Control of Blood Water Potential (AQA A Level Biology): Revision Note

Osmoregulation

• The control of the water potential of body fluids is known as osmoregulation

  • Osmoregulation is an example of homeostasis

• Specialised sensory neurones, known as osmoreceptors, monitor the water potential of the blood

  • Osmoreceptors are located in the hypothalamus of the brain

Low blood water content

• When blood water content decreases, e.g. due to dehydration:

  1. osmoreceptors detect a decrease in the water potential of the blood

     • Water moves out of osmoreceptor cells down a water potential gradient, causing the cells to shrink slightly

  2. nerve impulses are sent along sensory neurones to the posterior pituitary gland

  3. nerve impulses stimulate the posterior pituitary gland to release antidiuretic hormone (ADH) into the blood

  4. ADH increases the water permeability of the collecting ducts

  5. the kidneys reabsorb more water

     • water molecules move from high water potential in the collecting duct, to low water potential in the medulla

  6. a small volume of concentrated urine is produced

ADH and the kidneys

• ADH alters water reabsorption by increasing the water permeability of the cells lining the collecting duct; this occurs as follows:

  1. ADH molecules bind to receptors on the surface of cells lining the collecting duct

  2. vesicles containing aquaporins fuse with the membranes of the collecting duct cells

  3. the number of aquaporins in the cell membranes is increased, increasing the permeability of the membrane to water

High blood water content

• When blood water content increases, e.g. due to high fluid intake

  1. osmoreceptors detect an increase in the water potential of the blood

  2. nerve impulses are sent along sensory neurones to the posterior pituitary gland

  3. these nerve impulses cause the posterior pituitary gland to release less ADH

  4. water permeability of the collecting ducts decreases

  5. the kidneys reabsorb less water

  6. a large volume of dilute urine is produced