5.6 - Kidneys - Excretion

Excretion and the excretory organs

• Excretion is the removal of the waste products made by the body's chemical reactions, and of substances in excess of the body's needs.
• As cells carry out their metabolism, they make substances the body can't use, which would be harmful if they built up - so they must be removed.
• The main waste products are:
  • Carbon dioxide - Made by respiration
  • Urea - Made when the body breaks down excess amino acids

Organs involved in excretion

• Lungs - Remove carbon dioxide, which leaves the body when you breathe out
• Skin - Loses some water, ions and urea in sweat, but the body doesn't control how much is lost this way
• Kidneys - The main organs of excretion: they remove urea, excess water and excess ions from the blood and get rid of them in urine

The amount of water and ions the kidneys remove is carefully adjusted, so the kidneys both excrete waste and help keep the body's internal conditions steady.

The urinary system

• Blood is carried to each kidney by a renal artery and returned by a renal vein.
• Inside, each kidney has an outer layer (the cortex) and an inner region (the medulla), with a space (the pelvis) where urine collects.
• Urine made by each kidney drains down a tube called the ureter to the bladder, where it is stored.
• Urine leaves the body through the urethra.