Osmoregulation

• The water potential of the blood has to be maintained regardless of the water and solutes taken in as you eat and drink, and the water and mineral salts lost by sweating, defaecation and urination.
• The amount of water lost in the urine is controlled by ADH in a negative feedback system.
• ADH is produced by the hypothalamus and secreted and stored in the posterior pituitary gland.
• ADH increases the permeability of the collecting duct to water.
• When water potential is low:

  1. ADH binds to receptors on the cells of the collecting duct
  2. this triggers the formation of cAMP
  3. cAMP causes vesicles containing aquaporins in their membrane to fuse with the cell membrane on the side of the cell in contact with the tissue fluid of the medulla
  4. These aquaporin channels now become part of the cell’s membrane
  5. water can move out of the cells into the tissue fluid by osmosis
  6. as the cell now has low water potential it will take in water from the collecting duct
• When ADH levels fall, levels of cAMP fall, then the water channels are removed from the cell membranes and enclosed in vesicles again. The collecting duct becomes impermeable to water once more so no water can leave.