The Digestive System

1. The Digestive System

3. Digestive System: Overview • The alimentary canal or gastrointestinal (GI) tract digests and absorbs food. • Alimentary canal – mouth, pharynx, esophagus, stomach, small intestine, and large intestine. • Accessory digestive organs – teeth, tongue, gallbladder, salivary glands, liver, and pancreas.

4. Greater Omentum • Greater - covers small intestines like an apron. • This is the visceral fat that may be detrimental to your health.

5. Where does she carry most of her weight?

6. Mesentery and Mesocolon • Mesentery of small intestines contain the nerves and the blood vessels • Mesocolon anchors colon to posterior body wall

7. Peritoneum – serous membrane of the abdominal cavity Visceral – covers external surface of most digestive organs Parietal – lines the body wall Peritoneal cavity- small space between visceral and parietal peritoneum. Lubricates digestive organs Allows them to slide across one another Retroperitoneal structures are located behind the peritoneum.

8. The Digestive System Process • Ingestion: intake of food. • Digestion: Mechanical and chemical breakdown of food. • Absorption: Uptake of nutrients from the digestive tract into the blood and lymph. • Compaction: Absorbing water and consolidating indigestible food to form feces. • Defecation: Elimination of the feces.

9. Types of Digestion • Mechanical digestion • physical breakdown of food into smaller particles • teeth and churning action of stomach and intestines • Chemical digestion • series of hydrolysis reactions that break macromolecules into their monomers • enzymes from saliva, stomach, pancreas and intestines • Results • Polysaccharides into monosaccharides • Polypeptides into amino acids • Triglycerides into glycerol and fatty acids

10. Motility in the Intestines • Segmentation ensures mechanical digestion in the SI. • Ensure enzymes mix with proper substrates for maximal absorption.

11. Peristalsis Peristalsis moves contents steadily in one direction along the alimentary canal.

12. Histology of the Alimentary Canal • From esophagus to the anal canal the walls of the GI tract have the same four layers (tunics) • From the lumen outward they are the mucosa, submucosa, muscularis externa, and serosa • Each tunic has a predominant tissue type and a specific digestive function

13. Anatomy of the Alimentary Canal Diaphragm Serosa Mucosa Muscularis Submucosa Blood vessels

14. Mucosa • The innermost layer epithelial layer that lines the lumen of the alimentary canal • Stomach and small intestine mucosa contain: • Enzyme-secreting cells that aid in the absorption of nutrients for of digestion • Mucus producing cells protects the lumen from hydrolytic enzymes while aiding the movement food through the alimentary canal • Has many lymphatic vessels that protection against infectious disease

15. Other layers of the Alimentary Canal • Submucosa – • Dense connective tissue containing elastic fibers • Blood and lymphatic vessels • Nerves (Submucosal Nerve Plexus) • Muscularis externa – • Composed of involuntary smooth muscle innervated by the (Myenteric Nerve Plexus) • responsible for segmentation and peristalsis • Serosa • the protective visceral peritoneum • Outer most layer

16. Enteric Nervous Control Myenteric Nerve Plexus Submucosal Nerve Plexus

17. Enteric Nervous Control • Is often referred to the second brain because it can regulate digestive tract mobility, secretion and blood flow independently of CNS. • It is thought that there are more neurons in the ENS than the spinal cord. • Composed of two nerve networks • submucosal plexus • controls glandular secretion and muscular contraction of the mucosa • myenteric plexus • controls peristalsis • contractions of muscularis external

18. Autonomic Function in Digestion • Parasympathetic nervous system has many neural connections to the ENS. • It stimulates digestive activities via the vagus nerve. • Sympathetic nervous system inhibits digestive activity by shunting blood away from the gut and to the heart and skeletal muscles.

19. Phases of Digestion

20. Cephalic phase • Prepares the digestive system before the ingestion of food. Stimulation of the senses (sight, smell and taste) or thought of food will activate the Parasympathetic Nervous System (PNS) via the vagus nerve. • The medulla will stimulate the both digestive organs and glands to increase their metabolic activity. • Excitatory events include: • Sight or thought of food • Stimulation of taste or smell receptors • Inhibitory events include: • Loss of appetite or depression • Decrease in stimulation of the parasympathetic division

21. Digestive Processes in the Mouth • Mechanical digestion begins in the mouth (chewing) • Salivary amylase begins chemical breakdown of starch. • Mouth is at a fairly neutral ph. Ideal for amylase • Propulsion is initiated by swallowing • The pharynx and esophagus serve as a passage way for food from the mouth to the stomach.

