🩸 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.