Chapter 23: Digestive System Flashcards
Digestive Tract:
o About 9 meters or 30 feet long (in cadaver).
o Accessory organs: Teeth, Tongue, Salivary glands, Liver, Gall bladder, Pancreas.
o Most of “stuff” inside tube has not yet entered body tissues!
Nutrient Disassembly:
o Carbohydrates become monosaccharides (Glucose, Fructose, Galactose).
o Triglycerides become Fatty acids + glycerol, some monoglycerides.
o Proteins become amino acids (few di- and tri- peptides).
o Vitamins stay as vitamins.
o Minerals stay as minerals.
o Water stays as water.
Processes of Digestive System:
o Ingestion. o Mechanical Digestion (mixing). o Chewing = Mastication. o Propulsion: o Deglutition = swallowing. o Peristalsis. o Mass movements. o Secretion (water, mucus, bile, enzymes, hormones). o Chemical Digestion = breaking nutrients down into small pieces. o Absorption (small intestine). o Defecation.
Histology of the Digestive Tract:
o 4 Tunics (Layers): Mucosa, Submucosa, Muscularis Externa, and Serosa.
Mucosa:
o Mucous epithelium: o Many types. o Exocrine cells. o Endocrine cells. o Lamina propria: o Very vascular!! o Lymphatic follicles. o Glands. o Muscularis mucosae.
Submucosa:
o Thick c.t. layer. o Bigger blood and lymphatic vessels. o Glands. o Lymphatic follicles. o Submucosal plexus (Meissner’s plexus).
ENS (Enteric Nervous System):
o Nervous network regulating digestive motility, secretions, and blood flow
o More neurons than spinal cord!
o Definitely influenced by ANS:
o Parasympathetic:
o Enhances gland secretion.
o Enhances motility.
o Sympathetic:
o Inhibits gland secretions.
o Inhibits smooth muscle contraction (exception: some sphincters are excited by sympathetic nerv. system).
o Causes vasoconstriction of blood vessels to the digestive tract.
Regulation of Digestive System:
o Local neuronal control: o Enteric sensory neurons (detect stretch & chemicals). o Enteric motor neurons (activate glands or change motility). o CNS control: o ANS: vagus and sympathetic control. o Conscious sight/smell/taste. o Hormonal control: o Systemic (gastrin, CCK, secretin). o Paracrine (histamine, PGs).
Muscularis Externa:
o Circular layer of smooth muscle. o Can form sphincters! o Myenteric (Auerbach) plexus. o Longitudinal layer of smooth muscle. o Exceptions.
Peristalsis:
o Adjacent segments of alimentary tract organs alternately contract and relax moving food along the tract distally.
Serosa:
o Outermost layer.
o Usually represents visceral peritoneum.
o Sometimes not part of peritoneum = adventitia.
Peritoneum:
o Largest serous membrane of body.
o Visceral peritoneum.
o Peritoneal cavity.
o Parietal peritoneum.
o Mesentery = double layer of peritoneum that stabilizes intestines/keeps them from becoming entangled during digestion.
o Examples: Mesentary “proper”, mesocolons, lesser omentum, greater omentum, and falciform ligament.
Retroperitoneal Organs:
o Intraperitoneal: organ is almost completely surrounded by peritoneum.
o Retroperitoneal: lies against the posterior body wall and is covered by peritoneum only on the anterior side:
o Examples: Duodenum, Pancreas, Ascending and descending colon, Rectum, Kidneys, ureters, urinary bladder and Adrenal glands.
Blood Supply of the Digestive System:
o Arteries: o Celiac trunk o Superior mesenteric a. o Inferior mesenteric a. o Veins from pancreas, stomach, GB, spleen, small and large intestines drain into the HEPATIC PORTAL VEIN. o Hepatic portal system.
Tongue:
o Skeletal muscle. o Lingual papillae: o Filiform. o Fungiform. o Foliate. o Circumvallate. o Lingual tonsils. o Lingual glands: o Mucus and lingual lipase.
