The Digestive System

1. The Digestive System

2. Functions of the Digestive System • Ingest food • Break down food into nutrient molecules • Absorb molecules into the bloodstream • Rid the body of indigestible remains

4. Main Divisions of the Digestive System • Alimentary Canal • Continuous, muscular digestive tube winding throughout the body • Digests and absorbs food particles • Contains the following organs: • Mouth, Pharynx, Esophagus, Stomach, Small and Large Intestines • Accessory Digestive Organs • Contains the following organs: • Teeth, Tongue, Gallbladder, Salivary Glands, Liver, and Pancreas

5. Digestive System Divisions

6. Digestive Processes • Ingestion • Propulsion • Mechanical digestion • Chemical digestion • Absorption • Defecation

7. Circulation of the Alimentary Canal/GI Tract • Splanchnic Circulation • Includes all arteries branching off the abdominal aorta and the hepatic portal circulation • Arterial supply: • Celiac Trunk (hepatic, splenic, and left gastric) • Mesenteric Arteries: sup. and inf. • Receives ¼ of the blood volume (CO); increases after a meal

8. Histology of the Alimentary Canal/GI Tract • From esophagus to anus, the walls of the alimentary canal have the same 4 layers: • Mucosa (secretes hormones and mucus, absorbs end products of digestion, and protects against infection • Submucosa (contains lymphoid follicles and elastic tissue) • Muscularis externa (segmentation and peristalsis, contains inner circular layer and outer longitudinal layer, area where valves are found) • Serosa (same as visceral peritoneum, made of areolar connective tissue)

10. Histology of the GI Tract

11. Peritoneum • Peritoneum is the serous membrane lining the abdominopelvic cavity • Visceral peritoneum covers the external surfaces of most digestive organs and is continuous with the parietal peritoneum that lines the body wall • Between the two peritoneums is the peritoneal cavity • Mesentery is a double layer peritoneum; provides routes for BV, lymphatics, nerves • Alimentary canal organs are classified as • Retroperitoneal - no mesentery and organs lies posterior to the peritoneum (SADPUCKER) • Intraperitoneal - mesentery and organs lies within the peritoneal cavity

13. Features and Functions of the Mouth • Buccal/oral cavity • Contains stratified squamous epithelium • Vestibule: area bounded by lips and cheeks externally and teeth and gums internally • Lips: posses no sweat or oil glands • Palate: forms roof of the mouth, soft and hard palate, uvula

14. Anatomy of the Mouth

15. Features and Functions of the Tongue • Helps grind food into a bolus which contains partially digested food and saliva • Helps form words and is a sensory organ for taste • Three surface features: • Filiform papillae (roughness and grip) • Fungiform papillae (contains taste buds) • Circumvallate papillae (contains taste buds)

16. Anatomy of the Tongue

17. Features and Functions of the Salivary Glands • Main functions: • Produces and secretes saliva • Cleanses the mouth • Dissolves food chemicals so they can be tasted • Moistens food, compacting it into a bolus • Begins the chemical breakdown of food • Salivary amylase: starch

18. Types of Salivary Glands • Submandibular Glands • Found underneath the mandible • Sublingual Glands • Found underneath the tongue • Parotid Glands • Found anterior to the ear between masseter and skin • Saliva travels to oral orifice via ducts from all three glands

19. Anatomy of the Salivary Glands

20. Composition of Saliva: 97-99.5% water • pH 6.75-7.0 • Sodium, potassium, chloride, phosphate, and bicarbonate • Mucin • Salivary amylase

21. Features and Functions of the Teeth • Break food into smaller parts, increasing surface area for digestion • Types of Teeth • Deciduous Teeth (“baby” teeth) • Permanent Teeth • Incisors- cutting and shredding • Canines- piercing and tearing • Molars- grinding • Premolars- grinding and crushing

22. Tooth Structure • Crown: exposed portion of tooth covered by enamel which covers dentin • Root: internal portion that is beneath the gums (gingiva) and is anchored by periodontal ligaments

23. Deglutition and the Pharynx • Deglutition = swallowing • Oropharynx and Laryngopharynx are common passageways for food and air • Pharynx contains stratified squamous epithelium (friction-resistant)

24. Features and Functions of the Esophagus • Muscular tube that propels food to stomach; bolus enters stomach through esophageal hiatus • Skeletal muscle (upper third for swallowing) and smooth muscle (lower third) for peristalsis • Esophageal glands – produce mucus to lubricate bolus • Esophageal sphincter – prevents backflow into oral cavity • Cardiac sphincter- prevents backflow into esophagus

25. Anatomy of the Esophagus

26. Microscopic Anatomy of the Esophagus

27. Digestive Processes in the Mouth, Pharynx, and Esophagus • Mouth processes: • Ingestion • Mechanical digestion (e.g. salivary amylase) • Initiation of Propulsion • Mastication: chewing • Pharyngeal processes: • Deglutition = swallowing • Voluntary Buccal phase • Involuntary Pharyngeal-Esophageal Phase • Esophageal processes: • Peristalsis (rhythmic contractions, involuntary)

