ADH & Composition of Urine (Edexcel IGCSE Biology)

• Water reabsorption occurs along the nephron tubules in the kidneys
  • Water reabsorption is important for osmoregulation
• 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
• Any change to the water level of the blood is detected by the hypothalamus, which then sends a signal to the pituitary gland
• The pituitary gland in the brain constantly releases a hormone called 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 collecting ducts of the kidney by osmosis (the collecting ducts 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 collecting ducts of the kidney by osmosis (the collecting ducts become more permeable to water)
  • As a result, the kidneys produce a small volume of concentrated urine

The negative feedback loop of ADH controlling the level of water in the blood

• 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