The Digestive System

Components the Digestive Tract. Oral cavity Pharynx Esophagus Stomach Small intestine Large intestine

The Small Intestine What are the Regions of the Small Intestine? Duodenum Jejunum Ileum

Small Intestine Digestion & absorption Duodenum: (1st section) digestive juices, major chemical digestion Jejunum (2nd): absorb nutrients Ileum (3rd): absorb Vit. B12, bile salts, remaining nutrients

Digestive Glands Secrete into SI (duodenum) Pancreas: neutralize acidic chyme (bicarbonate), enzymes (carbs, proteins, fats) Bile salts: made in liver, stored in gallbladder Emulsify fats (make smaller droplets)

Pancreas Exocrine: Endocrine: Acini: Secrete pancreatic juice. Islets of Langerhans: Secrete insulin and glucagon. Insert fig. 18.26

The Pancreas The exocrine pancreas produces a mixture of buffers and enzymes essential for normal digestion.

Contains H20, HC03- and digestive enzymes. Pancreatic Juice Contains H20, HC03- and digestive enzymes.

The exocrine pancreas is organized much like the salivary glands: It resembles a bunch of grapes, with each grape corresponding to a single acinus.

The acinus, which is the blind end of a branching duct system, is lined with acinar cells that secrete the enzymatic portion of the pancreatic secretion.

The ducts are lined with ductal cells. Ductal epithelial cells extend into a special region of centroacinar cells in the acinus. The centroacinar and ductal cells secrete the aqueous HCO3- -containing component of the pancreatic secretion.

Enzymatic component of pancreatic secretion (acinar cells). Pancreatic amylase and lipases are secreted as active enzymes. Pancreatic proteases are secreted in inactive forms and converted to their active forms in the lumen of the duodenum; for example, the pancreas secretes trypsinogen, which is converted in the intestinal lumen to its active form, trypsin. Trypsin (when activated by enterokinase) triggers the activation of other pancreatic enzymes.

Aqueous component of pancreatic secretion (centroacinar and ductal cells). Pancreatic juice is an isotonic solution containing Na+, Cl-, K+, and HCO3- (in addition to the enzymes). The Na+ and K+ concentrations are the same as their concentrations in plasma, but the Cl- and HCO3- concentrations vary with pancreatic flow rate. Centroacinar and ductal cells produce the initial aqueous secretion, which is isotonic and contains Na+, K+, Cl-, and HCO3-.

Aqueous component of pancreatic secretion (centroacinar and ductal cells). This initial secretion is then modified by transport processes in the ductal epithelial cells as follows: In the presence of carbonic anhydrase, CO2 and H2O combine in the cells to form H2CO3. H2CO3 dissociates into H+ and HCO3-. The HCO3- is secreted into pancreatic juice by the Cl--HCO3- exchanger in the apical membrane. The H+ is transported into the blood by the Na+-H+ exchanger in the basolateral membrane. The net result, or sum, of these transport processes is net secretion of HCO3- into pancreatic ductal juice and net absorption of H+; absorption of H+ causes acidification of pancreatic venous blood.

Like gastric secretion, pancreatic secretion is divided into cephalic, gastric, and intestinal phases. In the pancreas, the cephalic and gastric phases are less important than the intestinal phase. Briefly, the cephalic phase is initiated by smell, taste, and conditioning and is mediated by the vagus nerve. The cephalic phase produces mainly an enzymatic secretion. The gastric phase is initiated by distention of the stomach and is also mediated by the vagus nerve. The gastric phase produces mainly an enzymatic secretion.

The intestinal phase is the most important phase and accounts for approximately 80% of the pancreatic secretion Acinar cells (enzymatic secretion). The pancreatic acinar cells have receptors for CCK and muscarinic receptors for ACh. During the intestinal phase, CCK is the most important stimulant for the enzymatic secretion.

