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Digestion

Learn about the process of digestion, including the reduction of complex food substances through mechanical and chemical means, their absorption into the internal environment, and the functions of the digestive system. Explore the major organs involved, the histology of the digestive tract, and the control of GI tract activity. Understand the roles of saliva, stomach acid, pancreatic juice, and bile in chemical digestion, as well as the absorption of carbohydrates, proteins, lipids, and nucleic acids.

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Digestion

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  1. Digestion General Physiology Tony Serino, Ph.D. Biology 346 Misericordia University

  2. Digestion • The reduction through mechanical and chemical means (hydrolysis) of complex food substances into simple monomers and their absorption into the internal environment.

  3. Functions of Digestive System • Motility(Propulsion) • Ingestion –food enters tract • Mastication -chewing • Deglutition -swallowing • Transportation through tract (peristalsis) • Mixing • Egestion (Defecation) • Secretion • Endocrine and Exocrine secretions • Digestion • mechanical and chemical breakdown of food • Absorption • Passage of food particles from external to the internal environment

  4. Major Organs of System

  5. Teeth Accessory Organs

  6. Basic Histology of Digestive Tract (Auerbach’s) (Meissner’s) (LOCI)

  7. Tube Movements Peristalsis Segmentation* *majority of contractions of SI

  8. Control of Overall GI Tract Activity

  9. Mouth –ingestion, mastication, mechanical dig. Salivary glands secrete saliva –mixture of water, mucus, electrolytes, antibodies and enzymes. Enzymes are salivary amylase (pytalin) which breaks down starches and maltase. Tongue –mixes food and tastes Teeth –mechanical digestion

  10. Pharynx –junctional tube Pharyngeal Constrictors Nasopharynx Oropharynx Laryngopharynx

  11. Deglutition (Swallowing)

  12. Deglutition (cont’d)

  13. J-shaped muscular pouch • Receives bolus and produces chyme • Liquefies food by mixing it with HCl and vigorous churning • Produces gastric juice: mucous, water, HCl, pepsin, rennin (in infant), GIF (binds to vit. B12 in diet) Stomach • Low pH stops amylase activity, but secretes pepsinogen (pepsin) that begins break down of proteins • Absorbs little except imbibed water, electrolytes, and some drugs (ie. alcohol and aspirin)

  14. Stomach Acid • HCl acid is produced by the parietal cells of the gastric glands

  15. Regulation of Gastric Juice

  16. Small Intestine • Largest amount of digestion and absorption of gut • Several structures to increse surface area: plica, villli, length, microvilli • Secrete or have bound many enzymes that complete digestion process • Intestinal juice also contains mucous and antibacterial compounds • Absoprtion directly into capillary bed of villi or into villus lacteal

  17. Secretin’s receptors are found in the pancreas, which responds with additional bicarbonate delivery: gastric motility and secretion are inhibited.

  18. Cholecystokinin’s receptors are located: • in the pancreas, which • responds with additional • enzyme delivery • in the gallbladder, which • contracts to deliver more • bile • in the sphincter • of Oddi, which relaxes to • facilitate delivery of the enzymes and bile salts

  19. Large Intestine (Colon) • Massive re-absorption of water and electrolytes by active absorption of Na+ • Microbe action produces Vit. K and is absorbed • Responsible for egestion(defecation reflex)

  20. Defecation Reflex

  21. Liver • Metaboblizes all food groups • Storage of Fe and Cu and other metals • Storage of Vit. A, B12, D, E, K • Produces bile and most plasma proteins • Detoxifies the blood, storage of toxins

  22. Bile formation by cells in the liver includes 6 components: bile salts, lecithin, bicarbonate ions, cholesterol, bile pigments, and trace metals. The bile is stored in the gallbladder and then delivered into the duodenum upon stimulation from CCK.

  23. Bile Salts Up to 95% of the cholesterol-based bile salts are “recycled” by reabsorption along the intestine.

  24. Secretion of Bile

  25. Pancreas • Heterocrine gland secretes pancreatic juice and hormones • Pancreatic juice is sodium bicarbonate and digestive enzymes including trypsin, chymotrypsin, carboxypeptidase, lipase, etc. • Hormones: insulin, glucagon, somatostatin

  26. Secretion of Pancreatic Juice

  27. Chemical Digestion -CHO • Ingest 200-400 grams per day • Digestible sugars are the disaccharides: sucrose, lactose, and maltose and the polysaccharides: glycogen and starch

  28. Chemical Digestion -Proteins • Major proportion of our food intake

  29. Amino Acid Absorption

  30. Chemical Digestion -Lipids • Small intestine is only major site of digestion since pancreas is largest producer of lipase • Dietary fat accumulate as large globule; bile salts break globule into small droplets (emulsify the fats) providing greater access to enzymes

  31. Micelles and Fat absorption Emulsifying Fat

  32. Fat Absorption Big Droplets of Fat Small Droplets of Fat Micelles Fatty Acids and Monoglycerides Chylomicron Assembly Distribution and Processing

  33. Chemical Digestion –Nucleic Acids

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