22. Salivary Glands

23. Saliva • Functions of saliva • moisten, begin starch and fat digestion, cleanse teeth, inhibit bacteria, bind food together into bolus • Composed of 99.5% water and solutes • salivary amylase, begins starch digestion • lingual lipase, digests fat activated by stomach acid • mucus, aids in swallowing • lysozyme, enzyme kills bacteria • immunoglobulin A, inhibits bacterial growth • electrolytes = Na+, K+, Cl-, phosphate and bicarbonate • pH of 6.8 to 7.0

26. Stomach • Chemical breakdown of proteins and to a lesser extent fats begins and food (bolus) is converted to chyme. • Cardiac region –connects the esophagus to stomach. • Muscularis –Is composed of 3 muscular layers • Allows the stomach to churn, mix, and pummel food physically • Rugae: folds in the stomach when empty stomach • The pylorus is continuous with the duodenum through the pyloric sphincter • Regulates how rate that food enters into the small intestines.

27. Digestion in the Stomach • The stomach: • Holds ingested food • Digests food both mechanically and chemically • Delivers chyme to the small intestine • Enzymatically digests proteins with pepsin • Lingual lipase activated. • Secretes intrinsic factor required for absorption of vitamin B12 • Vitamin B12 is necessary for the production of many different type of cells including nerve and red blood cells • Pernicious anemia can result from a lack of either Vitamin B12 or intrinsic factor.

28. Glands of the Stomach • Gastric glands of the fundus and body have a variety of secretory cells • Mucous neck cells – secrete mucus to protect stomach from acid environment. • Parietal cells – secrete H+Cl- , intrinsic factor and ghrelin • Chief cells – produce pepsinogen and gastric lipase • Pepsinogen is activated to pepsin by H+Cl- in the stomach • Enteroendocrine cells – secrete hormones that regulate digestion and gastric motility. • G-cells secretegastrin : hormone that increases secretory activity and gastric mobility

29. Microscopic Anatomy of the Stomach

30. Gastric Phase • Excitatory events include: Stimulate Gastrin release • Stomach distension (stomach expands as bolus enters) • Activation of stretch receptors stimulates the Vagus nerve • Activation of chemoreceptors: presences of proteins buffer stomach acid ↑pH • Inhibitory events include: • As food exits out the pylorus the pH may drop below 2 • There will be less distension. • Emotional upset that overrides the parasympathetic division and activate the sympathetic division

31. Control of Gastric Secretion

32. Stomach Acid Functions • Activate enzymes pepsin and lingual lipase. • Breakdown plant connective tissues and cell walls. • Denatures proteins and converts minerals to a more bioavailable form. • Contributes to immunity by destroying ingested pathogens.

33. Healthy Mucosa and Peptic Ulcer • Gastric and Peptic ulcers are linked to the acid tolerant bacteria Helicobactor pylori and excessive use of NSAID’s. • Antibiotics function to kill the H pylori bacteria • Proton pump inhibitors block the H+K+ATPase pump thus reducing acid production in the stomach.

34. Hiatal Hernia • Part of the esophagus and upper part of stomach get trapped in the thoracic cavity. • LES sphincter fails allowing acid in the esophagus. • mimic symptoms of a heart attack. • Over time it will erode the esophagus resulting in strictures, hemorrhage or even cancer.

35. Hormones and Gastric Emptying • Once food (Chyme) enters the small intestines duodenal enteroendocrine cells secrete various hormones that inhibit gastric secretion : • CCK, (Cholecystokinin)VIP (Vasoactive Intestine Peptide ) GIP( Gastric inhibitory peptide) and Secretin • These collectively inhibit stomach secretions of HCL and gastrin from the G-cells thus reducing stomach motility. • Pyloric sphincter tightens limiting admission of more food into the duodenum. • Allows the duodenum time to work on the chyme that has already entered.