Salivary Glands:
o Parotid: below ear/over masseter muscle.
o Submandibular: lower edge of mandible.
o Sublingual: deep to tongue in mouth floor.
o All have ducts emptying into oral cavity.
o Secrete salivary amylase, lysozyme, IgA.
Functions of Saliva:
o Deglutition: Swallowing. o Taste. o Buffer. o Chemical digestion. o Salivary amylase. o Protection: o Lysozyme enzyme. o IgA. o Rinsing action—1 to 1.5 quarts/day!!!
Tooth Composition:
o Enamel: o Calcium phosphate or carbonate. o Hardest substance in the body. o Dentin = calcified c.t. that is harder than bone! o Cementum: o Bone-like. o Attaches root to periodontal ligament.
Dentition:
o Primary = deciduous = baby teeth:
o 20 teeth that start erupting at 6 months.
o Permanent teeth:
o 32 teeth that erupt between 6 to 12 years.
o Differing structures indicate function.
o Incisors for biting.
o Canines for tearing.
o Premolars and molars to crush/grind.
Pharynx:
o Skeletal muscle lined by mucous membrane.
o DEGLUTITION pushes bolus of food into oropharynx.
o Soft palate elevated.
o Larynx lifted as epiglottis closes.
Digestive Processes of Esophagus:
o 1. Buccal Phase: The upper esophageal sphincter is contracted (closed). The tongue presses against the hard palate forcing the food bolus into the oropharynx.
o 2. Pharyngeal-esophageal phase begins: The tongue blocks the mouth. The soft palate and its uvula rise, closing off the nasopharynx. The larynx rises so that the epiglottis blocks the trachea. The upper esophageal sphincter relaxes; food enters the esophagus.
o 3. Pharyngeal-esophageal phase continues: The constrictor muscles of the pharynx contract, forcing food into the esophagus inferiorly. The upper esophageal sphincter contracts after food enters.
o 4. Peristalsis moves food through the esophagus to the stomach.
o 5. The gastroesophageal sphincter surrounding the cardial orifice opens. After the food enters the stomach, the sphincter closes, preventing regurgitation.
Esophageal Disorders:
o Dysphagia: difficult swallowing.
o Esophageal obstructions (tumors, constrictions).
o Impaired peristalsis (neuromuscular disorders).
o Acute esophagitis: ingestion of irritating substances, viral inflammation, intubation.
o Barrett’s esophagus (dysplasia).
o GERD = gastroesophageal reflux disease.
o LES fails to close adequately.
o Stomach contents “reflux”.
o HCl from stomach irritates esophageal lining = heartburn (radiating substernal pain).
Risk Factors for GERD:
o Hiatal hernia
o Recurrent vomiting (bulimia)
o Lying down right after a meal
o Delayed gastric emptying (common in DM)
o Increased abdominal pressure: Pregnancy or obesity, Tight fitting clothing, Large meals.
o Reduced LES muscle tone: Smoking, OCAs, Pregnancy (or even late menstrual cycle stages), Large amounts of dietary fat, Alcohol and caffeine, Carminatives (peppermint and spearmint).
Hiatal Hernia:
o Protrusion of stomach through the esophageal hiatus of the diaphragm.
o 2 types of Hiatal Hernia:
o Sliding hiatal hernia (95 percent); Very common.
o Paraesophageal.
o Most common GI d/o !!!!!
o LES fails to close adequately →stomach acid “refluxes” leads to irritation esophageal mucosa.
o Causes severe esophagitis due to excessive gastric reflux: GERD.
Gross Anatomy of the Stomach:
o Size:
o 50 mL – 1.5 L
o Regions:
o Cardiac region, Fundus of stomach, Body of stomach, Pyloric region.
o Gastric emptying:
o Solid food has been liquified to chime.
o Pyloric stenosis.
