13.2 Urinary System

Subject: Cambridge O Level Biology (5090)

2026 📋 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Identify the kidneys, ureters, bladder, and urethra on a diagram, and state the function of each
  2. Explain why excretion is necessary, focusing on why urea is harmful
  3. Outline the structure of a nephron and its blood vessels (Bowman's capsule, glomerulus, tubules, loop of Henle, collecting duct)
  4. Outline the function of a nephron and its blood vessels, including:
    • (a) Filtration in the glomerulus
    • (b) Reabsorption in the nephron
    • (c) How urine is formed
  5. Describe how the liver uses amino acids to build proteins
  6. Describe deamination — how the liver breaks down excess amino acids and produces urea

📖 Main Content

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1. What is Excretion?

Excretion is the removal of waste products that are made by the body's own chemical reactions (called metabolism). These waste products build up inside your body and must be removed before they cause harm.

🔑 Important distinction: Excretion is NOT the same as egestion (getting rid of undigested food as faeces). Excretion only refers to waste made by the body's own processes.

One of the most important waste products that must be excreted is urea.

2. Why Does the Body Need to Excrete Urea? (The Toxicity of Urea)

Your body digests proteins from food (like meat, eggs, and beans) into amino acids. Amino acids are the building blocks of protein. Your body uses many of these amino acids to build its own proteins.

However, your body cannot store amino acids. If there are more amino acids than the body needs, the liver breaks them down. This process is called deamination (explained in detail below in Section 7).

During deamination, a toxic (poisonous) substance called urea is produced.

Why is urea dangerous?

  • Urea is toxic — this means it is poisonous to cells if it builds up in the blood.
  • If urea is not removed from the body, it will accumulate (build up) in the blood and reach dangerously high levels.
  • High levels of urea in the blood can damage body cells and organs, and can eventually be fatal (cause death).

This is why the body must continuously remove urea — and this is the main job of the urinary system.

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