Chapter 24, Part 2! The Digestive System!

Transcription

1 Chapter 24, Part 2! The Digestive System! SECTION 24-6! The small intestine digests and absorbs nutrients, and associated glandular organs assist with the digestive process 2 1!

2 Small Intestine Regions! 21 feet long in cadaver; 10 feet long in living person (What accounts for this difference?)! 1. Duodenum ( 12, length of 12 finger widths)! Entrance = pylorus! Has Brunner s (alkaline mucous) glands (Why alkaline?)! 2. Jejunum ( empty in cadaver)! No Brunner s glands or Peyer s patches! 3. Ileum ( twisted )! Many Peyer s patches (M.A.L.T.)! Opens into colon at ileocecal valve! 3 Small Intestine Histology! Lining is modified to increase surface area for secretion and absorption! Compared to a simple tube:! 1. Plica circulares (circular folds)! Permanent folds of mucosa and submucosa! Folds contain submucosa (identifies a plica in lab)! Increase luminal surface area by 3X Submucosa! 4 2!

3 Small Intestine Histology 2! 2. Villi! Folds of mucosa! Fold contains lamina propria (not submucosa use for identification in lab)! Contain lacteals! Lymphatic capillaries! ü Transport chylomicrons (lipid + protein) to blood! Increase luminal surface area by 10X 5 The Intestinal Wall Villi Figure 24-17a and b! 6 3!

4 Intestinal Villus Figure 24-17c and d! Lamina propria! 7 Small Intestine Histology 3! 3. Microvilli! Folds of epithelial cell membrane! Fold contains cytoplasm (not lamina propria or submucosa) and actin filaments! Form brush border! Contains brush border enzymes - digestion! Increase surface area by 20X Total increase in surface area (for digestion and absorption) compared to a simple, unmodified tube is: 3 x 10 x 20 = 600X!!! (i.e. Your small intestine would need to be 600 times longer to do the same amount of work.)! 8 4!

5 Intestinal Glands! a.k.a. crypts of Lieberkühn! Epithelial cells lost to lumen replaced by stem cells! Some shed cells release brush border enzymes into lumen! Enteroendocrine cells! Gastrin and GIP! Secretin! Cholecystokinin! Crypt 9 Intestinal Secretions! About 1.8 l/day from:! Osmosis from mucosa to chyme in lumen! Secretions of intestinal glands! Buffer acidic chyme! Protect against digestive enzymes! Increase secretion in response to:! Local reflexes! Enterocrinin from neuroendocrine cells! ( mucus production)! Vagus stimulation! 10 5!

6 The Pancreas Figure 24-18! 11 Pancreas Anatomy! Heterocrine gland: both exocrine and endocrine Head, body, tail, exocrine ducts! Pancreatic duct (of Wirsung) divides:! 1. Pancreatic duct! Joins common bile duct! Becomes hepatopancreatic ampulla (of Vater)! Discussed in lab or later in notes! Empties into duodenum! 2. Accessory duct (of Santorini)! Opens into duodenum in < 10% of people! 12 6!

7 Pancreas Histology! 1. Exocrine portion = acini ( bunch of grapes )! Secretes pancreatic juice (Not a good lab test answer! be more specific.)! l/day! ü Digestive enzymes (see below)! ü Sodium bicarbonate (neutralizes chyme)! 2. Endocrine portion - islets of Langerhans! Glucagon, insulin, somatostatin, pancreatic polypeptide (Chapter 18)! 13 Pancreatic Juice (ph )! Acinar cells Secrete digestive enzymes as proenzymes! E.g. trypsin secreted as trypsinogen! Trypsinogen enteropeptidase -----> trypsin! (in brush border)! Trypsin activates chymotrypsin, carboxypeptidase and elastase! Pancreatic duct cells Secrete ions including bicarbonate! Water follows! 14 7!

8 Pancreatic Enzymes (Discussed in detail later)! Pancreatic alpha-amylase! Carbohydrates! Similar to salivary amylase! Pancreatic lipase! Lipids fatty acids! Pancreatic nucleases! DNA, RNA! Pancreatic proteolytic enzymes! proteases, peptidases! 15 Gross Anatomy of the Liver Figure 24-19b! Discussed in lab or later in notes! Largest visceral organ! Falciform ligament separates R. and L. lobes! Round ligament = remnant of fetal umbilical vein! Liver cells = hepatocytes! 16 8!

9 Gross Anatomy of the Liver 2 Figure 24-19c! Discussed in lab or later in notes! 17 Liver Histology Figure 24-20! Blood tracing Sublobular veins 18 9!

