Organs involved in excretion:
- Liver
- Kidneys
- Lungs
- Skin

Functions of liver:
- Liver creates proteins from amino acids through a process called assimilation.
  - Assimilation of amino acids is a process where amino acids in the bloodstream are taken up by the liver and linked together to form proteins.
  - Fibrinogen is one of the important proteins that the liver produces. It is responsible for blood clotting.
- Liver also breaks down the components of dead red blood cells into bilirubin.
  - Bilirubin is released as part of bile and eventually further broken down in the intestine into a component that gives faeces its brown colour.
- Likewise, the liver breaks down excess amino acids to produce energy through a process called deamination.
  - Ammonia is one of the major waste products produced during deamination.
  - It is toxic.
  - The liver converts ammonia into urea.
  - Urea is not as toxic as ammonia.
  - It is excreted by our kidneys as part of urine.

Role of skin in excretion:
- Skin regulates the body temperature by sweating.
- Sweat contains salt and water along with a trace amount of urea.

Role of lungs in excretion:
- Lungs exhale water vapour and carbon dioxide.
- Carbon dioxide is a by-product of respiration and can be harmful to our body in large amounts.
  - Excess carbon dioxide can dissolve in water and form carbonic acid.
  - Carbonic acid is a relatively weak acid.
  - However, being an acid, it can make our blood more acidic and, in turn, affect processes such as enzymatic reactions.
  - This will affect the normal functioning of our body.

13.01 - Liver and Lungs

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