13.1 - 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.

Deamination and the formation of urea

• Proteins in food are broken down during digestion into amino acids.
• These amino acids are used to build the body's own proteins, such as enzymes - taking up and using them like this is called assimilation, and the liver does much of it.
• The body cannot store amino acids, so any that are surplus to needs have to be got rid of.

How excess amino acids are removed

• Excess amino acids are dealt with in the liver by deamination.
• Deamination is the removal of the nitrogen-containing part of an amino acid, which produces ammonia.
• Ammonia is very toxic, so the liver immediately converts it into a less harmful waste product called urea. This step is called detoxification.
• The urea passes into the blood and is carried to the kidneys, which filter it out and remove it from the body in urine.
• Getting rid of urea matters because, if it were allowed to build up, it would be toxic and harm the body's cells.