Revision Notes

13.1 The Excretory System

Excretory Products

  • 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


Organs involved in excretion, IGCSE & GCSE Chemistry revision notesOrgans involved in excretion


The Excretory System table, IGCSE & GCSE Biology revision notes

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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
Extended Only

The Role of the Liver

  • Many digested food molecules absorbed into the blood in the small intestine are carried to the liver for assimilation (when food molecules are converted to other molecules that the body needs)
  • These include amino acids, which are used to build proteins such as fibrinogen, a protein found in blood plasma that is important in blood clotting
  • Excess amino acids absorbed in the blood that are not needed to make proteins cannot be stored, so they are broken down in a process called deamination
  • Enzymes in the liver split up the amino acid molecules
  • The part of the molecule which contains carbon is turned into glycogen and stored
  • The other part, which contains nitrogen, is turned into ammonia, which is highly toxic, and so is immediately converted into urea, which is less toxic
  • The urea dissolves in the blood and is taken to the kidney to be excreted
  • A small amount is also excreted in sweat


Amino acid groups, IGCSE & GCSE Chemistry revision notesAmino acid groups


  • In deamination, the nitrogen-containing amino group is removed and converted into ammonia and then urea to be excreted

Exam Tip

Excretion and egestion are two terms that often get confused:

Excretion is the removal from the body of waste products of metabolic reactions, toxic substances and substances in excess of requirements.

Egestion is the expulsion of undigested food waste from the anus.

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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