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Stomach stuff (stomach (Gastric pit cells (D cells (somatostatin (inhibits…

- - - - Denatures protein
      - Kills bacteria in food
      - Provides optimum pH for activation of pepsins
      - Help Fe2+, Ca2+ absorption
      - Activates pepsinogen --> pepsin
      - Promotes pancreatic, small intestinal, bile secretion
    - - Histamine (ECL cells)
        
        stimulates parietal cells via H2 histamine receptors on parietal cells
        
        e.g. Tagamet and Zantac block H2 receptors to reduce acid reflux
      - Parasympathetic neurons
        
        use ACh to stimulate both parietal and ECL cells (both direct and indirect stimulation)
      - Gastrin (G-cells)
        
        carried directly to parietal cells via blood (for direct stimulation)
        
        also indirect stimulation of parietal cells:
        stimulates ECL cells to make histamine
  - - - somatostatin
        
        inhibits gastric acid secretion
      - ghrelin
        
        regulates hunger
    - - histamine
        
        stimulates gastric acid secretion
        
        indirect stimulation via H2 histamine receptors on parietal cells
      - serotonin
        
        stimulates gastric acid secretion
    - - gastrin
        
        stimulates gastric acid secretion
    - - gastric acid
        
        activates pepsin; kills bacteria
      - intrinsic factor
        
        complexes with vitamin B12 to permit absorption
    - - mucus
        
        physical barrier between lumen and epithelium
      - bicarbonate
        
        buffers gastric acid to prevent damage to epithelium
    - - gastric lipase
        
        digests fats
      - pepsinogen
        
        digests proteins
  - - - simple diffusion
      - facilitated diffusion
      - active transport
      - endocytosis
      - paracellular transport
    - - GI lumen via secreted enzymes
      - surface of enterocytes by membrane-bound enzymes
- - - - fenestrated (plasma proteins leak out), wider than capillaries, lined with epithelial cells, blood flow
  - - - veins from pancreas, gallbladder, stomach, omentum, spleen join with hepatic portal vein
        
        blood leaves via hepatic vein to join regular venous circulation after circulating through liver capillaries
  - - - chemical alteration of biologically active molecules (hormones and drugs)
        
        production of urea, uric acid, other molecules that are less toxic than parent compounds
      - phagocytosis by kupffer cells
      - excretion of molecules in bile
    - - production of glucose from liver glycogen and other molecules (amino acids, lactic acid) by gluconeogenesis
      - secretion of glucose into blood
      - conversion of blood glucose --> glycogen and fat
    - - excretion of cholesterol in bile
      - production of ketone bodies from fatty acids
      - synthesis of triglycerides and cholesterol
    - - production of plasma transport proteins
      - production of clotting factors (fibrinogen, prothrombin, others)
      - production of albumin
    - - synthesis of bile salts
      - conjugation and excretion of bile pigment (bilirubin)
    - - hepatocytes absorb/store excess nutrients in blood (glucose, iron, retinol, calciferol)
    - - RBC's rupture --> hemoglobin in blood plasma --> hemoglobin absorbed by phagocytosis by Kupffer cells in liver --> split into heme groups (iron recycled to bone marrow, bilirubin to bile) and globins (hydrolyzed to AA's, returned to blood)
- - - - propels contents from one segment to next, downstream
    - - evacuation of colonic content through anal canal (involves voluntary and involuntary reflexes)
      - rectal pressure increases --> internal anal sphincter relaxes --> need to defecate rises --> external anal sphincter voluntarily relaxes --> longitudinal rectal muscles contract to increase pressure --> aided by contraction of abdominal and pelvic skeletal muscles (Valsalva's manuever)
    - - contents are continuously mixed
  - - - water absorbed passively following osmotic gradient set up by active Na+/K+ pumps (aldosterone helps)
  - - - secretion of defensin by paneth cells in crypts
      - secretion of IgA antibodies by plasma cells
      - mucus from goblet cells prevents contact with epithelial cells
    - - make fatty acid from cellulose --> energy by colon epithelial cells --> help absorb electrolytes (Na, Ca, HCO3-, Mg, Fe)
      - outcompete harmful species of bacteria
      - make vitamin K, some B vitamins
      - maintain healthy epithelial barrier
      - limit inflammation
      - promote repair of damaged epithelium
- - - - propulsion of contents into large intestine
      - peristalsis
        
        responsible for forward movement: rhythm of contractions paced by specialized cells (interstitial cells of cajal) = pacemaker cells, joined by gap junctions to each other and to smooth muscle lining GI tract. Pacemaker cells influenced by autonomic nervous system
      - segmentation
        
        muscular constrictions - simultaneously at different segments mixing and cutting, no forward movement
      - maximizing exposure of contents to membranes of intestinal cells --> help absorption and digestion
      - mixing contents with enzymes and other secretions --> help digestion
    - - brush border enzymes
        
        hydrolyze disaccharides, polypeptides, other substrates to simple nutrient molecules
        
        disaccharidase
        
        maltase
        
        maltose --> glu + glu
        
        lactase
        
        lactose --> glu + galac
        
        deficiency: gastrointestinal disturbances (lactose intolerance)
        
        sucrase
        
        sucrose --> glu + fruc
        
        deficiency: gastrointestinal disturbances
        
        phosphatase
        
        Ca2+, Mg2+, -ATPase
        
        absorption of dietary calcium; enzyme activity regulated by vitamin D
        
        Alkaline phosphatase
        
        removes phosphate groups from organic molecules; enzyme activity may be regulated by vitamin D
        
        peptidase
        
        enterokinase
        
        activates trypsin (+ indirectly other pancreatic juice enzymes)
        
        deficiency: protein malnutrition
        
        aminopeptidase
        
        --> free AA's, dipeptides, tripeptides
  - - - lymph vessel (lacteal) absorbs fats
      - capillaries absorb monosaccharides and amino acids
- - - - Enzymatic component: essential for proper digestion and absorption of carbohydrates, fats, proteins
        
        pancreatic enzymes
        
        chymotrypsin (polypeptides)(internal peptide bonds)
        
        lipase (triglycerides)
        
        trypsin (protein)
        
        amylase (starch)
      - Aqueous component (contains HCO3-): neutralizing stomach acid in duodenum so pancreatic enzymes can function properly