Protection of Stomach Mucosa:
o Thick coat (1 to 3 mm) of alkaline mucus.
o Tight junctions of epithelial cells.
o External membranes of glandular cells are not permeable to HCl.
o Pepsin is secreted in an inactive form (pepsinogen).
o Damaged mucosal cells are SHED every 3 to 6 days.
o Constant balance between acid and mucus.
Peptic Ulcer Disease (PUD):
o Erosion from chronic inflammation.
o 80 percent of peptic ulcers are actually in duodenum.
o 10 percent of US adults will have an ulcer.
o Caused by: Hypersecretion of HCl, hyposecretion of mucus, Helicobacter pylori infection.
Mucosa and Gastric Glands:
o No goblet cells.
o Mucous neck cells.
o Parietal cells— secrete:
o HCl, and Intrinsic factor.
o Chief cells— secrete inactive pepsinogen enzyme (gastric lipase and rennin in infants).
o HCl converts pepsinogen to pepsin (active form).
Parietal Cell Secretion of HCl:
o Stimulus: Ach, Histamine, and Gastrin. (All three are needed)
Parietal Cells:
Secrete Intrinsic Factor:
o B-12 bound to dietary protein in food.
o Pepsin and acid free it from this dietary pro.
o B-12 combines with cobalophilin (a protein from saliva).
o Pancreatic proteases cleave this bond.
o B-12 binds with intrinsic factor.
o Receptor mediated endocytosis in ileum.
Enteroendocrine Cells:
o Gastrin hormone (g cell)
o “get it out of here”
o Target tissue = stomach, LES, ileocecal sphincter, large intestine
o Release more gastric juice
o Increase gastric motility
o Relax pyloric sphincter
o Constrict esophageal sphincter preventing entry
o Histamine: target tissue = parietal cells, which have H-2 receptors.
o Serotonin: target tissue = gastric smooth muscle (contraction).
o Ghrelin: increases appetite.
Chemical Digestion in Stomach:
o Protein digestion begins:
o HCl denatures (unfolds) protein molecules.
o HCl transforms pepsinogen into pepsin that breaks peptides bonds between certain amino acids.
o Rennin = Chymosin, important in kids, enzyme that acts on casein in milk.
o Fat digestion continues:
o Gastric lipase splits the triglycerides in milk fat.
o most effective at pH 5 to 6 (infant stomach).
o HCl lowers stomach pH to 1.5 to 3.5, kills microbes in food and inactivates some natural enzymes found in food.
Absorption in Stomach:
o Alcohol (usually only a little).
o Meals containing large amounts of fat will slow the rate of ETOH absorption.
o ETOH adheres to the fat droplets in chyme!
o Most alcohol absorbed in small intestine.
o Aspirin and some lipid soluble meds.
o Short chain fatty acids.
o Water and electrolytes.
Cephalic Phase:
Stomach Getting Ready
o Cerebral cortex =sight, smell, taste & thoughts of food.
o Stimulate parasympathetic nervous system.
o Vagus nerve:
o Increases stomach muscle and glandular activity.
o Sympathetic N.S.:
o Inhibits stomach activity.
Gastric Phase:
Stomach Working
o Nervous control keeps stomach active.
o Stretch receptors & chemoreceptors provide information.
o Vigorous peristalsis and glandular secretions continue (myenteric and vagal reflexes).
o Chyme is released into the duodenum.
o Endocrine influences over stomach activity
o Distension and presence of caffeine or protein cause G cells to secrete GASTRIN into blood.
o GASTRIN hormone increases stomach glandular secretion.
o GASTRIN hormone increases stomach churning and pyloric sphincter relaxation.
o Histamine and ACh (parasympathetic nervous system) also stimulate gastric secretions (acid).
2 Central Reflexes Triggered by Stimulation of Stretch Receptors in Stomach Wall:
o Gastroenteric Reflex: Stimulates motility and secretion along the entire small intestine.
o Gastroileal Reflex: Triggers the opening of the ileocecal valve, allowing materials to pass from the small intestine into the large intestine.
