The Kidneys

• The kidneys make urine by taking waste products out of your blood
• Filtration

  substances are filtered out of the blood as it passes through the kidneys
• Selective reabsorption

  Useful substances like glucose, some ions and the right amount of water are absorbed back into the blood
• Substances removed from the body in urine:

  • urea
  • ions
  • Water
• Urea:

  • Proteins can't be stored in the body, so any excess amino acids are are converted into fats and carbohydrates, which can be stored - this happens in the liver and is called deamination
  • ammonia is a produced as a waste product from deamination
  • ammonia is toxic so it's converted to urea in the liver
  • urea is then transported to the kidneys, where it's filtered out of the blood and excreted from the body in urine
• Deamination

  the process of turning amino acids into fats and carbohydrates to store in the body
• A small, unregulated amount of urea is also lost from the skin in sweat
• Ions:

  • ions are taken into the body by food, and then absorbed into the blood
  • if the ion (or water) content of the body is wrong, this could upset the balance between ions and water, meaning too much or too little water is absorbed by osmosis
  • having the wrong amount of water can damage cells or mean they don't work as well as normal
  • some ions are lost in sweat, however, this amount is not regulated, so the right balance of ions in the body must be maintained by the kidneys
  • the right amount of ions is reabsorbed into the blood after filtration and the rest is removed from the body in urine
• Water:

  • the body has to constantly balance the water coming in against the water going out
  • we lose water from the skin in sweat and from the lungs when breathing out
  • we can't control how much we lose in these ways, so the amount of water is balanced by the the amount we consume and the amount removed by the kidneys in urine
• The concentration of urine is controlled by a hormone called anti-diuretic hormone (ADH)
• ADH is released into the bloodstream by the pituitary gland
• The brain monitors the water content of the blood and instructs the pituitary gland to release ADH into the blood according to how much is needed
• The whole process of water content regulation is controlled by negative feedback
• Water content is too high:

  • a receptor in the brain detects that the water content is too high
  • the coordination centre in the brain receives the information and coordinates a response
  • the pituitary gland releases less ADH, so less water is reabsorbed from the kidney tubules
• Water content is too low:

  • a receptor in the brain detects that the water content is too low
  • the coordination centre in the brain receives the information and coordinates a response
  • the pituitary gland releases more ADH, so more water is reabsorbed from the kidney tubules
• If the kidneys don't work properly, waste substances build up in the blood and you lose your ability to control the levels of ions and water in your body, which eventually results in death
• People with kidney failure can be kept alive by having dialysis treatment or a kidney transplant
• Dialysis treatment

  where machines do the job of the kidneys