Osmoregulation

• What is osmoregulation?

  The mechanism by which the balance of water and dissolved solutes is maintained
• Water balance is a negative feedback system that relies on the hypothalamus in the brain. The hypothalamus contains osmoreceptor cells, which can detect changes in the water potential of the blood passing through the brain
• The hypothalamus controls the sensation of thirst and secretion of ADH. ADH is stored in the pituitary gland, and its target cells are the endothelial cells of the collecting ducts of the kidney nephrons
• How is ADH involved in the osmoregulation of a dehydrated person?

  • ADH travels in the blood of the capillary from the pituitary gland to the collecting duct
  • ADH is detected by receptors on the collecting duct endothelial cells
  • this causes porous vesicles containing aquaporins to move and fuse with the cell membrane of the endothelial cells
  • so the cell membrane becomes porous (permeable to water)
  • there is a high water potential in the collecting duct lumen and a low water potential in the capillary
  • so water moves from the collecting duct lumen to the blood in the capillary
• Low water potential in the blood -> more ADH secreted -> more water is reabsorbed into the blood -> a small volume of concentrated urine is produced
• High water potential in the blood -> less ADH secreted -> less water is reabsorbed into the blood -> a large volume of dilute urine is produced