Excretion
Structures and Functions in Living Organisms
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: 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:
- Blood from one of the patient's arteries flows out of the body and into the dialysis machine.
- Inside the machine, the blood passes through a long network of partially permeable tubes (the artificial filter).
- 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, excess ions and excess water diffuse out of the blood through the partially permeable membrane into the dialysis fluid (down a concentration gradient).
- 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).
- 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
Common exam question
Explaining how dialysis removes urea but keeps glucose
Question: Explain how the composition of the dialysis fluid removes urea but keeps glucose and ions in the blood, or how the dialyser is designed to speed up the removal of urea (2–4 marks).
Asked in 2 of the 23 papers. Urea leaves because the fluid contains no urea (a lower concentration of urea, excess salts and other waste), so there is a concentration gradient and urea diffuses out of the blood across the partially permeable membrane. Glucose and ions stay because the fluid holds them at the same concentration as plasma: no gradient, no net loss, and the body needs glucose for respiration and water and ions for water balance.
The design question marks in pairs, a feature with its reason, so give both: fluid kept moving to maintain the gradient; long, coiled tubing for a large surface area; a thin membrane for a short diffusion distance; fluid kept warm so particles move faster.
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 :
- 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.
Exam tip
Transplant questions: tissue match and credited advantages
Asked why a kidney from a close relative is less likely to be rejected (2 marks), give two of: the relative is genetically similar; so the kidney has the same or similar antigens, tissue type or blood type; so there is less immune response and fewer antibodies. Asked for advantages of a transplant over dialysis, credited points are that it is permanent, allows a normal life and diet with no repeated hospital visits, and that the kidney does its other functions. Repeating a reason the passage already gave earns nothing, and "cheaper in the long run" is ignored.
| Dialysis | Kidney transplant | |
|---|---|---|
| What it does | Filters the blood with an external machine | Replaces the failed kidney with a working donor kidney |
| How often | Three sessions of around 4 hours per week, for life | One surgery; the new kidney then works continuously |
| Lifestyle restrictions | Strict diet and fluid limits, frequent hospital visits | Relatively normal life; daily immunosuppressant tablets |
| Donor needed? | No | Yes (living relative or deceased donor) |
| Risk of rejection | None (no donor tissue involved) | Yes; must be tissue-matched and take immunosuppressants |
| Long-term cost | High (machine time, staff time, transport) | Lower in the long run, once surgery is paid for |
| Quality of life | Substantially reduced | Close to normal if the kidney is not rejected |