Excretion

Each kidney has so much spare filtration capacity that a person can live a normal life with only one working kidney. But if both kidneys fail (a condition called renal failure), urea, water and ions accumulate in the blood within days, and the patient will die without treatment. There are two main treatments: dialysis and kidney transplant.

Dialysis (an artificial kidney)

Dialysis is a machine-based treatment that artificially performs the kidney's filtration job. The most common type is haemodialysis:

1. Blood from one of the patient's arteries flows out of the body and into the dialysis machine.
2. Inside the machine, the blood passes through a long network of partially permeable tubes (the artificial filter).
3. Outside these tubes flows a dialysis fluid designed to match the normal levels of glucose, mineral ions and water that should be in healthy blood. The fluid contains no urea.
4. Urea, excess ions and excess water diffuse out of the blood through the partially permeable membrane into the dialysis fluid (down a concentration gradient).
5. Useful substances such as glucose and the right amount of ions stay in the blood, because the dialysis fluid already has the same concentration of these (no gradient → no net diffusion).
6. The cleaned blood returns to a vein, back into the body.

A typical haemodialysis session lasts about 4 hours and is done three times a week at a hospital or clinic. Patients have to:

• Watch their diet closely (especially salt and protein intake, since the machine is not as efficient as a real kidney at controlling these)
• Limit how much fluid they drink between sessions
• Accept the time commitment, the restrictions on travel, and the long-term risks of infection at the access point

Dialysis keeps people alive, sometimes for many years, but it is not a cure. Without it, kidney failure is fatal within days.

Kidney transplant

A kidney transplant replaces the failed kidney with a healthy one from a donor. The donor can be:

• A close living relative (who can donate one kidney and survive normally with the other)
• A recently deceased person whose family has consented to organ donation

The surgery places the donor kidney into the lower abdomen, connects its blood vessels to the patient's arteries and veins, and connects its ureter to the patient's bladder. If everything works, the patient's body fluid balance returns to normal within days.

Advantages of a transplant over dialysis:

• A working transplanted kidney is far more efficient than a dialysis machine
• The patient does not need to attend the clinic three times a week
• Diet and fluid restrictions can usually be relaxed
• The cost per year is much lower in the long run

Disadvantages and challenges of a transplant:

• The donor kidney is recognised by the recipient's immune system as foreign because its antigens do not match. The recipient's lymphocytes can attack the transplant, a process called rejection.
• To reduce the chance of rejection, the donor must be a tissue match (the donor's antigens must be as close to the recipient's as possible). Close relatives are usually the best matches.
• The recipient must take immunosuppressant drugs for life to dampen the immune response. This makes them more vulnerable to infections and some cancers.
• There is a shortage of donor kidneys, and the waiting list is often long. Many patients spend years on dialysis before a suitable kidney becomes available.