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Unformatted text preview: DIGESTIVE SYSTEM Digestion Absorption Secretion Motility Defense Mechanisms Regulation Hormones + ANS Excretion Figure 21-1 Digestion Mechanical Chewing, Mixing, Segmentation Chemical Enyzmes Carbohydrates, Proteins, Lipids, Nucleic acids Different enzymes are active in various organs Objective: Breakdown of foodstuffs into smaller pieces that the cells can use Absorption Movement of substances from the lumen of the digestive tract across the intestinal epithelium into the blood or lymph Active and passive transport mechanims Secretion Release of substances from glandular epithelium of the digestive tract and accessory organs into the lumen of the digestive tract Release of hormones into the blood that affect target cells that regulate digestive functions Motility Movement as a result of contraction of smooth muscle in walls of digestive organs Propulsion Mechanical digestion Defense Mechanisms Importance open to exterior (easy access for foreign invaders) Mucus secreted by epithelium Enzymes Acidic environment of stomach Lymphoid tissue (GALT gutassociated lymphoid tissue) collections of lymphocytes along the digestive tract Mouth Pharynx Esophagus Stomach Small Intestine Liver Gallbladder Pancreas Large intestine Rectum Anus
Figure 21-2a Histology of the Alimentary Canal Figure 23.6 4 Layers of Digestive (GI) tract Mucosa Submucosa Muscularis Externa Serosa Mucosa Mucous membrane Modifications to increase SA Epithelial cells with underlying CT (lamina propria) Mitotic rapid turnover of cells Projections, invaginations Stomach Small intestine Rugae, gastric glands Muscularis mucosae Plicae circularis, villi, microvilli, crypts Smooth muscle cells Figure 21-2b Figure 21-2c Figure 21-2d Figure 21-2e Submucosa Connective tissue Blood vessels Glands Nerves Submucosal (Meissner's) Plexus Histology of the Alimentary Canal Figure 23.6 Muscularis Externa 2 layers of smooth muscle Inner circular layer Outer longitudinal layer Stomach additional inner oblique layer Nerves Myenteric (Auerbach's) Plexus Serosa Outer covering Connective tissue membrane continuous with lining of abdominal cavity Extensions form mesentery that supports abdominal organs (keeps them in place) Enteric Nervous System Submucosal plexus Myenteric plexus Controls glandular secretion and smooth muscle contraction Can act independently of ANS Intrinsic vs. Extrinsic neurons Long vs. Short reflexes Parsympathetic stimulatory Sympathetic inhibitory Figure 21-11 Mouth Stratified squamous epithelium Salivary glands Mechanical digestion Saliva mucus, antibacterials (antibodies, lysozyme), amylase Mastication (chewing), Deglutition (swallowing) Salivary amylase Carbohydrates Chemical digestion Pharynx Food + air Stratified squamous epithelium Skeletal muscle for movement into esophagus (Remember: epiglottis over larynx forces bolus down into esophagus rather than into larynx) Esophagus Skeletal muscle transition to smooth muscle Peristalsis wave of contraction to propel bolus into stomach Above bolus contract circular (squeeze boluw onward Below bolus contract longitudinal (widen lumen to receive bolus) Peristaltic waves continue along entire length of digestive tract Figure 21-4a Figure 21-25 Stomach Simple columnar epithelium Mucus! Decrease friction Barrier to acid Stomach Cells Mucous cells Mucus HCO3 Chief cells Pepsinogen activated to pepsin requires acid protein digestion Gastric lipase (limited fat digestion) Parietal cells HCl defense mechanism denatures proteins (more susceptible to protein digestion) activates pepsin inactivates salivary amylase Intrinsic factor Vitamin B12 absorption in intestine ECL (enterochromaffinlike) cells Histamine stimulates Parietal cells to release HCl G cells Gastrin (hormone) stimulates gastric secretions Figure 21-27
Gastric Motility 3 layers of smooth muscle Mixing! (mechanical digestion) Emptying into small intestine Cephalic Phase Sight, smell, thought, taste of food Parasympathetic stimulation of: Salivary glands Stomach Intestine Accessory organs Stimulates secretions and motility getting ready for food! Figure 21-23 Gastric Phase Presence of food in the stomach Distension Protein digestion products (peptides, amino acids) Increase gastric secretions + motility PNS Gastrin Figure 21-26 Intestinal Phase Entry of chyme Stimulation of intestinal digestion + motility Inhibition of gastric activity * Important to limit gastric emptying into small intestine Figure 21-28 Intestine Duodenum Jejunum Ileum Simple columnar epithelium Contributions to lumen from cells of intestine AND accessory organs (liver, gallbladder, pancreas) Intestinal Epithelial Cells Goblet cells mucus Enterokinase (enteropeptidase) Brush border enzymes Enzyme that activates trypsin Hormones Peptidases (proteins) Disaccharidase (carbohydrates) Nucleosidases, Phosphatases (nucleic acids) Secretin CCK (cholecystokinin) Figure 21-2e Secretin Released due to presence of acidic chyme Function: Neutralize acid Actions: Inhibit gastric secretions + motility Stimulates HCO3 secretion from pancreas (duct cells) Brunner's glands in submucosal layer of duodenum secrete