13.1 Excretion in humans

2026 Syllabus Objectives

  1. State that carbon dioxide is excreted through the lungs
  2. State that the kidneys excrete urea and excess water and ions
  3. Identify in diagrams and images the kidneys, ureters, bladder and urethra
  4. Identify in diagrams and images the structure of the kidney, limited to the cortex and medulla
  5. Outline the structure and function of a nephron and its associated blood vessels, limited to:
    • (a) the role of the glomerulus in the filtration from the blood of water, glucose, urea and ions
    • (b) the role of the nephron in the reabsorption of all of the glucose, some of the ions and most of the water back into the blood
    • (c) the formation of urine containing urea, excess water and excess ions (details of these processes are not required)
  6. Describe the role of the liver in the assimilation of amino acids by converting them to proteins
  7. State that urea is formed in the liver from excess amino acids
  8. Describe deamination as the removal of the nitrogen-containing part of amino acids to form urea
  9. Explain the importance of excretion, limited to toxicity of urea

What is Excretion? 🔑

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Excretion is the removal of the waste products of metabolism and substances in excess of requirements from the body.

All living cells carry out metabolic reactions continuously. These reactions, such as respiration, produce substances that cells may not need. Some of these substances can be toxic if allowed to accumulate, so they must be removed from the body.

Example: Carbon Dioxide as a Waste Product

Respiration provides energy but also produces water and carbon dioxide. While cells can use the energy and water, carbon dioxide is a waste product that must be excreted.

Why must carbon dioxide be removed?

  • Carbon dioxide dissolves in water to form a weak acid
  • This would lower the pH of cells and blood
  • Low pH would be toxic to cells and affect enzyme function
  • Enzymes are essential for controlling metabolic reactions

🔑 Key Point: In mammals, carbon dioxide from respiration is excreted through the lungs. This process is described in detail in Chapter 11.

Excretion in Plants

Plants also excrete waste materials:

  • During daylight hours, plants use carbon dioxide produced in respiration for photosynthesis, so it is not a waste product
  • At night, plants cannot photosynthesise but continue to respire, so carbon dioxide becomes a waste product
  • Plants excrete carbon dioxide through their stomata

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