The Kidneys

• The kidneys are very important in homeostasis
• Kidneys act as filters to 'clean the blood'
• The kidneys make urine by taking waste products out of the blood. Sustances are filtered out of the blood as it passes through the kidneys. This process is called filtration
• Useful substances like glucose, some ions and the right amount of water are then absorbed back into the blood after filtration. This is called selective reabsorption
• The substances removed from the body in urine include:

  Urea

  • Proteins (and the amino acids that they are broken down into) can't be stored by the body - so any excess amino acids are converted into fats and carbohydrates, which can be stored
  • Ammonia is produced as a waste product from this process
  • 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
• A small unregulated amount of urea is also lost from the skin in sweat
• The substances removed from the body in urine include:

  Ions

  • Ions such as sodium are taken into the body in 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 / little water is drawn into cells by osmosis. This could damage cells
  • 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
• The substances that are removed from the body in urine include:

  Water
• The substances that are removed from the body in urine include:

  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 form 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 amount we consume and the amount removed by the kidneys in urine
• The concentration of urine us controlled by a hormone
• The concentration of urin is controlled by a hormone called anti-diuretic hormone (ADH). This 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