7.21 Hormones in Osmoregulation

• The control of the water potential of body fluids is known as osmoregulation
  • Osmoregulation is a key part of homeostasis
• Specialised sensory receptors, 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 sensory neurones to the posterior pituitary gland, located 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

Low blood water content

• Blood water content might drop as a result of reduced water intake, sweating, or diarrhoea
  • Low blood water content can also be referred to as high blood solute concentration, or low blood water potential
  • If blood water content gets too low it can lead to dehydration
• A reduction of blood water content is detected by the hypothalamus in the brain
• The hypothalamus causes the pituitary gland to secrete ADH into the blood
  • The target cells of ADH are in the distal convoluted tubule and collecting duct in the kidneys
• ADH increases the permeability of the walls of the distal convoluted tubule and collecting duct in the kidneys to water
  • The permeability of the walls of the distal convoluted tubule and collecting duct are increased by increasing the number of channel proteins called aquaporins in the cell surface membranes of the cells lining the nephron lumen; 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 causes the vesicles to move to and fuse with the luminal membranes of the collecting duct cells
    • This increases the permeability of the membrane to water
• More water is reabsorbed into the blood via the distal convoluted tubule and collecting duct
• The reabsorption of water leaves a concentrated filtrate that passes through the collecting duct and into the renal pelvis
  • This remaining filtrate is the urine; from the renal pelvis it passes along the ureter to the bladder
• The blood water content increases and a small quantity of concentrated urine is produced

ADH increases the permeability of the walls of the collecting duct to water by increasing the number of aquaporins in the cell surface membranes of the collecting duct cells.

High blood water content

• Blood water content might increase due to increased water intake or loss of salts during sweating
  • High blood water content can also be referred to as low blood solute concentration, or high blood water potential
  • If blood water content gets too high it can lead to overhydration
• High blood water content is detected by the hypothalamus
• The hypothalamus no longer stimulates the pituitary gland to release ADH and ADH levels in the blood drop
• The distal convoluted tubule and collecting duct walls become less permeable to water
  • Fewer aquaporins are present
• Less water is reabsorbed from these regions of the nephron into the blood, and the water instead passes down the collecting duct into the renal pelvis along with the rest of the filtrate
• Blood water content decreases and a large quantity of dilute urine is produced