The I cells are stimulated to secrete CCK by the presence of amino acids, small peptides, and fatty acids in the intestinal lumen. Of the amino acids stimulating CCK secretion, phenylalanine, methionine, and tryptophan are most potent. In addition, ACh stimulates enzyme secretion and potentiates the action of CCK by vagovagal reflexes.

Ductal cells (aqueous secretion of Na+, HCO3-, and H2O). The pancreatic ductal cells have receptors for CCK, ACh, and secretin. Secretin, which is secreted by the S cells of the duodenum, is the major stimulant of the aqueous HCO3-- rich secretion. Secretin is secreted in response to H+ in the lumen of the intestine, which signals the arrival of acidic chyme from the stomach. To ensure that pancreatic lipases will be active (since they are inactivated at low pH), the acidic chyme requires rapid neutralization by the HCO3--containing pancreatic juice. The effects of secretin are potentiated by both CCK and ACh.

The Liver What is the Overview of the Liver? Largest visceral organ Four Lobes Over 200 known functions

The liver is the body’s center for metabolic regulation. It produces bile that will be ejected by the gallbladder into the duodenum under stimulation of CCK. Bile is essential for the efficient digestion of lipids; it emulsifies fats so that individual lipid molecules can be readily attacked by digestive enzymes.

Liver largest internal organ. Hepatocytes form hepatic plates that are 1–2 cells thick. Arranged into functional units called lobules. Plates separated by sinusoids. More permeable than other capillaries. Contains phagocytic Kupffer cells. Secretes bile into bile canaliculi, which are drained by bile ducts.

Hepatic Portal System Products of digestion that are absorbed are delivered to the liver. Capillaries drain into the hepatic portal vein, which carries blood to liver. ¾ blood is deoxygenated. Hepatic vein drains liver.

Enterohepatic Circulation Compounds that recirculate between liver and intestine. Many compounds can be absorbed through small intestine and enter hepatic portal blood. Variety of exogenous compounds are secreted by the liver into the bile ducts. Can excrete these compounds into the intestine with the bile. Insert fig. 18.22

Major Categories of Liver Function

Bile Production and Secretion The liver produces and secretes 250–1500 ml of bile/day. Bile pigment (bilirubin) is produced in spleen, bone marrow, and liver. Derivative of the heme groups (without iron) from hemoglobin. Free bilirubin combines with glucuronic acid and forms conjugated bilirubin. Secreted into bile. Converted by bacteria in intestine to urobilinogen. Urobilogen is absorbed by intestine and enters the hepatic vein. Recycled, or filtered by kidneys and excreted in urine.

Bile Production and Secretion (continued) Bile acids are derivatives of cholesterol. Major pathway of cholesterol breakdown in the body. Principal bile acids are: Cholic acid. Chenodeoxycholic acid. Combine with glycine or taurine to form bile salts. Bile salts aggregate as micelles. 95% of bile acids are absorbed by ileum.

Gallbladder Sac-like organ attached to the inferior surface of the liver. Stores and concentrates bile. When gallbladder fills with bile, it expands. Contraction of the muscularis layer of the gallbladder, ejects bile into the common bile duct into duodenum. When small intestine is empty, sphincter of Oddi closes. Bile is forced up to the cystic duct to gallbladder.

The Small Intestine What is the Intestinal Wall? Plicae have small projections, villi Both increase surface area of mucosa for absorption

Folds, villi and microvilli increase surface area for absorption

The Small Intestine The Intestinal Wall Each villus is a fold in the mucosa. Covered with columnar epithelial cells interspersed with goblet cells. Epithelial cells at the tips of villi are exfoliated and replaced by mitosis in crypt of Lieberkuhn. Lamina propria contain lymphocytes, capillaries, and central lacteal.

Absorption Glucose and galactose are absorbed by mechanisms involving Na+-dependent cotransport. Fructose is absorbed by facilitated diffusion.

Absorption of Lipids Free fatty acids, monoglycerides, and lysolecithin leave micelles and enter into epithelial cells. Resynthesize triglycerides and phospholipids within cell. Combine with a protein to form chylomicrons. Secreted into central lacteals.

