Digestive System Lecture 2 PDF

Chapter 24 DIGESTIVE SYSTEM Part 2 Lecture Overview The stomach The small intestine The pancreas The liver The gallbladder Dr. Kevin Tipper...

Chapter 24 DIGESTIVE SYSTEM Part 2 Lecture Overview The stomach The small intestine The pancreas The liver The gallbladder Dr. Kevin Tipper, ND REVIEW FROM LAST CLASS Digestive System Anatomy Consists of: 1. Gastrointestinal (GI) Tract muscular tube also called digestive tract or alimentary canal Food passes along length from mouth to anus 2. Accessory organs produce or store secretions that flow into GI tract through ducts Functions of the Digestive Tract Mechanical Digestion Includes all movements that help digestion: Mastication (chewing) Swallowing Mixing Peristalsis Muscular contractions in the wall of the GI tract physically break down food by churning it and propelling it along the tract and also help dissolve foods by mixing them with fluids secreted into the GI tract Functions of the Digestive Tract Chemical Digestion Enzymes secreted by accessory organs and cells that line the tract break down the food chemically Mainly accomplished by using water to break down chemical bonds (hydrolysis) Fats are broken down into fatty acids and glycerol Carbohydrates are broken down into monosaccharides Proteins are broken down into amino acids Layers of the GI Tract Four major layers of the digestive tract Same basic layers from esophagus to anal canal The layers are: 1. Mucosa 2. ​Submucosa 3. Muscular Layer 4. Serosa or adventitia Digestive System Anatomy TODAY’S LECTURE The Stomach Function It connects the esophagus to the duodenum Serves as mixing chamber and holding reservoir Mechanical digestion In the stomach, digestion of starch continues (briefly), digestion of proteins begins, and digestion of triglycerides increases and certain substances are absorbed Chemical digestion Produces intrinsic factor The Stomach Structure a muscular, expandable, J-shaped organ with three layers in the muscular layer Highly variable shape depending on contents Empty: muscular tube with constricted lumen Full: can expand to contain 1–1.5 liters of material (chyme) Viscous, highly acidic, soupy mixture formed from the combination of food, saliva, and gastric gland secretions The Stomach Lesser curvature forms the medial surface Attached to the mesentery of the lesser omentum Greater curvature forms the lateral and inferior surfaces Attached to the mesentery of the greater omentum The Stomach There are 4 stomach regions: 1. ​Fundus Superior to the junction between the stomach and esophagus 2. Cardia Superior, medial portion within 3 cm of gastroesophageal junction Secretes mucus to protect esophagus from stomach acid/enzymes 3. Body Largest region Between the fundus and pylorus Functions as a mixing bowl 4. ​Pylorus Sharp curve of “J” of the stomach Frequently changes shape with mixing movements The Stomach The pylorus is made up of the Pyloric antrum (antron, cavity) Portion connected to stomach body Pyloric canal Empties into the duodenum (proximal segment of small intestine) Andchyme leaving the stomach is controlled by the pyloric sphincter The Stomach The Stomach Rugae Wrinkles Prominent, temporary mucosal folds Allow gastric expansion by flattening out with stomach expansion The Stomach Histology Mucosa Composed of simple columnar epithelium Produces layer of alkaline mucus Protects epithelial cells against acid and enzymes in gastric lumen Epithelial cells extend down into lamina propria to form columns of secretory cells called gastric glands which open up into gastric pits Secretions flow from gastric glands  gastric pits  lumen The Stomach Histology Mucosa Gastric pits Shallow depressions opening onto the gastric surface Active stem cells at the base of each pit replace superficial cells shed into the chyme Each pit communicates with several gastric glands The Stomach Histology Mucosa Gastric glands Glands in the fundus and body Secrete most of the acid and enzymes enabling gastric digestion Dominated by parietal cells and chief cells (more soon) Secrete ~1.5 L of gastric juice each day Glands in the pylorus Secrete mucus and hormones that coordinate and control digestive