36. Small Intestine – 3 Segments • Has three subdivisions: duodenum, jejunum, and ileum • Runs from pyloric sphincter to the ileocecal valve

37. Small Intestine • Duodenum (10 in.) • Receives stomach contents through the pyloric sphincter. • Pancreatic juice and bile aid in chemical digestion of chyme. • Neutralizes stomach acids, emulsifies fats, pepsin inactivated by pH increase, pancreatic enzymes hydrolyze their specific substrate. • Mostly digestion with minimal absorption occurs here. • Jejunum - next 8 ft. (in upper abdomen) • has large tall circular folds; walls are thick, muscular • nutrient digestion and absorption occurs here • Ileum - last 12 ft. (in lower abdomen) • has peyer’s patches – clusters of lymphatic nodules • ends at ileocecal junction with large intestine • Primarily nutrient absorption occurs here

38. Microscopic Anatomy of the Small Intestine

39. Small Intestine: Histology • Lined simple columnar epithelium which functions in absorption and secretion. • Villi • fingerlike extensions of the mucosa containing blood and lymph vessels for the absorption of nutrients • Microvilli • tiny folds in the individual simple columnar epithelial cells that further increase digestive surface area. • Brush-border enzymes • hydrolytic enzymes responsible for chemical digestion of organic molecules and activation of pancreatic enzymes. • Pancreatic enzymes and bile enter the duodenum through for the sphincter of Oddi

41. Liver: Functions • The largest gland in the body • Liver cells( Hepatocytes) functions include: • Production of bile (emulsification of fat) • Detoxification( removes or dilutes toxins in the body • Storage of fat-soluble vitamins ( ADEK) • Increases BGLs: gluconeogenesis and glycogenolysis. • Decreases BGL’s: Stores excess glucose as glycogen • Formation of cholesterol including HDL and LDL • Used to transport lipids throughout the circulatory system.

42. Pancreas • Exocrine function • Acini (clusters of secretory cells) digestive enzymes • Secretes pancreatic juice which breaks down all categories of foodstuff • The pancreas also has an endocrine function • release of insulin and glucagon

43. Acinus of the Pancreas

44. Hormonal Control of the Intestinal Phase • Baroreceptor and Chemoreceptors detects distension chemical changes in the intestines results in enteroendocrine cells to secrete two hormones • Secretin causes the release of HCO3– from the pancreas neutralizing acid chyme • Alkaline pH provides an optimal environment for pancreatic enzymes • CCK stimulates the pancreas to release the following digestive enzymes: Amylase ,Lipase and Proteases : (trypsinogen, chymotrypsinogin, procarboxypeptidase) • contraction of the Gall bladder • Relaxes the Sphincter of Oddi into the duodenum. • Promotes short term satiety

45. The Gallbladder • A green muscular sac on the ventral surface of the liver is palpable in the right upper quadrant. • Stores and concentrates bile that was produced in the liver • Bile is released in response to the hormone CCK. • Main role of bile is to emulsify fat (break into smaller pieces) • Emulsified fat droplets have a greater surface area for pancreatic lipase to work.

46. Emulsification of Fats

47. Fatty Acid Absorption

48. Chemical Digestion: Fats • Absorption: bile salts and pancreatic lipase results in FFA and Glycerol • FFA diffuse into intestinal cells where they: • Combine with proteins and form chylomicrons • Large chylomicrons must enter lacteals and are transported to systemic circulation via lymphatic system.

49. Carbohydrate Metabolism • Salivary amylase gets denatured in stomach (pH < 4.5) • Pancreatic amylase further hydrolyzes carbohydrates. • Brush border enzymes maltase, sucrase, lactase complete hydrolysis resulting in monosaccharides. • Maltase → Hydrolyzes maltose into 2 glucose molecules • Sucrase → hydrolyzes sucrose leaving fructose and glucose • Lactase → hydrolyzes lactose into Galactose and glucose • Absorption: via co-transport with Na+, and facilitated diffusion • Enter the capillary bed in the villi • Transported to the liver via the hepatic portal vein

50. Carbohydrate Digestion - Small Intestine