Unlike plants, humans have organs which are specialised for the removal of certain excretory products
They include the lungs and kidneys
The liver also has a vital role in excretion
The Need for Excretion
Excretion is the removal of the waste substances of metabolic reactions (the chemical reactions that take place inside cells), toxic materials and substances in excess of requirements
Carbon dioxide must be excreted as it dissolves in water easily to form an acidic solution which can lower the pH of cells
This can reduce the activity of enzymes in the body which are essential for controlling the rate of metabolic reactions
For this reason, too much carbon dioxide in the body is toxic
Urea is also toxic to the body in higher concentrations and so must be excreted