Excretion

Excretion is the removal of the toxic waste products of metabolism, and substances in excess of the body's needs, from the body

Living cells produce waste all the time, simply by being alive. Respiration produces carbon dioxide and water. Breaking down excess amino acids produces urea. Lots of other metabolic reactions produce small amounts of other unwanted molecules. If these waste products are not removed they build up and start to cause problems.

Excretion is not the same as egestion

A point that gets tested in exams almost every year:

• Excretion removes the chemical waste made inside the cells: carbon dioxide, urea, excess water and excess mineral ions. These are products of metabolism.
• Egestion removes undigested food as faeces from the gut. Faeces have never entered any cell. They are not metabolic waste.

So faeces are not an example of excretion, even though they leave the body.

Why excretion matters

If metabolic waste built up in the body, several things would go wrong:

• Toxic effects. Carbon dioxide dissolves in water to form a weak acid that lowers cellular pH. A drop in pH denatures enzymes and shuts down metabolism.
• Osmotic effects. A build-up of dissolved waste raises the solute concentration of the blood and tissue fluid. This pulls water out of cells by osmosis, leaving cells dehydrated and unable to function.
• Wasted storage space. Cells have a limited internal volume. Letting waste accumulate steals room that could be used for storing useful molecules.