Osmoregulation

• Levels of water in the collecting duct are regulated by the hormone ADH which acts to retain water in the body, by enabling it to be reabsorbed back into the blood.
• ADH - Antidiuretic Hormone - stimulates the collecting duct to retain water.
• ADH allows aquaporins to be inserted into the epithelium allowing water to be reabsorbed back into the blood, therefore making urine more concentrated.
• A dehydrated person will produce more ADH. Making urine more concentrated.
• Overhydrated?

  A lack of ADH will mean that the collecting duct is impermeable to water. Excess water passes out into the urine.
• As blood flows through the brain osmoreceptors in the hypothalamus monitor the water potential of the blood.
• When the water potential is negative the osmoreceptors lose water by osmosis and shrink. This stimulates neurosecretory cells in the hypothalamus to secrete ADH into the bloodstream.
• Neurosecretory cells - Produce & release ADH
• Neurosecretory cells are specialised neurones.
• ADH is synthesised in the cell body, lying within the hypothalamus.
• ADH travels down the axon to the terminal bulb of the posterior pituitary where it is stored in vesicles.
• Stimulation generates an action potential which causes the release of ADH from the vesicles, into the bloodstream.
• Once ADH is released it binds to target cells of the collecting duct causing more water to be reabsorbed into the bloodstream, raising the water potential.
• Once AHD is released and the water potential increases the changes are detected by osmoreceptors, they lose less/ no water and secretory cells are not stimulated.