activity 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 Stomach Cells of gastric glands (continued) Parietal cells secrete: Intrinsic factor Glycoprotein that aids in vitamin B12 absorption Hydrochloric acid (HCl) Activates pepsinogen Keeps stomach at pH 1.5–2 G cells (enteroendocrine cells) Secretes gastrin which regulates digestion (more later) The Stomach Cells of gastric glands (continued) Chief cells secrete: Pepsinogen Activated by HCl to become pepsin Active proteolytic (protein-digesting) enzyme Gastric lipase Fat digesting enzyme Mucous neck cells Secrete mucus to protect epithelial layer of the stomach from high acid contents The Stomach Histology Submucosa composed of areolar connective tissue Muscular layer 3 layers of smooth muscle: an outer longitudinal, middle circular, and an inner oblique layer Serosa part of visceral peritoneum From lesser curvature, it forms the lesser omentum From the greater curvature, it forms greater omentum The Stomach Layers of the muscularis externa Oblique layer Strengthens the stomach wall Assists in mixing and churning to form chyme Circular layer Longitudinal layer Digestion in the Stomach Digestion that started in the mouth continues in the stomach, both mechanical and chemical Mechanical digestion in the stomach Mixing waves gentle, rippling peristaltic movements creates chyme by breaking down food, mixing it with secretions of the gastric glands Once the food reaches the pyloric sphincter, each mixing wave forces about 3mL of chyme into the duodenum this is called gastric emptying Digestion in the Stomach Chemical digestion in the stomach Salivary amylase It continues acting in the stomach until the churning mixes it with the acidic gastric juices Lingual lipase Becomes more active in the stomach acts to digest triglycerides into fatty acids and diglycerides Hydrochloric acid (HCl) secreted by parietal cells kills many microbes, denatures proteins, and stimulates the release of hormones that promote flow of bile and pancreatic juice Digestion in the Stomach Chemical digestion in the stomach Pepsin secreted by chief cells Secreted as pepsinogen (inactive form) WHY? It is activated once it comes in contact with HCl Acts to break down certain peptide bonds between amino acids which leaves smaller peptide fragments Gastric lipase Secreted by chief cells splits triglycerides into fatty acids and monoglycerides Absorption in the Stomach Thereis only a small amount of nutrient absorption that takes place in the stomach Some water Ions short chain fatty acids certain drugs (aspirin) alcohol The Small Intestine The small intestine Plays key role in nutrient digestion and absorption 90 percent of nutrient absorption occurs in small intestine Most of remaining 10 percent occurs in large intestine Average overall length = 6 m (19.7 ft) Diameter ranges from 4 cm (1.6 in.) near stomach to 2.5 cm (1 in.) near large intestine Three segments 1. ​Duodenum 2. ​Jejunum 3. ​Ileum The Small Intestine The Small Intestine: Duodenum Segments of the small intestine 1. ​Duodenum 25 cm (10 in.) in length Segment closest to the stomach Acts as a “mixing bowl” Receives chyme from stomach and digestive secretions from liver, gallbladder, and pancreas Mostly retroperitoneal Duodenal glands produce mucous secretions Has few circular folds and small villi Main function is to neutralize acidic chyme The Small Intestine: Jejunum Segments of the small intestine 2. ​Jejunum Between the duodenum and ileum ~2.5 m (8.2 ft) in length In the peritoneal cavity Has numerous circular folds and abundant, long villi Majority of chemical digestion and nutrient absorption occurs in jejunum The Small Intestine: Ileum Segments of the small intestine 3. ​Ileum Final segment of small intestine ~3.5 m (11.5 ft) in length Ends at the ileocecal valve Sphincter controlling flow from the ileum into the cecum of the large intestine Has few circular folds (none in the distal portion) Villi are relatively stumpy Submucosa contains aggregated lymphoid nodules The Small Intestine Histology Mucosa simple columnar epithelium and contains many different cells Absorptive cells (digest and absorb) goblet cells (produce mucus) intestinal glands (intestinal juice) Paneth cells (lysozyme) enteroendocrine cells S cells: secretes secretin CCK cells: secretes CCK K cells: secretes glucose-dependent insulinotropic peptide (GIP) The Small Intestine Histology Mucosa Abundance of MALT Groups of lymphatic nodules called Peyer’s patches that are present in the ileum The Small Intestine Histology Submucosa Duodenal (Brunner’s) glands secrete alkaline mucus that helps neutralize gastric acid in the chyme Muscularis Has an outer longitudinal layer and an inner circular layer Serosa Completely surrounds the small intestine except for majority of duodenum Retroperitoneal, has adventitia Special Structures of the Intestinal Tract Many intestinal structures add surface area to increase absorption Circular folds (plicae circulares) Series of transverse folds along the intestinal lining Permanent feature compare to rugae Roughly 800 folds in the small intestine Mostly in the jejunum Increase AS for absorption Special Structures of the Intestinal Tract Villi (singular, villus) Small mucosal projections that increase surface area for absorption Covered by epithelial cells Surfaces covered with microvilli Contains arteriole, venule, blood capillary, and lacteal Special Structures of the Intestinal Tract Internal structure of a villus Extensive capillary network in the lamina propria Carries absorbed nutrients to the hepatic portal circulation Special Structures of the Intestinal Tract Internal structure of a villus (continued) Lymphatic capillary, or lacteal (lacteus, milky) Transports materials that cannot enter blood capillaries Example: Absorbed fatty acids assembled into protein–lipid packages (chylomicrons) too large to diffuse into bloodstream Transported by lymphatic system to venous circulation Special Structures of the Intestinal Tract Internal structure of a villus (continued) Smooth muscle in muscularis mucosae and within the villi Moves villi back and forth, exposing surfaces to intestinal contents Squeezes lacteal, assisting in lymph movement Special Structures of the Intestinal Tract Microvilli Projections of apical membrane of absorptive cells Creates a brush border with brush border enzymes Brush border Carpet of microvilli on the surface of the epithelial cells Increases surface area for absorption Contains enzymes that digest materials Epithelial cells can then absorb breakdown products The Pancreas The pancreas has endocrine and exocrine functions The exocrine functions of the pancreas contribute to digestion Pancreas Location: retroperitoneal organ that lies posterior to the greater curvature of stomach Three major parts: Head, body, tail The Pancreas Pancreatic duct (duct of Wirsung) Delivers exocrine secretions to the duodenum Joins the common bile duct from the liver and gallbladder and enters duodenum as a common duct at the hepatopancreatic ampulla (ampulla of Vater) The ampulla opens at the major duodenal papilla Accessory pancreatic duct (Santorini duct) Branches from the pancreatic duct and empties separately into the duodenum Occurs in 3–10 percent of the population The Pancreas The Pancreas Pancreatic tissue is made up of small clusters of glandular epithelial cells 99% are pancreatic acini Contain pancreatic acinar cells Secrete pancreatic enzymes and buffers EXOCRINE 1% are pancreatic islets Secrete hormones (glucagon, insulin, somatostatin, and pancreatic polypeptide) ENDOCRINE Discussed in detail in Endocrine Section The Pancreas Pancreatic juice About 1200 - 1500 mL of pancreatic juice are produced each day Combination of mostly water and ions (secreted by epithelial cells lining the duct) and exocrine secretions (enzymes and buffers) Sodium bicarbonate: buffers acidic stomach chyme Pancreatic juice is slightly alkaline (7.1-8.2) which stops the action of pepsin and creates proper pH for enzymes to act in the small intestine Enzymes Pancreatic amylase: digests starch Proteolytic enzymes: digests protein Pancreatic lipase: main fat-digesting enzyme Nucleases (Ribonuclease and deoxyribonuclease): digest RNA and DNA Trypsinogen: digests protein The Pancreas: Enzymes Pancreatic amylase Carbohydrase (breaks down certain starches) Almost identical to salivary amylase