Digestive System PDF

Digestive System Digestive System Anatomy of Gastrointestinal System The gastrointestinal (GI) system includes following: 1. Mouth 2. Pharynx 3. Esophagus 4. Stomach 5. Small intestine 6. Large intestine 7. The accessory organs that are not part of the tract but secrete...

Digestive System Digestive System Anatomy of Gastrointestinal System The gastrointestinal (GI) system includes following: 1. Mouth 2. Pharynx 3. Esophagus 4. Stomach 5. Small intestine 6. Large intestine 7. The accessory organs that are not part of the tract but secrete substances via connecting ducts, include the following: 1. Salivary glands, 2. Liver, 3. Gallbladder 4. Pancreas. Functions of the gastrointestinal organs The functions of the gastrointestinal system can be described in terms of four processes: 1. Digestion 2. Secretion 3. Absorption 4. motility  Digestion = The breaking down of large particles containing macromolecules (such as proteins and polysaccharides) into small molecules. This is accomplished by the action of HCl in the stomach, bile from the liver, and a variety of digestive enzymes that are released by the system’s exocrine glands.  Secretion = The releasing of the above substances into the lumen of the GI tract  Absorption = The process of moving the molecules produced by digestion from the lumen of the gastrointestinal tract across a layer of epithelial cells and enter the blood or lymph.  Motility = The contractions of smooth muscles in the gastrointestinal tract wall. These contractions serve two functions: 1. they mix the luminal contents with the various secretions 2. they move the contents through the tract from mouth to anus.  Food Stomach Chyme Digestive action fragment fragment fat droplet of protein of polysaccharide salt water none of these molecules, except water, can cross the epithelium of the gastric wall, and thus little absorption of organic nutrients occurs in the stomach. Small intestine = tube ≈ 2.4 cm in diameter & 3m in length Structure of the Gastrointestinal Tract Wall 1) Luminal surface (inside) from the mid-esophagous anus 2) Single layer of epithelial cells (cover the luminal surface) from the stomach on Contain: - Exocrine cells: secrete mucus, acid, ions, enzymes into the lumen - Endocrine cells: release hormones into the blood 3) Lamina propria layer of connective tissue, contain: blood vessels, nerve fibers & lymphatic vessels 4) Muscularis mucosa (thin layer of smooth muscle) Mucosa = Epithelium layers + lamina propria + muscularis mucosa 5) Sub-mucosa: connective tissue layer, contains: Nerve cells (submuscosal plexus) + blood vessels + lymphatic vessels 6) Muscularis externa: smooth muscle layer + myentric nerve plexes Circular muscle Longitudinal muscle (thick inner layer) (thinner outer layer) Contraction Contraction Narrowing the lumen shortens the lumen 7) Serosa: thin layer of connective tissue 8) After serosa the Abdominal cavity Serosa linked to abdominal cavity by sheet of connective tissue.  Villi = A fingerlike projection/extension of the luminal surface covered by a layer of epithelial cells Microvilli = small projections of epithelial cell layer, also known as the brush border Villus contain: 1. Lacteal (lymphatic vessels) 2. Capillary network  Mucosa + villi + microvilli ↑ the intestine surface area about 600 fold over that of a flat-surface tube having the same length & diameter Length of small intestine (total surface area)≈ 250-300m2 = the area of a tennis court. Epithelial surfaces in the gastrointestinal tract are continuously replaced by new ones. cells at the base of the villi are devided into new cells & differentiate as they migrate to the top of the villus & replace the older dead one cells this process is called turn over → ≈ 17 billion epithelial cells are replaced each day → The entire epithelium of the small intestine is replaced approximately every 5 days  The rapid cell turnover is damaged by: 1. Anti-cancer drugs 2. Radiation therapy Nutrients absorbed from the intestinal capillaries are processed by the liver (enter the hepatic portal vein liver) before entering the circulation, in contrast to the lacteal The materials (fat) absorbed by lacteals are emptied into the lymphatic system & this eventually emptying into the thoratic duct. Digestion & Absorption 1) CHO : CHO found in food are of 3 types: - Fiber not breakdown by enzymes pass to the large intestine metabolized by bacteria. - Starch salivary amylase Disaccharides + mixture of mouth short chain of glucose mol.s - Disaccharides Pancreatic amylase Monosaccharides in small intestine Transport from intestinal lumen accros the epithelial layers (cells) into blood by facilitative diffusion 2) Proteins: protein ingested for 2 main reasons: 1. Supply essential a.a.s 2. Replace the N2 contained in a.a, bec. is brokendown & the N2 released is converted into urea Protein pepsin peptide fragments stomach Trypsin + chymotrypsin Protein (pancreas) peptide fragments Small intestine peptide fragments carboxypeptidase free a.a.s (pancreas) peptide fragments amino peptidase free a.a.s (luminal membrane of epithelial cells of small intestine) Absorption: Free a.a.s active transport epithelial cells facilitative diffusion Blood 3) Fat In the upper portion of stomach fats aggregate into large lipid droplets (bec. Fats are water insoluble) Emulsification (see fig. 17-10) [Division of large lipid droplets into smaller one (↑ surface area)] Emulsion (suspension of small lipid droplets) Susceptible to lipase (pancreatic) in small intestine Monoglyceride + 2 F.A.s Micelles (see fig. 17-11) (simillar to emulsion, but smaller; contain bile salt, F.A, monoglyceride & P.L) Pass thru. the epithelial cells of small intestine Free F.A + monoglyceride aggregate into T.G & pass to blood as chylomicrous. (see fig. 17-12)  Emulsification requires: 1. Mechanical disruption of the large fat droplets into smaller droplets This is achieved by: contractile activity occur in lower portion of the stomach & small intestine 2. Emulsifying agent, which acts to prevent the smaller droplets from re-aggregating back into large droplets This is achieved by: phospholipids food secreted in the bile (fig. 17-10) (fig. 17-11) (fig. 17-12)  The regulation of Gastrointestinal Process (1) Neural Regulation 1. The NS found in gastrointestinal tract is called Enteric NS, it is consists of 2: - Myenteric plexus - sub-mucosal plexus Influences smooth muscle influence secretory activity activity (Exocrine glands) 2. Nerve fibers from both sympathetic & para-sympathetic of the CNS enter the intestinal tract & synapse with neurons in both plexuses. Therefore the CNS can influence the motility & secretory activity of the gastrointestinal tract. (2) Hormonal Regulation Endocrine cells (scattered throughout the epithelium of the stomach and small intestine) Secrete hormones that control the gastrointestinal system The surface of each endocrine cell is exposed to the lumen, and various chemical substances stimulate the cell to release its hormones from the opposite side of the cell into the blood, by which it reach their target cells. Ex. On gastrointestinal hormones are: 1. secretin 2. cholecystokinin (CCK) 3. gastrin 4. glucose-dependent insulinotropic peptide (GIP)