10 Liver Histology 2! Organized into roughly six-sided lobules! Hepatic triad at corners of lobules contain:! Branch of hepatic portal vein! Branch of hepatic artery! Branch of bile duct! Hepatocytes surround sinusoids! Sinusoids are large leaky capillaries! Arterial and portal vein blood empties into sinusoids Sinusoids empty into central vein! 19 Liver Histology 3! Reticuloendothelial cells = fixed macrophages! Phagocytize worn out RBCs, bacteria, etc.! Bile canaliculi! Intercellular canals between hepatocytes! Hepatocytes secrete bile! Canaliculi hepatic ducts common hepatic duct cystic duct gallbladder (storage)! You will need to know the hepatic portal circulation (and when to use it) for the blood tracing test.! 20 10!

11 Liver Functions! 1. Metabolic regulation A. CH 2 O metabolism - glycogenolysis, gluconeogenesis! B. Lipid metabolism - regulate blood [lipid]! C. Amino acid metabolism! Amino acids lipids glucose! D. Waste removal (e.g. ammonia less toxic urea)! E. Vitamin storage (fat-soluble vitamins)! F. Mineral storage (iron stored on ferritin)! G. Drug/toxin inactivation (smooth ER)! 21 Liver Functions 2! 2. Hematological regulation A. Phagocytosis and antigen presentation (Kupffer cells)! B. Synthesize plasma proteins (hepatocytes)! C. Remove circulating hormones! D. Remove circulating antibodies! E. Remove (or store) toxins! 3. Synthesis and excretion of bile 22 11!

12 3. Bile Functions! ml produced per day by hepatocytes! ph *** Effect on duodenum ph?! Contains:! Water, bile salts, bile acids, cholesterol, lecithin, pigments, ions! Functions:! A. Digestive! Bile salts emulsify fats (do NOT themselves digest fats)! Increases surface area for digestion by lipases! Solubilizes cholesterol! 23 Bile Functions 2! Bile salts! Bile acids (derived from cholesterol combined with sodium or amino acids! Acids = Bilcholic, deoxycholic, chenodeoxycholic, and lithocholic acids (just FYI, do not memorize) 24 12!

13 Bile Functions 3! B. Excretory! Bile pigment = modified bilirubin (from heme of hemoglobin)! Bilirubin (liver) (small intestine to large intestine) bilirubin urobilinogen stercobilin (excreted in feces) + urobilin (excreted in urine)! C. 90% of bile salts reabsorbed by ileum! 25 The Gallbladder Figure 24-21! Discussed in lab or later in notes! 26 13!

14 Gallbladder 2! Functions:! 1. Bile storage (holds ml)! 2. Bile modification (does not make bile!)! Reabsorbs water, concentrates bile! 3. Bile release! Presence of lipids in small intestine causes CCK release! Hepatopancreatic sphincter (of Oddi) relaxes (part of duodenal papilla (lab))! Gallbladder contracts! 27 Major Duodenal Hormones Figure 24-22! 28 14!

15 Activities of Major Digestive Tract Hormones! Figure 24-23! Food in stomach! Chyme in duodenum! Material arrives in jejunum! 29 SECTION 24-7! The large intestine is divided into three parts with regional specialization! Cecum! Colon! Rectum! 30 15!

16 The Large Intestine Figure 24-23! Discussed in lab or later in notes! 31 Large Intestine! Functions:! 1. Reabsorption of water - forming feces! 2. Absorption of vitamins (e.g., vitamin K - some produced by bacteria)! 3. Storage of feces! Anatomy:! About 5 feet long! Mesocolon attaches it to posterior abdominal wall! Ileocecal valve (sphincter) connects with small intestine! 32 16!

17 Large Intestine Anatomy 2! Cecum opens into colon! Blind pouch! Appendix attached! Rectum/Anus! Temporary storage of feces! Anus has stratified squamous EPI! Internal anal sphincter! Smooth muscle = involuntary! Stretch initiates urge to defecate! External anal sphincter! Skeletal muscle = voluntary! 33 Large Intestine Histology! Mucosa! No villi or permanent folds! Has intestinal (mucous) glands! Epithelial cells! a) Absorptive cells (water)! b) Goblet cells (lubricating mucus)! c) Do NOT produce digestive enzymes! Lymph nodules (lots)! 34 17!

18 Large Intestine Histology 2! Submucosa! Many lymphoid nodules! Muscularis externa! 3 longitudinal smooth muscle bands = taeniae coli! Serosa! Contractions produce haustra ( pouches )! Has epiploic (fatty) appendages! 35 The Large Intestine Figure 24-25! No villi! Many goblet cells! Discussed in lab or later in notes! 36 18!

19 Large Intestine Physiology! 1. Absorption! A. Water! B. Others! Electrolytes! Vitamins (K, biotin, B 5 )! Much produced by bacteria! Bile salts (especially cecum)! Urobilinogen (excreted in urine)! 37 Large Intestine Physiology 2! 2. Movements! Gastroileal and gastroenteric reflexes ileum! A. Peristalsis in small intestine! Move materials into cecum! B. Haustral churning or segmentation movements! Chyme forced from one haustrum to the next! C. Mass movements (mass peristalsis)! Stimulus = stretch of stomach or duodenum! Strong contractions in transverse colon onward! Move contents into rectum! 38 19!

