ADH (Edexcel GCSE Biology)

The concentration and volume of urine

• Urine produced by the kidneys contains a mixture of
  • Urea
  • Excess mineral ions
  • Excess water

• The colour and quantity of urine produced in the body can change quickly
  • Large quantities of urine are usually pale yellow in colour because it contains a lot of water and so the urea is less concentrated
  • Small quantities of urine are usually darker yellow / orange in colour because it contains little water and so the urea is more concentrated

• There are various reasons why the concentration of urine will change, including:
  • Water intake - the more fluids drunk, the more water will be removed from the body and so a large quantity of pale yellow, dilute urine will be produced
  • Temperature - the higher the temperature the more water is lost in sweat and so less will appear in the urine, meaning a smaller quantity of dark yellow, concentrated urine will be produced
  • Exercise - the greater the level of exercise, the more water is lost in sweat and so less will appear in the urine, meaning a smaller quantity of dark yellow, concentrated urine will be produced

The role of ADH in regulating water content of the blood

• Water reabsorption occurs along the nephron tubules in the kidneys
• The control of water reabsorption by the tubules is an example of negative feedback
• If the water content of the blood is too high then less water is reabsorbed, if it is too low then more water is reabsorbed
  • This is controlled by the hormone ADH

• The pituitary gland in the brain constantly releases ADH
• How much ADH is released depends on how much water the kidneys need to reabsorb from the filtrate
• ADH affects the permeability of the tubules to water
• If the water content of the blood is too high:
  • The pituitary gland releases less ADH which leads to less water being reabsorbed in the tubules of the kidney (the tubules become less permeable to water)
  • As a result, the kidneys produce a large volume of dilute urine

• If the water content of the blood is too low:
  • The pituitary gland releases more ADH which leads to more water being reabsorbed in the tubules of the kidney (the tubules become more permeable to water)
  • As a result, the kidneys produce a small volume of concentrated urine

The kidney controls water levels using the hormone ADH. It is an example of a negative feedback mechanism.

Remember the negative feedback cycle covered in 7.2.1: the brain detects if the level is too high or low and modulates how much ADH the pituitary gland releases