Pancreatic lipase Breaks down certain complex lipids Main fat digesting enzyme Nucleases Ribonuclease and deoxyribonuclease: Break down RNA or DNA Proteolytic enzymes Break proteins apart into mixture of dipeptides, tripeptides, and amino acids Secreted as inactive proenzymes Activated in the duodenum Active forms include trypsin, chymotrypsin, carboxypeptidase, elastase The Pancreas: Enzymes A note on trypsin Trypsinogen is released from the pancreas and will reach the small intestine and will encounter enterokinase (a brush-border enzyme) which activates trypsin, which then acts on the other inactive enzymes to activate them The Liver Liver Largest visceral organ Weighs 1.5 kg (3.3 lb) It is inferior to the diaphragm and occupies most of the right hypochondriac and part of the epigastric region of the abdominopelvic cavity The gallbladder is a pear shaped sac that is located in a depression of the posterior surface of the liver and hangs from the anterior margin Wrapped in tough fibrous capsule Covered by layer of visceral peritoneum The Liver Functions 1. Carbohydrate metabolism Important in maintaining a normal blood glucose level 2. Lipid metabolism Store some TG, breakdown fatty acids, synthesize lipoproteins, cholesterol 3. Protein metabolism Hepatocytes deaminate amino acids and will convert toxic ammonia to urea 4. Processing of drugs and hormones The liver detoxifies alcohol and excretes drugs 5. Excretion of bilirubin Bilirubin is absorbed by the liver from the blood and secreted into bile The Liver Functions (continued) 6. Synthesis of bile salts Bile salts are used in the small intestine for emulsification and absorption of lipids 7. Storage Storage of glycogen and certain vitamins and minerals 8. Phagocytosis Act to phagocytize aged WBC, RBC and some bacteria 9. Activation of vitamin D The skin, liver, and kidneys participate in synthesizing active form of vitamin D The Liver The Liver There are 4 lobes of the liver: 1. ​Left lobe 2. ​Right lobe Separated from the left lobe by the falciform ligament Round ligament is a thickening in the posterior margin of the falciform ligament Marks the path of the fetal umbilical vein 3. ​Caudate lobe On the posterior surface of the liver 4. ​Quadrate lobe Located between the left lobe and the gallbladder The Liver Associated structures Gallbladder Temporarily stores bile produced by the liver Bile duct Carries bile from the liver and gallbladder to the duodenum The Liver Cells of the liver Hepatocytes Make up 80% of the liver major functional cells of liver Wide variety of metabolic, secretory, and endocrine functions secrete bile (excretory product and digestive secretion) Stellate macrophages (Kupffer cells) Immune function The Liver Bile canaliculi ducts between hepatocytes that collect bile exits livers as common hepatic duct Hepatic sinusoids highly permeable blood capillaries receiving oxygenated blood from hepatic artery and deoxygenated nutrient-rich blood from hepatic portal vein Hepatic sinusoids will converge and deliver blood to a central vein which then drains into the inferior vena cava Portal triad a bile duct a branch of the hepatic artery a branch of the hepatic portal vein The Liver Portal triad Contains three structures 1. A branch of the hepatic portal vein Receives blood from the hepatic portal system bringing blood from abdominal viscera 1. A branch of the hepatic artery proper 2. A bile duct Branches from the arteries and veins of each portal area deliver blood to liver (hepatic) sinusoids of adjacent liver lobules The Liver The Liver There are 3 different ways to organize liver tissue into functional units, all relating to portal triads 1. Hepatic lobule 2. Portal lobule 3. Hepatic acinus The Liver 1. Hepatic lobules Liver contains ~100,000 liver lobules Hexagonal shape in cross section Surrounded by six portal triads (one at each corner of the lobule) Each roughly 1 mm in diameter 2. Portal Lobule Triangle cornered by three central veins The Liver 3. Hepatic acinus the smallest functional contains a small portal tract at the center and central veins at the periphery divided into zones 1, 2, and 3 zone 1 surrounds the portal tract and zone 3 surrounds the hepatic