alkaline mucus to help neutralize acidic chyme (Why don't you need Brunner's glands in other parts of small intestine?) CCK Presence of fats Actions: Inhibit gastric secretions + motility Stimulates enzyme secretion from pancreas (acinar cells) By decreasing gastric secretions + motility, complete digestion and absorption of foodstuffs can take place in the small intestine. Figure 21-7 Pancreas Endocrine function Secretion of insulin + glucagon from islets of Langerhans to regulate blood levels of glucose Exocrine function Secretion of enzymes to aid in chemical digestion in duodenum (acinar cells) Secretion of HCO3 to aid in neutralizing acidic chyme (duct cells) Pancreatic Enzymes Proteolytic enzymes secreted in inactive form must be activated Trypsinogen Trypsin activated by enterokinase and trypsin Chymotrypsinogen Chymotrypsin activated by trypsin Procarboxypeptidase Carboxypeptidase activated by trypsin Figure 21-29 Other Pancreatic Enzymes Pancreatic amylase (carbohydrates) Pancreatic lipase (fats) Also procolipase and prophospholipase (activated by trypsin) and used in fat digestion Pancreatic nucelases (Nucleic acids) Figure 21-10a Liver Hepatocytes produce bile Bile salts (*fat digestion + absorption) Bile pigments (from hemoglobin degradation) Wastes to be excreted in feces Bile is secreted into hepatic ducts that carry the bile directly to the duodenum or to the gallbladder for storage Figure 21-10b Main pancreatic duct + Common bile duct converge and dump secretions into duodenum Controlled by Sphincter of Oddi CCK relaxes this sphincter Role of Bile Salts Emulsification of fats Larger to smaller fat droplets Amphipathic properites Increase SA for pancreatic lipase to work Figure 21-20a Lipids in our Diet *Triglycerides: Pancreatic lipase + colipase Phospholipids: Phospholipase Cholesterol: No digestion required Figure 21-18 Gallbladder Stores and concentrates bile made by liver (hepatic ducts to cystic duct) CCK stimulates contraction Secretin stimulates HCO3 secretion from duct cells Figure 21-10b Motility of Small Intestine Segmentation Alternating contractions of smooth muscle in areas of chyme Enhances mixing and absorption increases contact with epithelial cells Peristalsis Migrating motor complex (between meals) Figure 21-4b * Most digestion and absorption takes place in the small intestine Absorption = Transepithelial Transport From lumen into epithelial cell (apical surface) Out of epithelial cell (basolateral surfaces) into interstitial fluid and on into blood or lymph within the villi Figure 21-13 Absorption of Proteins Amino acids (some di and tripeptides) into villi capillaries Cotransport or transcytosis (>3 amino acids) Figure 21-17 Absorption of Carbohydrates Monosaccharides into villi capillaries Cotransport and facilitated diffusion Figure 21-14 Figure 21-15 Absorption of Nucleic Acids Bases, monosaccharides, phosphate ions into villi capillaries Active transport, cotransport + facilitated diffusion Absorption of Lipids Lipid digestion products are transported from the lumen to the epithelial cells as micelles Diffusion (lipophilic) into epithelial cells Triglycerides are resynthesized TG's + cholesterol form chylomicrons Exocytosis of chylomicrons Figure 21-20b Figure 21-19 - Overview Chylomicrons are too big to enter blood capillaries Absorption into lacteals (lymphatic capillaries in the villi) Returned to venous blood Small fatty acids are absorbed into blood capillaries Other Things Absorbed WATER!! Electrolytes Vitamins, minerals Figure 21-5 Large Intestine Anatomy: Ileocecal valve at entrance Cecum, Appendix Ascending, Transverse, Descending, Sigmoid Colon Rectum Anus Figure 21-31 (2 of 4) Tenia coli modification in longitudinal muscle layer (3 bands) Contractions form haustra (increases absorption surface area) Goblet cells of mucosa Mucus! Figure 21-31 (3 of 4) Absorption + Digestion in LI Water + electrolytes Absorbs most of water that it receives (1.5 L .1 L in feces) Bacteria Contributes to chemical digestion Vitamin production (K) Flatus Motility Peristalsis Segmentation continues Mass movements 34 times/day Propulsion Gastrocolic reflex Reflexes Enterogastric reflex Presence of chyme in small intestine inhibits PNS decreases gastric secretions + motility Increased stomach activity results in increased segmentation in the ileum and relaxation of the ileocecal valve promotes emptying into large intestine Presence of food in the stomach increases motility in the colon Gastroileal reflex Gastrocolic reflex Figure 21-31 (4 of 4) Defecation Rectal distension Internal anal sphincter relaxes (smooth muscle, involuntary) External anal sphincter relaxes (skeletal muscle, voluntary) Figure 21-22 Hepatic Portal System Liver has 2 sources of blood flow: Hepatic artery Hepatic portal vein Oxygenrich blood from aorta Oxygenpoor blood from viscera (stomach, intestines, pancreas, spleen) Nutrientrich Hepatic clearance due to enzymes in Figure 21-30 Figure 21-10c Figure 21-10d ...
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This note was uploaded on 04/29/2009 for the course BIOL 425 taught by Professor Tondi during the Fall '08 term at George Mason.
- Fall '08