Excretion in Humans (Edexcel IGCSE Biology: Double Science)

• Many of the necessary metabolic reactions that take place within the cells of organisms produce waste products
• Excretion is the removal of the waste substances of metabolic reactions, toxic materials and substances in excess of requirements
• Metabolic wastes produced by the human body include:
  • Carbon dioxide and water from aerobic respiration in cells
  • Urea produced by the breakdown of excess proteins (amino acids) in the liver
  • Other substances in excess such as medical drugs (e.g. antibiotics) and dietary minerals (e.g. sodium)

The danger of waste products

• If waste products are allowed to build up they can have a range of negative effects on the body:
  • Toxicity - waste products can have toxic effects if they are allowed to reach high concentrations
    •  Carbon dioxide 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

  • Osmotic effect - body fluids can become more concentrated due to higher amounts of waste products
    • Concentrated body fluids can cause water to move out of cells, changing their water potential and preventing them from carrying out essential reactions

  • Using up necessary storage - space within an organism is limited and is required for the storage of more useful molecules

• Humans have organs that are specialised for the removal of certain excretory products
• Excretory organs of the human body include:
  • The kidneys for the excretion of urea, water and excess salts
    • The kidneys excrete these substances in the urine
    • Note that the urea comes from the breakdown of excess amino acids in the liver
  • The lungs for the excretion of carbon dioxide and water
    • During exhalation carbon dioxide leaves the lungs, as well as water in the form of vapour
  • The skin for the excretion of excess mineral ions (e.g. sodium) and water, as well as some urea
    • Excretion at the skin surface occurs via sweating

Excretory organs of the human body