4. Absorption of Water, Minerals, and Vitamins - Alcon

• the greatest volume of intestinal secretion comes from?

  bottom of crypts of Lieberkühn
• • Water absorbed in the intestine is isosmotic
  • Water absorbed in gut is passive following osmotic gradient
• In the gut water move osmotically - follows Na+
• ** Jejunum is the major site of Na+ absorption in small intestine

  • use Na+ dependent coupled transporter - entry into enterocyte
• Ileum absorbs NaHCO3

  • HCO3- secreted
  • Cl- is absorbed
• in colon

  • aldosterone = induces synthesis of Na+ channels
  • increase Na+ absorption
  • increase K+ secretion
• Chloride secretion is the primary driving force for water secretion across epithelium

  • two ways
  • ** CFTR - activated by cAMP
  • Ca2+ activated chloride channel (CaCC) - opened by cytosolic Ca2+
• Fasting has no effect on secretory diarrhea

  • but stops osmotic diarrhea
• Osmotic diarrhea - unabsorbed dietary nutrients --> osmotically pull water into lumen

  • Ex: lactase deficiency
• Secretory diarrhea - due to increased endogenous secretion of fluid and electrolytes
• Secretory diarrhea in cholera

  1. increase adenylate cyclase activity
  2. increase cAMP
  3. activate PKA
  4. inhibit NaCl absorption
  5. phosphorylates CFTR
  6. increase ATP mediated efflux
  7. pulls water out of intestinal cells
  8. watery diarrhea
• CFTR defect - cause thick mucus secretion

  • decrease secretion of Cl
  • increase resorption of Na
• absorption of Ca2+ in the intestine requires?

  active form of vitamin D
• two mechanism for the absorption of Ca2+ in the small intestine

  1. passive paracellular absorption of Ca2+ - not required vitamin D
  2. active transcellular - only occur at the duodenum
  3. ** Ca2+ enter via TRPV6
• Absorption of iron (heme)

  • Fe2+ binds to mobilferrin for transit
  • Hephaestin convert Fe2+ to Fe3+
  • Fe3+ exits vis Ferroportion
  • Fe3+ circulates in blood bound to transferrin
• Iron is stored as ferritin

  • hemosiderin - a complex aggregate of ferritin "insoluble" iron
• Fe2+ is better absorbed than Fe3+
• Duodenal enterocyte is responsible for all dietary iron absorption
• Hepcidin - produced by liver

  • control release of iron from enterocyte
  • regulate systemic iron metabolism
• All lipid soluble vitamin require micelles except vitamin K1
• Water soluble vitamin absorbed in duodenum and jejunum
• Absorption of folate

  • found in diet as polyglutamyl conjugates
  • degraded by brush border to PteGlu1
  • can be released into blood
  • directly
  • converted to 5-MTHF
• Vitamin B12 is absorbed in ileum

  • requires intrinsic factor
• Vitamin B12 deficiency leads to pernicious anemia
• ** Gastrectomy will result in loss of gastric parietal cells - which is a source of intrinsic factor

  • loss of vitamin B12
• Folate and cobalamin deficiencies cause megaloblastic anemia