🧪 Kidneys & Excretion

💧 Water balance & osmosis

  • Osmosis = Movement of water particles from an area of high concentration to an area of low concentration across a partially permeable membrane.

  • If blood too dilute → water enters cells → swell (and potentially burst).

  • If blood too concentrated → water leaves cells → shrink.

  • Both can stop cells working properly.

  • Maintaining constant water levels = homeostasis.

🚰 Ways the body loses water

  1. Lungs (exhalation) – water vapour lost, not controlled.

  2. Skin (sweat) – water, ions (e.g. sodium), and urea lost, not controlled (linked to temperature control).

  3. Kidneys (urine) – controlled.

    • Excess water, ions, urea excreted.

🩸 Role of the kidneys

  • Blood enters kidney via artery, containing urea, ions, water, glucose.

  • Kidneys:

    • Filter small molecules (urea, ions, water, glucose).

    • Selective reabsorption:

      • All glucose reabsorbed.

      • Some water and ions reabsorbed (depending on body’s needs).

      • Urea not reabsorbed.

  • Blood leaves kidney via vein with no urea and balanced water/ions.

  • Waste products = excreted as urine, stored in bladder.

🔬 Higher tier – Excess amino acids

  • Proteins → digested to amino acids.

  • Often more amino acids than needed.

  • Deamination (in liver):

    • Excess amino acids → ammonia.

    • Ammonia = toxic → immediately converted to urea.

  • Urea excreted by kidneys in urine.

✅ Exam tips:

  • Learn osmosis definition word-for-word.

  • Be able to compare water loss routes (lungs/skin = uncontrolled, kidneys = controlled).

  • Higher tier: deamination = breakdown of excess amino acids, ammonia → urea.

  • Always mention selective reabsorption for glucose, water, and ions.