LEC 23: DIGESTIVE SECRETIONS

• Phases of regulation of gastric secretion and stomach motility

  1. Cephalic phase

  2. Gastric phase

  3. Intestinal phase
• Cephalic phase


  Triggered by sensory stimuli (aroma, taste, sight, thought), stimulates gastric secretions and motility
• Gastric phase


  Initiated by stretch & chemical stimuli in the stomach, promotes gastric secretions & stomach muscle contractions
• Intestinal phase


  Begins with brief stimulation of gastric secretions, followed by inhibition due to intestinal conditions
• Gastric acid secretion

  1. Protons (H+) are secreted into the stomach lumen by proton pumps

  2. Bicarbonate (HCO3-) is pumped into the blood via Cl- and HCO3- antiporters

  3. Chloride ions (Cl-) are pumped out to the lumen to join with protons (H+), forming HCl

  4. Proton pumps powered by ATP secrete H+

  5. Cl- diffuses into the stomach lumen through Cl- channels
• Mechanisms facilitating stomach function

  • Reflex-mediated receptive relaxation of stomach muscle & stretch of smooth muscle allows it to expand
  • Enteric pacemaker cells set stomach's electrical rhythm
  • Stomach emptying rate depends on contents and small intestine readiness
• Alkaline tide


  1. Protons (H+) are secreted into the stomach

  2. Bicarbonate (HCO3-) pumped into blood via Cl- and HCO3- antiporters

  3. Increase of HCO3- in blood leaving the stomach

  4. Leads to alkalinity

  5. Helps neutralize the acidic environment created by the stomach during digestion

  6. Maintains blood pH within a narrow range
• Liver anatomy


  • Composed of 4 lobes
  • Liver lobules: Plates of liver cells (hepatocytes) surround central vein and meet at portal triads
  • Portal triads: Consists of hepatic artery, hepatic portal vein, bile duct
• Liver functions

  • Hepatocytes secrete bile, process nutrients, store fat-soluble vitamins, play roles in detoxification
  • Liver disorders: Hepatitis (inflammation), Cirrhosis (scar tissue replaces hepatocytes), Kwashiorkor (enlarged fatty liver and edema caused by protein deficiency)
• Pancreas anatomy & function


  • Has both endocrine and exocrine functions
  • Exocrine function: Secretes pancreatic juices containing enzymes and bicarbonate into the small intestines
  • Pancreatic duct joins with bile duct from liver to deliver juice to intestines
• Role of bile

  Emulsifies fat, aids in digestion and absorption of fats
• Role of pancreatic juice

  • Contains enzymes that break down carbohydrates, proteins, and fats
  • Neutralizes stomach acid in the duodenum
  • Facilitates digestion in the small intestines
• Role of the gallbladder


  Stores bile, concentrates bile by absorbing water and ions, releases bile into the small intestines to aid in fat digestion when needed
• Regulation of bile and pancreatic juice secretion into the small intestine

  1. Regulated by hormonal and neural mechanisms

  2. Secretin stimulation: Triggered by acidic chyme, stimulates secretion of bicarbonate-rich pancreatic juice

  3. Cholecystokinin (CCK) stimulation: Triggered by fatty chyme, stimulates secretion of enzyme-rich pancreatic juice

  4. Neural stimulation: Nervous signals from vagus nerve & enteric nervous systems can stimulate pancreatic secretion, especially in response to sight, smell, or taste of food
• Enteroendocrine G cells


  Produce gastrin, which stimulates gastric secretions and promote movement of food along the GI tract
• Somatostatin


  From stomach mucosa and duodenum, has the opposite effect of gastrin
• Stomach mucosal cells


  Can release histamine to stimulate parietal cells to release HCl and serotonin to stimulate stomach muscle contraction
• Ghrelin


  The hunger hormone, released by the stomach mucosa, with highest levels just before a meal
• what phase is triggered by sensory stimuli and stimulates gastric secretion and motility?

  cephalic
• what promotes gastric secretions and stomach muscle contractions?

  gastric
• what phase begins with a brief stimulation of gastric secretions, followed by inhibition due to intestinal conditions?

  intestinal
• during gastric acid secretion, how are protons secreted into the stomach lumen?

  proton pumps
• what is pumped into the blood via chloride and bicarbonate secretions?

  bicarbonate
• what joins with protons to form hydrochloric acid?

  chloride ions
• what are proton pumps powered by?

  ATP
• what do proton pumps secrete?

  protons
• during alkaline tide, what leads to alkalinity?

  increase of bicarbonate in blood leaving the stomach
• what helps neutralize the acidic environment created by the stomach during digestion and maintains blood pH?

  alkaline tide
• what secretes bile, process nutrients, stores fat-soluble vitamins and play roles in detoxification?

  hepatocytes
• what is inflammation of the liver?

  hepatitis
• what liver disorder causes scar tissues to replace hepatocytes?

  cirrhosis
• what liver disorder causes an enlarged fatty liver and edema caused by protein deficiency?

  kwashiorkor
• what organ contains both endocrine and exocrine functions?

  pancreas
• what function secretes pancreatic juices contains enzymes and bicarbonate into small intestines?

  exocrine
• what duct joins with bile duct from liver to deliver juice to intestines?

  pancreatic duct
• what emulsifies fat, aids in digestion, and absorption of fats?

  bile
• bile aids in digestion, absorbs fat, and what else?

  emulsifies fat
• where does pancreatic juice neutralize stomach acid?

  duodenum
• what breaks down carbohydrates, proteins, and fats?

  pancreatic juice
• how is secretin stimulated?

  acidic chyme