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Digestive System (Accessory Digestive Organs (Pancreas (Exocrine Function:…

- - - - Liver cells are known as hepatocytes. Hepatocytes produce about 900ml of bile per day. Hepatocytes also process bloodborne nutrients (for example they store glucose as glycogen and make plasma proteins), store fat-soluble vitamins, and perform detoxification (for example converting ammonia to urea)
      - Bile is a yellow-green alkaline solution that contains bile salts and bilirubin, as well as cholesterol, triglycerides, phospholipids, and electrolytes.
        
        Bilirubin is a pigment formed from heme. Bilirubin is broken down in intestine to stercobilin. This gives feces its brown color
        
        95% of bile salts are recycled by Enterohepatic Circulation on each trip through the small intestine
        
        Bile Salts are cholesterol derivatives that function in fat emulsification and absorption
        
        Bile salts returning from enterohepatic circulation are the most powerful stimulus for bile secretion. Secretin is a minor stimulus for bile secretion
  - - - Pancreatic Ducts produce water and bicarbonate and secrete to duodenum via main pancreatic duct
      - Acini (clusters of secretory cells) produce zymogen granules that contain proenzymes
- - - - Fat digestion: minor. Began by lingual and gastric lipase
      - Protein digestion: major
        
        Pepsin begins. HCl denatures (unfolds) proteins in preparation small intestine
        
        Milk protein (casein) is broken down by rennin in infants
    - - Duodenum prevents overfilling by controlling how much chyme enters; duodenal receptors respond to stretch and chemical signals. Enterogastric reflex and enterogastrones (secretin + cholecystokinin) inhibit gastric secretion and duodenal filling
  - - - Secrete HCl
        
        1) Carbonic anhydrase: CO2 + H2O -> H2CO3
        
        2) H2CO3 is disassociated into H+ and HCO3-
        
        3) H+ -K+ ATPase pumps H+ into stomach lumen and K+ into cells. K+ returns into lumen through membrane channels
        
        4) Cl- from interstitial fluid (exchange for intracellular HCO3-) -> parietal cells -> stomach lumen -> Cl- meets with H+ -> HCl
      - Secrete intrinsic factors
    - - Secrete pepsinogen (together with HCl from parietal cells) -> activates pepsin
      - Secrete gastric lipase (15% fat digestion)
    - - somatostatin: interacts with G protein to inhibit insulin and glucagon secretion
      - histamine: helps to induce the production of HCl in the stomach
      - serotonin: inhibits gastric secretion
- - - - Enterocytes: Simple columnar, absorptive cells, tight junctions, many microvilli.
        Villi:absorb nutrients and electrolytes
        Crypts: produce intestinal juice, watery mixture of mucus, carrier fluid for chyme
      - Goblet cells are mucus-secreting cells
      - Enteroendocrine cells are the source of enterogastrones (CCK and Secretin)
      - Paneth cells secrete antimicrobial agents (defensins and lysozyme) deep in the crypts
      - Stem cells continuously divide to produce other cell types. The villus epithelium is renewed every 2-4 days.
- - - - A spinal reflex is initiated in which parasympathetic motor (efferent) fibers stimulate contraction of the rectum and sigmoid colon, and relaxation of the internal anal sphincter
      - If it is convenient to defecate, voluntary motor neurons are inhibited, allowing the external anal sphincter to relax so feces may pass. Muscles of the rectum contract to expel feces.
      - Feces move into and distend the rectum, stimulating stretch receptors there. The receptors transmit signals along afferent fibers to spinal cord neurons
- - - - Vitamin Synthesis: Synthesizes B complex and some vitamin K needed by the liver to produce clotting factors
    - - Mass movements forces feces toward rectum; parasympathetic signals stimulate contraction of sigmoid colon and rectum. The Defecation Reflex is initiated by distension.
    - - Haustral Contractions: Most colon contractions, haustra sequentially contract from distension, slow segmentation movements lasting about 1 minute for every 30 minutes
      - Gastrocolic Reflex: Initiated by presence of food in the stomach. Results in mass movements: slow, powerful peristaltic waves, activated 3-4x per day