ADH & Composition of Urine (Edexcel IGCSE Biology (Modular))

The Role of ADH

• 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

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