peritoneal cavity: slit-like potential space between visceral and parietal peritoneum • Serous fluid – lubricating

15. New definitions • Mesentery • Double layer of peritoneum • Extends to digestive organs from body wall • Hold organs in place • Sites of fat storage • Route by which circulatory vessels and nerves reach organs • Most are dorsal • Extend dorsally from gut to posterior abdominal wall • Ventral mesentery – from stomach and liver to anterior abdominal wall • Some mesenteries are called “ligaments” though not technically such

16. Mesenteries • Note dorsal, ventral and formation of retroperitoneal position

17. Mesenteries • Two ventral mesenteries • Falciform “ligament” • Binds anterior aspect of liver to anterior abdominal wall and diaphragm • Lesser omentum (=“fatty skin”) – see diagram* • All other mesenteries are dorsal (posterior) *

18. Mesenteries continued (all these are dorsal) • Greater omentum • Connects stomach to posterior abdominal wall – very roundabout • Wraps around spleen: gastrosplenic ligament • Continues dorsally as splenorenal ligament • A lot of fat • Limits spread of infection by wrapping around inflamed e.g. appendix • “Mesentery” or mesentery proper • Supports long coils of jejunum and ileum (parts of small intestine) • Transverse mesocolon • Transverse colon held to posterior abdominal wall • Nearly horizontal sheet fused to underside of greater omentum • Sigmoid mesocolon • Connects sigmoid colon to posterior abdominal wall see next slides for pics…

19. Note mesenteries: falciform ligament, lesser omentum, greater omentum

20. Note: greater omentum, lesser omentum, falciform ligament, transverse mesocolon, mesentery, sigmoid mesocolon

21. Some organs are “retroperitoneal” • Are “behind the peritoneum” • Fused to posterior (dorsal) abdominal wall • Lack a mesentery • Include: • Most of duodenum (1st part of small intestine) • Ascending colon • Descending colon • Rectum • Pancreas • Tend to cause back pain, instead of abdominal pain (This is as opposed to the organs which are intraperitoneal, or just “peritoneal”)

22. The Mouth • Mouth = oral cavity • Lining: thick stratified squamous epithelium • Lips- orbicularis oris muscle • Cheeks – buccinator muscle

23. “Vermillion border” or red border • Between highly keratinized skin of face and mucosa of mouth • Needs moisture • Note frenulums (folds of mucosa) • Palate – roof of mouth • Hard plate anteriorly • Soft palate posterioly • Uvula

24. Tongue • Mostly muscles • Grip and reposition food • Forms “bolus” of food (lump) • Help in swallowing • Speech – help form some consonants • Note frenulum on previous slide: can be too tight • Taste buds contained by circumvallate and fungiform papillae • Lingual tonsil – back of tongue

25. Teeth • Called “dentition” (like dentist) • Teeth live in sockets (alveoli) in the gum-covered margins of the mandible and maxilla • Chewing: raising and lowering the mandible and moving it from side to side while tongue positions food between teeth

26. Teeth • Two sets • Primary or deciduous • “Baby” teeth • Start at 6 months • 20 are out by about 2 years • Fall out between 2-6 years • Permanent: 32 total • All but 3rd set of molars by end of adolescence • 3rd set = “wisdom teeth” • Variable • Some can be “impacted” (imbedded in bone)

27. Teeth are classified according to shape and function • Incisors: chisel-shaped for chopping off pieces • Canines: cone shaped to tear and pierce • Premolars (bicuspids) and • Molars - broad crowns with 4-5 rounded cusps for grinding incisor canine premolar molar Cusps are surface bumps

28. Tooth structure • Two main regions • Crown (exposed) • Root (in socket) • Meet at neck • Enamel • 99% calcium crystals • Hardest substance in body • Dentin – bulk of the tooth (bone-like but harder than bone, with collagen and mineral) • Pulp cavity with vessels and nerves • Root canal: the part of the pulp in the root A C B

29. Tooth structure • Cementum – bone layer of tooth root • Attaches tooth to periodontal ligament • Periodontal ligament • Anchors tooth in boney socket of the jaw • Continuous with gingiva (gums) • Cavities or caries - rot • Plaque – film of sugar, bacteria and debris A C B

30. Salivary glands(tuboalveolar glands) • Intrinsic salivary glands – within mucosa • Secrete saliva all the time to keep mouth moist • Extrinsic salivary glands • Paired (2 each) • Parotid • Submandibular • Sublingual • External to mouth • Ducts to mouth • Secrete saliva only right before or during eating Saliva: mixture of water, ions, mucus, enzymes keep mouth moist dissolves food so can be tasted moistens food starts enzymatic digestion buffers acid antibacterial and antiviral

31. Extrinsic salivary glands • Parotids* - largest (think mumps) • Facial nerve branch at risk during surgery here • Submandibular # - medial surface mandible • Sublingual + - under tongue; floor of mouth * + Compound = duct branches Tubo = tubes Alveolar = sacs #