Intestinal Phase:
Stomach Emptying
o Stretch receptors in duodenum slow stomach activity and increase intestinal activity.
o Distension, fatty acids or sugar signals medulla .
o Sympathetic nerves slow stomach activity.
o Decreased parasympathetic activity (vagus).
o Regulated by hormones from small intestine:
o Secretin hormone decreases stomach secretions.
o Cholecystokinin (CCK) decreases stomach emptying.
o Gastric inhibitory peptide (GIP) (secreted by small intestine) decreases. stomach secretions, motility and emptying.
Liver:
o Largest gland in body; 2nd largest organ, exocrine and an endocrine organ.
o Located just underneath the diaphragm in the upper right quadrant of abdominal cavity.
o Has 4 gross (macroscopic) lobes.
o Right lobe, left lobe (seen anteriorly), divided by the falciform ligament (mesentery).
o Quadrate lobe, caudate lobe (underview).
o Enclosed by thin c.t. capsule, very little c.t. in rest of liver compared to other organs (mainly supportive reticular fibers).
o Almost completely covered by visceral peritoneum.
Porta Hepatis:
o Inferior surface.
o Blood vessels, ducts, nerves, lymph vessels enter/exit the liver.
o Beginning here, the c.t. capsule sends a branching network of SEPTA into the liver, dividing it into hexagon-shaped lobules.
Portal Area (Portal Triad):
o Branch of common hepatic artery.
o Brings in oxygenated blood from aorta.
o Branch of hepatic portal vein.
o Brings in nutrient rich blood from intestines, as well as venous blood from the pancreas and spleen.
o Bile duct receiving bile from canaliculi in liver cells that will eventually form either the right or left hepatic duct.
o Lymphatic vessels.
o Liver produces large quantities of lymph!
Unique Blood Supply to Liver:
o Receives blood from two sources:
o Oxygenated blood from hepatic arteries.
o Venous blood from hepatic portal vein (coming from nutrient -rich venous blood of intestines, as well as the spleen and pancreas).
o Blood “percolates” through hepatic sinusoidal capillaries in between plates of liver cells (VERY leaky).
o Sinusoidal capillaries empty into a central vein at the center of a hepatic lobule.
o Central veins of all hepatic lobules drain eventually into several hepatic veins.
o Hepatic veins drain into inferior vena cava.
Specialized Cells of the Liver:
o Hepatocytes = 80 percent of cells of liver.
o Kupffer cells = fixed macrophages.
o Line hepatic sinusoids.
o Remove and recycle worn out RBCs.
o Attack pathogens and remove debris.
o Endothelial cells lining liver sinusoids.
o Ito cells = stellate shaped cells.
o Store Vitamin A and lipids.
o Acquire features of myofibroblasts with liver injury or liver disease (fibrosis).
Hepatocytes:
o Polyhedral cells, with 6 or more surfaces.
o Form hepatic plates (plates of hepatocytes in-between hepatic sinusoids).
o Round nuclei; 25% of cells binucleate; 50 percent of nuclei are oddly polyploid!!!
o Contain glycogen granules, lipid droplets, thousands of mitochondria, peroxisomes, lots of endoplasmic reticulum.
o MAKE BILE and secrete it into tiny ducts called bile canaliculi.
General Functions of the Liver:
o Production/secretion of bile.
o Excretion of bilirubin/other wastes.
o Phagocytosis by Kupffer cells (helps spleen filter the blood).
o Detoxification of alcohol, meds, hormones.
o Converts NH3 to urea (via ornithine cycle).
o Synthesizes clotting factors and TPO.
o Makes hormones: EPO, TPO, hepcidin, IGFs.
o Helps to activate Vitamin D (makes calcidiol).
o Makes 90 percent of plasma proteins.
o Stores stuff: glycogen, fat, Vit. B-12, iron.