20 Defecation Reflex! Mass movements push contents into rectum:! 1. Rectum distended! 2. Stretch receptors activated! 3. Reflexes initiated! A. Myenteric plexus stimulated! Increase peristalsis in rectum! B. Parasympathetic neurons stimulated in spinal cord! Increase peristalsis in rest of colon! Relax internal anal sphincter! 39 Defecation Reflex 2! C. Somatic motor neurons stimulated! Contract external anal sphincter! (Can be voluntarily relaxed)! 40 20!

21 The Defecation Reflex 3 Figure 24-26! 41 SECTION 24-8! Digestion is the mechanical and chemical alteration of food that allows the absorption and use of nutrients! The point is to get materials that can be used:! For ATP synthesis (catabolism) or! To make new organic molecules (anabolism)! 42 21!

22 Digestion and Absorption Figure 24-27! Great Figure! 43 Carbohydrate Digestion and Absorption! A. Salivary and pancreatic enzymes! Salivary amylase and pancreatic α-amylase! Optimum ph ! 1. Salivary amylase! Starches disaccharides and trisaccharides! Inactivated by stomach acid! 2. Pancreatic α-amylase! Starches disaccharides and trisaccharides! 44 22!

23 Carbohydrate Digestion and Absorption 2! B. Brush border enzymes! Are integral proteins on intestinal microvilli Some function in lumen after epithelial cells are shed! Produce monosaccharides! Maltase: maltose 2 glucose! Sucrase: sucrose glucose + fructose! Lactase: lactose glucose + galactose! 45 Carbohydrate Digestion and Absorption 3! C. Absorption of monosaccharides! Transport requires carrier proteins! 1. Facilitated diffusion and cotransport into cells! a) Facilitated diffusion! [higher] to [lower] with carrier! No ATP required! 46 23!

24 Carbohydrate Digestion and Absorption 4! b) Cotransport = secondary active transport! E.g. Na + pumped out of cell (Na + /K + ATPase) into lumen! Na + and glucose bind to carrier at lumen! Both enter cell down Na + concentration gradient! Na + pumped back out! 2. Facilitated diffusion into blood! 47 Carbohydrases Table 24-1! 48 24!

25 Lipid Digestion and Absorption! A. Lingual lipase and pancreatic lipase (with bile salts)! Triglycerides monoglycerides + fatty acids! Bile salts in small intestine s lumen:! Derived from cholesterol! Both hydrophilic and hydrophobic parts! Emulsify fats surface area for attack! Coat monoglycerides + fatty acids! Form micelles! 49 Bile-Salt Micelle! In lumen of intestine 50 25!

26 Lipid Digestion and Absorption 2! B. Micelle contacts epithelial cell! Lipid diffuses into cell! New triglycerides formed! Triglycerides now coated with protein! Form chylomicrons! C. Chylomicrons leave base of cell by exocytosis! Enter lacteals (lymphatics) in intestinal villus! Transported to circulation! 51 Lipases and Nucleases Table 24-1! 52 26!

27 Protein Digestion and Absorption! 1. Stomach - pepsin! Proteins to peptides! 2. Small intestine! A. Enzymes in lumen from pancreas! e.g. trypsinogen enteropeptidase trypsin! Trypsin, chymotrypsin, elastase Break only specific types of peptide bonds and produce polypeptides! Carboxypeptidase Lops off any terminal amino acid and produces free amino acids! 53 Protein Digestion and Absorption 2! B. Brush border enzymes! Exopeptidases and dipeptidases! Polypeptides amino acids! C. Facilitated diffusion and cotransport into cells! D. Facilitated diffusion and cotransport into blood! 54 27!

28 Proteases Table 24-1!?! 55 Secretion and Absorption of Water Figure 24-28! Small intestine: ml = 7800 ml reabsorbed Large intestine: ml = 1250 ml reabsorbed 56 28!

29 Secretion and Absorption of Water! Food, drink Saliva Gastric secretion Intestinal secretion Bile Pancreatic secretion 2000 ml! 1500 ml! 1500 ml! 2000 ml! 1000 ml! 1000 ml! Colon mucus 200 ml! 9200 ml! Lost in feces: 150 ml! Most reabsorption in small intestine (water follows salt)! 57 SECTION 24-9! Many age-related changes affect digestion and absorption! 58 29!

30 Effects of Aging! 1. Decreased rate of stem cell division! 2. Decreased smooth muscle tone! 3. Cumulative damage over time! Tooth loss, EtOH effects on liver! 4. Increased incidence of cancers! Affects epithelial stem cells! Colon, esophagus, throat, buccal cavity! 5. Increased susceptibility to dehydration! Osmoreceptors not as effective! 59 30!