venule Mostly descriptive reasons Although, zone 3 is more prone to ischemic damage than zone 1 WHY? The Liver Functional anatomy of a liver lobule 1. Blood enters the liver sinusoids from branches of the hepatic portal vein and hepatic artery proper About one-third of blood supply is arterial Remainder comes from hepatic portal vein The Liver Functional anatomy of a liver lobule (continued) 2. Hepatocytes adjacent to the sinusoids regulate solute and nutrient levels by absorption and secretion The Liver Functional anatomy of a liver lobule (continued) 3. Stellate macrophages (Kupffer cells) in the sinusoidal lining engulf pathogens, cell debris, and damaged blood cells Also store iron, some lipids, and heavy metals (tin, mercury) absorbed by the digestive tract The Liver Functional anatomy of a liver lobule (continued) 4. Sinusoids all drain into a central vein Central veins of all lobules merge to form the hepatic veins Hepatic veins empty into the inferior vena cava The Liver Functional anatomy of a liver lobule (continued) 5. Hepatocytes secrete bile into a network of narrow channels called bile canaliculi 6. Bile canaliculi merge to form bile ductules Bile ductules carry bile to bile ducts in nearest portal area Bile plays a role in the digestion of fats Bile Hepatocytes secrete 800-1000mL of bile daily Mostly water, bile salts, cholesterol, lecithin, bile pigments and several ions Bilirubin is the principal bile pigment Where does bilirubin come from? Partially excretory product/partially digestive secretion Bile salts play role in emulsification (the breakdown of large lipid globules into a suspension of small lipid globules) Also aid in absorption of lipids following digestion The Gallbladder Hollow, pear-shaped organ Located in the depression on the posterior surface of the liver’s right lobe Divided into three regions: the fundus, the body, and the neck The Gallbladder Mucosa layer consists of simple columnar epithelium arranged in rugae similar to that in the stomach Between meals, stores and concentrates bile secreted from the liver Contraction of smooth muscle fibers eject contents of gall bladder into cystic duct The Gallbladder Path of bile Right and left hepatic ducts collect bile from the liver bile ducts Hepatic ducts unite to form the common hepatic duct Bile flows from the common hepatic duct into: The common bile duct (to the duodenum) The cystic duct (to the gallbladder for storage) When needed, contraction of the gallbladder forces bile back along the cystic duct to the common bile duct The Gallbladder Path of bile (continued) The common bile duct penetrates the duodenal wall and meets the pancreatic duct at the hepatopancreatic ampulla (ampulla of Vater, duodenal ampulla) The hepatopancreatic sphincter encircles the lumens of these areas where they enter the duodenum Prevents flow of bile into the duodenum except at mealtime The Gallbladder Functional relationships in bile storage/ejection Liver produces about 1 L of bile per day Unless hepatopancreatic sphincter is open, bile flows into the gallbladder for storage Release of CCK by the duodenum triggers: Dilation of the hepatopancreatic sphincter Contraction of the gallbladder Bile is ejected into the duodenum Bile salts break apart lipid droplets through emulsification Flow and Function of Bile Summary of Enzymes Summary of Digestive Hormones Next week we put it all together by looking at how we digest and absorb individual nutrients. So make sure you review before class. REVIEW QUESTIONS  What are rugae? What is the function of rugae? The mucosa lies in folds in the stomach when empty. It acts to allow expansion of the stomach.  Which cells secrete pepsinogen? How is pepsinogen activated to pepsin?  Chief Cells The HCl of the stomach  What substances are absorbed in the stomach?  Alcohol some drugs small chain fatty acids Small amounts of water some ions  What is the name of the pancreatic cells that release pancreatic enzymes? pancreatic acini  Where do pancreatic enzymes act and how do they reach their destination?  Small Intestine they enter the duodenum via the pancreatic duct What are the 3 regions of the small intestine? Duodenum, jejunum, and ileum