IGCSE Biology / Homeostasis and excretion
Lungs, kidneys and skin as excretory organs
Edexcel International GCSE Biology (4BI1), chapter 8: Homeostasis and excretion, specification point 2.71, 2.79B. Free lesson.
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Excretion removes waste products made by metabolism, as well as substances present in excess. The lungs, kidneys and skin excrete different substances, while faeces leaves the body by egestion rather than excretion.
What excretion means
Excretion is the removal of the waste products of metabolism and of substances in excess of requirements. Metabolism means the chemical reactions in cells. Excretion removes substances produced by those reactions, which is why faeces is different. The lungs, kidneys and skin are the three human excretory organs.
What each excretory organ removes
Each organ removes substances that have a different source. The lungs remove waste gas made by respiration. The kidneys remove urea made in the liver, along with excess water and salts. The skin releases some wastes in sweat, but sweating mainly helps control body temperature. Lungs: carbon dioxide and some water vapour. Carbon dioxide is made by respiration in every cell. Kidneys: urea, excess water and excess salts. Skin: some urea, water and salts in sweat. Sweating is a side effect of temperature control.
How the body makes urea
Excess amino acids cannot be stored, unlike glucose and fat. In the liver, their amino groups are removed in a process called deamination. The rest of each molecule can be converted into carbohydrate and used for energy. The amino group forms toxic ammonia, which is converted into the less toxic urea. Urea enters the blood and is carried to the kidneys for removal.
Why faeces is not excretion
Faeces is mostly undigested food, fibre and dead cells from the gut lining. The undigested food was never absorbed, so it did not take part in reactions in cells and was not made by metabolism. Removing faeces is called egestion, not excretion.