ADH & Composition of Urine (Edexcel IGCSE Biology)

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The Role of ADH

  • 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
  • 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

ADH and osmoregulation 1, downloadable AS & A Level Biology revision notes ADH and osmoregulation 2, downloadable AS & A Level Biology revision notes

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

Examiner Tip

You must remember the key phrase "ADH increases the permeability of the collecting duct to water" for your exams. Mark schemes will expect you to use this exact terminology. 

The Composition 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

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Lára

Author: Lára

Expertise: Biology Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.