32. Pharynx ___oropharynx • Oropharynx and laryngopharynx • Stratified squamous epithelium • Three constrictor muscles* • Sequentially squeeze bolus of food into esophagus • Are skeletal muscles • Voluntary action • Vagus nerve (X) ___laryngopharynx * * *

33. Esophagus • Continuation of pharynx in mid neck • Muscular tube collapsed when lumen empty • Descends through thorax • On anterior surface of vertebral column • Behind (posterior to) trachea Esophagus___________ *

34. Esophagus continued • Passes through “esophageal hiatus” in the diaphragm to enter the abdomen • Abdominal part only 2 cm long • Joins stomach at cardiac orifice* • Cardiac sphincter at cardiac orifice to prevent regurgitation (food coming back up into esophagus) • Gastroesophageal junction and GERD ___________________esophageal hiatus (hiatus means opening) *

35. Microscopic anatomy of esophagus Contains all 4 layers (see right) • Epithelium: nonkeratinized stratified squamous epithelium • At GE junction – thin simple columnar epithelium • Mucus glands in wall • Muscle (muscularis externa) changes as it goes down • Superior 1/3 of esophagus: skeletal muscle (like pharynx) • Middle 1/3 mixture of skeletal and smooth muscle • Inferior 1/3 smooth muscle (as in stomach and intestines) • When empty, mucosa and submucosa lie in longitudinal folds

36. Esophagus histology

37. Stomach • J-shaped; widest part of alimentary canal • Temporary storage and mixing – 4 hours • Into “chyme” • Starts food breakdown • Pepsin (protein-digesting enzyme needing acid environment) • HCl (hydrochloric acid) helps kill bacteria • Stomach tolerates high acid content but esophagus doesn’t – why it hurts so much when stomach contents refluxes into esophagus (heartburn; GERD) • Most nutrients wait until get to small intestine to be absorbed; exceptions are: • Water, electrolytes, some drugs like aspirin and alcohol (absorbed through stomach)

38. Stomach epigastrium • Lies mostly in LUQ • But pain can be epigastric or lower • Just inferior to (below) diaphragm • Anterior (in front of) spleen and pancreas • Tucked under left lower margin of liver • Anchored at both ends but mobile in between • Main regions in drawing to right-------------------------------- • Capacity: 1.5 L food; max capacity 4L (1 gallon) dome junction with esophagus contains pyloric sphincter funnel shaped

40. Stomach Regions • Cardiac region • Fundus (dome shaped) • Body • Greater curvature • Lesser curvature • Pyloric region • Antrum • Canal • Sphincter dome junction with esophagus contains pyloric sphincter funnel shaped

41. Rugae: longitudinal folds on internal surface (helps distensibility) • Muscularis: additional innermost oblique layer (along with circular and longitudinal layers)

42. Histology of stomach • Simple columnar epithelium: secrete bicarbonate-buffered mucus • Gastric pits opening into gastric glands • Mucus neck cells • Parietal cells • HCL • Intrinsic factor (for B12 absorption) • Chief cells • Pepsinogen (activated to pepsin with HCL) • Stimulated by gastrin: a stomach hormone

43. Small intestine • Longest part of alimentary canal (2.7-5 m) • Most enzymatic digestion occurs here • Most enzymes secreted by pancreas, not small intestine • Almost all absorption of nutrients • 3-6 hour process • Runs from pyloric sphincter to RLQ Small intestine___________

44. Blood supply: superior mesenteric artery; Veins drain into hepatic portal vein • Small intestine has 3 subdivisions • Duodenum – 5% of length • Jejunum – almost 40% • Ileum – almost 60% Duodenum is retroperitoneal (stuck down under peritoneum); others are loose Duodenum receives bile from liver and gallbladder via bile duct* enzymes from pancreas via main pancreatic duct* * *

45. Small intestine designed for absorption • Huge surface area because of great length • Structural modifications also increase absorptive area • Circular folds (plicae circulares) • Villi (fingerlike projections) 1 mm high – simple columnar epithelium: velvety • Microvilli Absorptivie cell with microvilli to increase surface area & many mitochondria: nutrient uptake is energy-demanding * Lacteal*: network of blood and lymph capillaries -Carbs and proteins into blood to liver via hepatic portal vein -Fat into lymph: fat-soluble toxins e.g. pesticides circulate systemically before going to liver for detoxification

46. Intestinal crypts* (of Lieberkuhn) inbetween villi • Cells here divide every 3-6 days to renew epithelium (most rapidly dividing cells of the body) • Secrete watery intestinal juice which mixes with chyme (the paste that food becomes after stomach churns it) • Intestinal flora – the permanent normal bacteria • Manufacture some vitamins, e.g. K, which get absorbed -have many mitochondria: nutrient uptake is energy-demanding Duodenal glands* * • Mucus to counteract acidity from stomach • Hormones: • Cholecystokinin (stimulates GB to release stored bile, also pancreas) • Secretin (stimulates pancreatic ducts to release acid neutralizer) * -produce mucus

47. General histology of digestive tract

50. Large intestine Digested residue reaches it Main function: to absorb water and electrolytes Subdivisions Cecum Appendix Colon Rectum Anal canal