Role of Hormones in Osmoregulation

  • Osmoregulation is the maintenance of water potentials within the blood.
  • The amount of water potential found within the blood is detected by osmoreceptors located in the hypothalamus.

Water potential is reduced by:

  1. Sweating
  2. Limited Consumption of water
  3. Breathing
  4. Urinating
  5. A change in water potential is detected by osmoreceptors in the hypothalamus
  • When water potential is low, there is a high-water potential in the osmoreceptors relative to outside (the blood) therefore, water will move out of the osmoreceptors through osmosis and therefore reduce in size.
  1. The osmoreceptors triggers the hypothalamus to release Antidiuretic Hormone (ADH)
  2. ADH moves to the posterior pituitary gland before being secreted into the capillaries to travel to the kidneys
  • ADH causes an increase the permeability to water of the cell surface membrane of the cells lining the distal convoluted tubules and collecting duct by doing the following:
  1. Specific protein receptors on the cell surface membrane of these cells bind to the ADH leading to activation of phosphorylase within the cell
  2. This results in the vesicles within the cell to move to and fuse with the cell surface membrane
  3. Vesicles contain pieces of plasma membrane which have numerous water channel proteins, called aquaporin, so by fusing with the plasma membrane the number of water channels increase which thus increases the cell surface membranes permeability
  4. ADH increases the permeability of the collecting duct to urea which therefore passes out, further lowering the water potential of the fluid around the collecting duct.
  5. Overall, more water leaves the collecting duct by osmosis back into the blood
  • b. water potential of the blood does not, and cannot increase however can be maintained to reduce further water loss.

Antidiuretic Hormone (ADH)

  • Water potential is measured by osmoreceptors in the hypothalamus
  • A decrease in water potential results in:
    1. water will move out of the osmoreceptor cells by osmosis
    2. Causes the cells to decrease in volume
    3. Sends a signal to other cells in the hypothalamus
    4. Hypothalamus sends signal to posterior pituitary gland
    5. Posterior pituitary gland releases antidiuretic hormone into blood
    6. AHD binds to receptors on plasma membrane of cells in the Distal convoluted tubules and collecting duct
    7. Aquaporin (protein channels) are inserted into the plasma membrane which allow water to pass (osmosis) making the walls of the DCT and collecting duct more permeable to water
    8. More water is reabsorbed from these tubules into the medulla and into the blood by osmosis
    9. A small amount of concentrated urine is produced

Dehydration

  • Water content of the blood drops (water potential decreases)
  • Detected by osmoreceptors in hypothalamus
  • Posterior pituitary gland is stimulated to release more ADH into blood
  • More ADH means the DCT and collecting duct are more permeable to water
  • More water is reabsorbed
  • Small amount of concentrated urine

Hydration

  • Water content of the blood increases (water potential increases)
  • Detected by osmoreceptors in hypothalamus
  • Posterior pituitary gland is stimulated to release less ADH into blood
  • Less ADH means the DCT and collecting duct are less permeable to water
  • Less water is reabsorbed
  • large amount of dilute urine