🩸 Kidney Water Balance, Dialysis & Transplants (Triple Science)

🔄 ADH & Water Regulation

  • Problem:

    • Exercise / sweating → water lost → blood becomes too concentrated.

  • Response:

    • Pituitary gland releases ADH.

    • ADH → makes kidney tubules more permeable to water.

    • More water reabsorbed back into blood.

    • Less urine produced.

    • Blood water level returns to normal.

  • If blood too dilute (e.g. after drinking lots of water):

    • Pituitary stops releasing ADH.

    • Kidneys reabsorb less water.

    • More urine produced → water level in blood decreases to normal.

  • This is a negative feedback cycle (ADH secretion changes depending on blood water levels).

⚙ Kidney Dialysis

  • Used when kidneys fail (cannot filter blood properly).

  • Patient’s blood passes through semipermeable membrane:

    • Allows urea, ions, water to pass through.

    • Prevents proteins + blood cells escaping.

  • Dialysis fluid:

    • Normal concentrations of water & ions.

    • No urea.

  • Urea diffuses from blood → dialysis fluid (concentration gradient).

  • Some ions and water also diffuse to restore normal balance.

  • Fluid is refreshed to maintain concentration gradients.

🏥 Kidney Transplants

  • Diseased kidney replaced by a healthy donor kidney.

  • Risk: donor kidney may be rejected by immune system.

  • Patients must take immunosuppressant (anti-rejection) drugs for life.

⚖ Comparison – Dialysis vs. Transplant

Dialysis
✔ No shortage of machines.
✘ Inconvenient (hospital visits several times/week).
✘ Strict controlled diet.
✘ Expensive long-term.
✘ Time-consuming, ongoing treatment.

Transplant
✔ Patients can live a normal life.
✔ Only expensive initially.
✘ Shortage of donors.
✘ Surgery risk.
✘ Patients must take immunosuppressants for life.

✅ Exam tips:

  • Be able to explain ADH action (more permeable tubules → more reabsorption → less urine).

  • For dialysis, always mention: semipermeable membrane, urea diffuses out, dialysis fluid refreshed.

  • For evaluation questions: always give at least one advantage + one disadvantage of dialysis vs. transplant.