Digestive System Lecture - Vancouver College of Massage Therapy - PDF

Chapter 24 DIGESTIVE SYSTEM Part 3 Lecture Outline Continuing the small intestine The large intestine The defecation reflex Regulation of digestion Phases of digestion...

Chapter 24 DIGESTIVE SYSTEM Part 3 Lecture Outline Continuing the small intestine The large intestine The defecation reflex Regulation of digestion Phases of digestion Dr. Kevin Tipper, ND REVIEW FROM LAST CLASS The Stomach The Stomach Cells of gastric glands Four main cell types: 1. Parietal Cells (exocrine) 2. Chief Cells (exocrine) 3. Mucous neck cells (exocrine) 4. G cells (endocrine) The Small Intestine Summary of Enzymes TODAY’S LECTURE The Small Intestine Intestinal Juice Secreted by glands of the small intestine 1-2 L daily Contains water and mucus, slightly alkaline Provide liquid medium aiding absorption The Small Intestine Brush Border Enzymes Inserted into plasma membrane of absorptive cells Some enzymatic digestion occurs at apical surface rather than just in lumen Examples: Carbohydrates: α-dextrinase, maltase, sucrase, lactase Proteins: aminopeptidase, dipeptidase, phosphatases Nucleosides: nucleosidases, phosphatases Digestion in the Small Intestine Mechanical Digestion Segmentation Localized, mixing contractions Causes sloshing of the chyme back and forth Mix chyme with digestive juices and brings it in contact with mucosa for absorption No set pattern of contractions, so no particular direction of Digestion in the Small Intestine Chemical Digestion of Carbohydrates Must break down polysaccharides to monosaccharides before they can be absorbed Mouth: So far, carbs have been acted on by salivary amylase but the acidic pH of the stomach has inactivated it Small Intestine: From pancreas: pancreatic amylase will act on any remaining starch Along with brush border enzymes on the apical surface of the small intestine Digestion in the Small Intestine Chemical Digestion of Carbohydrates Small Intestine (brush border enzymes): brush border enzymes act to break down small starches (disaccharides) into monosaccharides -dextrinase: clips off one glucose molecule at a time Sucrase: breaks sucrose into glucose and fructose Lactase: breaks lactose into glucose and galactose Maltase: breaks maltose and maltotriose into 2 or 3 molecules of glucose Monosaccharides can then be absorbed Digestion in the Small Intestine Lactose Intolerance Mucosal cells of the small intestine fail to produce lactase Undigested lactose in chyme causes fluid to be retained in the feces Bacterial fermentation of the undigested lactose results in the production of gases Symptoms include diarrhea, gas, bloating and abdominal cramps after consumption of milk and other dairy products Either avoid dairy or take lactase supplement with dairy (LACTAID) Digestion in the Small Intestine Chemical Digestion of Proteins Must break down proteins to amino acids before they can be absorbed Stomach: So far, proteins have been broken down into smaller peptide molecules in the stomach (by pepsin) Small Intestine: From pancreas: trypsin, chymotrypsin, carboxypeptidase, and elastase from pancreas continue to break down proteins into small peptides Aminopeptidase and dipeptidase in brush border help complete the break down process by cleaving off single amino acids (which can then be absorbed) Amino acids can then be absorbed Digestion in the Small Intestine Chemical Digestion of Lipids Mouth and Stomach: So far, some lipid digestion has occurred by lingual lipase and gastric lipases Small Intestine: From pancreas: pancreatic lipase most important in triglyceride digestion which occurs in the small intestine From liver and gallbladder: bile salts emulsify fats into small lipid globules so pancreatic lipase can act more efficiently Fatty acids, glycerol, and monoglycerides can then be absorbed Digestion in the Small Intestine Chemical Digestion of Nucleic Acids Small Intestine: From pancreas: Ribonuclease and deoxyribonuclease in pancreatic juice Nucleosidases and phosphatases in brush border Absorption in the Small Intestine Alldigestion to this point is to produce small molecules that can be absorbed through the epithelial cells lining the lumen This includes: Monosaccharides (glucose, fructose, galactose) Single amino acids, dipeptides, and tripeptides Fatty acids, glycerol, and monoglycerides Different molecules are absorbed differently (diffusion, facilitated diffusion, osmosis, and active transport) About 90% of absorption occurs in the small intestine and the remaining 10% occurs in the stomach and large intestine Absorption in the Small Intestine: Monosaccharides Absorbed by facilitated diffusion or active transport into the absorptive cell FYI: Fructose is via facilitated diffusion and glucose and galactose is via active transport FYI: The absorption of glucose is coupled to the active transport of sodium Monosaccharides will move out of the absorptive cells and into capillaries via facilitated diffusion Absorption in the Small Intestine: Amino Acids Most absorbed as amino acids via active transport into blood Half of absorbed amino acids come from proteins in digestive juice and dead mucosal cells and the other half comes from food Amino acids will move out of the absorptive cells and into the capillaries via diffusion Absorption in the Small Intestine: Lipids All dietary lipids absorbed by simple diffusion Short-chain fatty acids are small enough that they can be absorbed via simple diffusion and then pass into the capillaries