Transport of Lipids In blood, lipoprotein lipase hydrolyzes triglycerides to free fatty acids and glycerol for use in cells. Remnants containing cholesterol are taken to the liver. Form Very-low-density lipoprotein which take triglycerides to cells. Once triglycerides are removed, Very-low-density lipoprotein are converted to low-density lipoprotein. low-density lipoprotein transport cholesterol to organs and blood vessels.  High-density lipoprotein transport excess cholesterol back to liver.

Free amino acids absorbed by cotransport with Na+. Absorption of Proteins Free amino acids absorbed by cotransport with Na+. Dipeptides and tripeptides transported by secondary active transport using a H+ gradient to transport them into the cytoplasm. Hydrolyzed into free amino acids and then secreted into the blood.

The Small Intestine What is Intestinal Secretion? Intestinal juice Moistens chyme Buffers stomach acid Dissolves digestive enzymes Dissolves products of digestion

The Small Intestine What are the Intestinal Hormones? Gastrin Secretin Cholecystokinin (CCK)

Paracrine Regulators of the Intestine Serotonin (5-HT): Stimulates intrinsic afferents, which send impulses into intrinsic nervous system; and activates motor neurons. Motilin: Stimulates contraction of the duodenum and stomach antrum. Guanylin: Activates guanylate cyclase, stimulating the production of cGMP. cGMP stimulates the intestinal cells to secrete Cl- and H20. Inhibits the absorption of Na+. Uroguanylin: May stimulate kidneys to secrete salt in urine.

Absorption in Small Intestine Duodenum and jejunum: Carbohydrates, amino acids, lipids, iron, and Ca2+. Ileum: Bile salts, vitamin B12, electrolytes, and H20.

Intestinal Enzymes Microvilli contain brush border enzymes that are not secreted into the lumen. Brush border enzymes remain attached to the cell membrane with their active sites exposed to the chyme. Absorption requires both brush border enzymes and pancreatic enzymes.

Intestinal Reflexes Intrinsic and extrinsic regulation controlled by intrinsic and paracrine regulators. Gastroileal reflex: Increased gastric activity causes increased motility of ileum and movement of chyme through ileocecal sphincter. Ileogastric reflex: Distension of ileum, decreases gastric motility. Intestino-intestinal reflex: Overdistension in 1 segment, causes relaxation throughout the rest of intestine.

The Large Intestine Appendix = extension of cecum, role in immunity What is the Overview of the Large Intestine? Reabsorbs water and compacts feces Absorbs vitamins made by bacteria Stores feces before defecation Consists of three parts Cecum Colon Rectum Appendix = extension of cecum, role in immunity

The Large Intestine The Large Intestine Figure 16-17(b)

The Large Intestine What are the Functions of the Large Intestine? Absorption Water Ions Vitamins Organic wastes Bile salts Bacterial growth

Large Intestine Outer surface bulges outward to form haustra. Little absorptive function. Absorbs H20, electrolytes, several vitamin B complexes, vitamin K, and folic acid. Intestinal microbiota produce significant amounts of folic acid and vitamin K. Bacteria ferment indigestible molecules to produce short-chain fatty acids. Does not contain villi. Secretes H20, via active transport of NaCl into intestinal lumen. Guanylin stimulates secretion of Cl- and H20, and inhibits absorption of Na+ (minor pathway). Membrane contains Na+/K+ pumps. Minor pathway.

Defecation Waste material passes to the rectum. Occurs when rectal pressure rises and external anal sphincter relaxes. Defecation reflex: Longitudinal rectal muscles contract to increase rectal pressure. Relaxation of internal anal sphincter. Excretion is aided by contractions of abdominal and pelvic skeletal muscles. Push feces from the rectum.

Aging and the Digestive System What are Age-Related Changes in the Digestive System? Thinner, more fragile epithelium Reduced epithelial stem cell division Weaker peristaltic contraction Reduced smooth muscle tone Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings