Osmoregulation (OCR A Level Biology)

• The control of the water potential of body fluids is known as osmoregulation
• Osmoregulation is a key part of homeostasis
• Specialised sensory neurones, known as osmoreceptors, monitor the water potential of the blood (these osmoreceptors are found in an area of the brain known as the hypothalamus)
• If the osmoreceptors detect a decrease in the water potential of the blood, nerve impulses are sent along these sensory neurones to the posterior pituitary gland (another part of the brain just below the hypothalamus)
• These nerve impulses stimulate the posterior pituitary gland to release antidiuretic hormone (ADH)
• ADH molecules enter the blood and travel throughout the body
• ADH causes the kidneys to reabsorb more water
• This reduces the loss of water in the urine

When osmoreceptors detect a decrease in blood water potential, nerve impulses stimulate the release of ADH at the posterior pituitary gland. This ADH then travels in the blood to the kidneys, causing them to increase water reabsorption.

The effect of ADH on the kidneys

• Water is reabsorbed by osmosis from the filtrate in the nephron
• This reabsorption occurs as the filtrate passes through structures known as collecting ducts
• ADH causes the luminal membranes (ie. those facing the lumen of the nephron) of the collecting duct cells to become more permeable to water
• ADH does this by causing an increase in the number of aquaporins (water-permeable channels) in the luminal membranes of the collecting duct cells. This occurs in the following way:
  • Collecting duct cells contain vesicles, the membranes of which contain many aquaporins
  • ADH molecules bind to receptor proteins, activating a signalling cascade that leads to the phosphorylation of the aquaporin molecules
  • This activates the aquaporins, causing the vesicles (with aquaporin-containing membranes) to fuse with the luminal membranes of the collecting duct cells
  • This increases the permeability of the membrane to water

• As the filtrate in the nephron travels along the collecting duct, water molecules move from the collecting duct (high water potential), through the aquaporins, and into the tissue fluid and blood plasma in the medulla (low water potential)
• As the filtrate in the collecting duct loses water it becomes more concentrated
• As a result, a small volume of concentrated urine is produced. This flows from the kidneys, through the ureters and into the bladder

How ADH affects water reabsorption in the collecting duct of the nephron