via diffusion Long-chainfatty acids and monoglycerides must be transported to the apical surface via micelles because they are large and hydrophobic Absorption in the Small Intestine: Lipids Long-chain fatty acids and monoglycerides Bile salts form micelles to ferry them to absorptive cell surface where they are absorbed and the micelle remains in the lumen Reform into triglycerides and are coated with a protein forming chylomicrons Leave cell by exocytosis Enter lacteals to eventually enter blood with protein coat of chylomicron keeping them suspended and separate Absorption in the Small Intestine: Lipids Long-chain fatty acids and monoglycerides (continued) An enzyme called lipoprotein lipase (it is attached to the apical surface of capillary endothelial cells) acts to break down the triglyceride in the chylomicron into fatty acids and glycerol These products will then diffuse into hepatocytes and adipose cells to reform triglycerides again! Absorption in the Small Intestine: Lipids Let’s watch a video to help with understanding Absorption in the Small Intestine Electrolytes From GI secretions or food/liquid Electrolytes absorbed by active transport Vitamins Fat-soluble vitamins A, D, E, and K are absorbed by simple diffusion and transported with lipids in micelles Most water-soluble vitamins also absorbed by simple diffusion except for B12 which combines with intrinsic factor and is absorbed by active transport Water 9.3L: comes from ingestion (2.3L) and GI secretions (7.0L) Most absorbed in small intestine, some in large intestine Only 100ml excreted in feces All water absorption by osmosis Absorption in the Small Intestine The Large Intestine Also known as large bowel Average length is ~1.5 m (4.9 ft), and diameter is 7.5 cm (3 in.) Major functions 1. Reabsorbing water (and some electrolytes) and compacting intestinal contents into feces 2. Produce and absorb some vitamins (vitamin K, vitamin B7) 3. Storing fecal material prior to defecation The Large Intestine Starts at the junction of the ileum and the cecum via the ileocecal valve Four segments: 1. Cecum 2. Colon 3. Rectum 4. Anal canal The Large Intestine Large intestine segments Cecum Expanded pouch distal to the ileum Collects and stores material Begins the process of compaction Compression into feces Opening between the cecum and ileum is the ileocecal valve The Large Intestine Large intestine segments Cecum (continued) Attached to the appendix (also called the vermiform appendix) Contains numerous lymphoid nodules Appendicitis is inflammation of the appendix *Something to consider if someone is experiencing RLQ pain during treatment * The Large Intestine Large intestine segments (continued) Colon Subdivided into four regions 1. Ascending colon 2. Transverse colon 3. Descending colon 4. Sigmoid colon Ascending and descending colon are retroperitoneal and attached to the abdominal wall Transverse and sigmoid colon are suspended by mesocolon The Large Intestine Large intestine segments (continued) 1. ​Ascending colon From the cecum along the right margin of the peritoneal cavity to the inferior surface of the liver Bends sharply to the left at the right colic (hepatic) flexure The Large Intestine Large intestine segments (continued) 2. ​Transverse colon Crosses the abdomen from right to left Supported by the transverse mesocolon Makes a 90º turn at the left colic (splenic) flexure The Large Intestine Large intestine segments (continued) 3. ​Descending colon Moves inferiorly along the body’s left side to the iliac fossa Ends at the sigmoid flexure The Large Intestine Large intestine segments (continued) 4. ​Sigmoid colon (sigmeidos, Greek letter S) S-shaped segment About 15 cm (6 in.) long Empties into the rectum The Large Intestine Large intestine segments (continued) Rectum Forms last 15 cm (6 in.) of digestive tract Expandable for temporary feces storage Movement of fecal material into the rectum triggers the urge to defecate The Large Intestine Histology of the Large Intestine Mucosa Lacks villi No circular folds Contains distinctive intestinal glands Dominated by mucin-secreting goblet cells Mucus lubricates feces as it becomes drier and more compact Mucosa does not produce enzymes The Large Intestine Histology of the Large Intestine Muscularis Has an outer longitudinal layer and an inner circular layer Portions of the longitudinal muscles are thickened, and form teniae coli Forms haustra: pouches which give the large intestine a puckered appearance The Large Intestine Histology of the Large Intestine Serosa and Andventitia Ascending and descending colon are retroperitoneal = adventitia Transverse and sigmoid colon = serosa Omental (fatty) appendices Teardrop-shaped sacs of fat in the serosa of the colon The Large Intestine Histology of the Large Intestine Internal anal sphincter Inner circular smooth muscle layer Not under voluntary control External anal sphincter Outer skeletal muscle layer Under voluntary control Anus (exit of the anal canal) Epidermis here becomes keratinized Digestion in the Large Intestine There is very little digestion occurring in the large intestine, mechanical or chemical Mucus is secreted by the glands in the LI but no enzymes are secreted Final stage of digestion through bacterial action Bacteria acts to ferment any remaining carbohydrates, decomposes bilirubin which gives feces its brown color, produces some B vitamins and vitamin K Remaining water absorbed along with ions and some vitamins Absorption in the Large Intestine Accounts for