28. Deglutition and the Pharynx

29. Peristalsis

30. Features and Functions of the Stomach • Temporary storage area for food and allows it to mix with gastric juice to produce chyme • Regions: cardiac, fundus, body, and pyloric • Greater and Lesser Curvatures: connected to greater and lesser omentums • Rugae folds: longitudinal folds in stomach wall - mucous b/w folds • Muscle layers arranged circularly, longitudinally, AND obliquely (aids in digestion)

31. Anatomy of the Stomach

32. Microscopic Anatomy of the Stomach • Simple columnar epithelium – contains gastric pits that secrete gastric juices • Goblet cells – secrete mucus that coats stomach and prevents it from being digested itself • Parietal cells – secrete hydrochloric acid (converts pepsinogen into pepsin) and intrinsic factor (necessary for absorption of vitamin B12) • Chief cells – secrete pepsinogen which is converted to pepsin to aid in protein digestion • Enteroendocrine cells – release hormones such as: • Histamine, Serotonin, Gastrin, Endorphins, and Somatostatin

33. Histamine - activates parietal cells to release HCl • Serotonin - contraction of stomach muscle • Gastrin - gastric glands to increase secretion • Endorphins - natural opiates • Somatostatin - sympathetic n.s. - inhibits gastric secretion - inhibits gastric emptying

34. Microscopic Anatomy of the Digestive System

35. Digestive Processes in the Stomach • Gastric Secretion • Cephalic Phase • Stimulated by the thought, sight, taste, or aroma or food • Inputs from olfactory receptors and taste buds travel to parasympathetic enteric ganglia which then stimulate stomach glands • Gastric Phase • Stomach distension activates stretch receptors • Food chemicals (e.g. peptides, rising pH) activate chemoreceptors which activate G cells which secrete gastrin • Intestinal Phase • Presence of low pH and partially digested foods in duodenum stimulates intestinal gastrin secretion

36. Digestive Processes in the Stomach

37. Gastric Motility and Emptying • Peristaltic waves approach stomach and become stronger near pyloric region • Pyloric sphincter allows ~ 3 mL of chyme to pass to duodenum and the rest to return to stomach for further mixing

38. Features and Functions of the Small Intestine • Receives chyme from stomach; performs majority of digestion and absorption of nutrients • Regions: • Duodenum (upper region receiving chyme from stomach and digestive enzymes from pancreas and bile from liver and gallbladder) • Jejunum/Ileum (lower regions where absorption occurs) • Plicae circulares (permanent folds in mucosa and submucosa that slow movement of chyme)

41. Anatomy of the Small Intestine

42. Microscopic Anatomy of Small Intestine • Villi: fingerlike projections that increase the surface area of the SI • Microvilli: tiny projections on the plasma membranes of columnar cells that appear fuzzy (i.e. brush border cells) • Crypts of Lieberkuhn: secrete intestinal juice and special lysozymes that protect against bacteria • Peyer’s Patches: aggregated lymphoid tissues containing lymphocytes

43. Microscopic Anatomy of the Small Intestine

44. Secretions of the Small Intestine • Secretin: released by enteroendocrine cells when acidic chyme enters SI; causes release of bicarbonate-rich pancreatic juices • Somatostatin: slows gastric motility and emptying and inhibits production of gastric secretions • Cholecystokinin (CCK): released when fatty, protein-rich chyme enters SI; causes release of enzyme-rich pancreatic juices and bile • Brush border enzymes: process long peptides, nucleic acids, and sugars into smaller ones

45. After Digestion: Absorption Protein Fats

46. Functions of the Liver • Largest internal organ • Functions: • Filters and processes nutrient-rich blood of carbohydrates, proteins, and lipids from intestine • Production and regulation of cholesterol • Production of bile which emulsifies fats • Removes drugs and hormones from circulation • Storage of vitamins and minerals

47. Anatomy of the Liver • Right and Left Lobes: separated by falciform ligament • Caudate and Quadrate Lobes: found on posterior side • Blood vessels: • Hepatic artery/vein and hepatic portal vein • Gallbladder: found underneath left lobe, stores bile

48. Gross Anatomy of the Liver

50. Microscopic Anatomy of the Liver • Liver Lobules: structural unit of liver • Hepatocytes: liver cells contained within the lobules • Hepatic portal vein & Hepatic Artery: the circulation of the liver, they bring blood into the liver where it is filtered through the liver sinusoidal capillaries • Kupffer cells: remove debris • Filtered blood drains into the central vein, then to the hepatic vein, and eventually to the inferior vena cava • Bile (produced by hepatocytes) drains into the bile duct after passing through portal triad • Bile then shipped to gallbladder for storage