What you'll learn
The kidneys keep the blood clean and balanced, so if they fail it becomes life-threatening — and the two main treatments are dialysis and kidney transplants. For AQA GCSE Biology you need to understand what the kidneys do, what happens in kidney failure, how dialysis works, how transplants work, and how the two treatments compare. This guide covers the role of the kidneys, kidney failure, dialysis, transplants, and the advantages and disadvantages of each. By the end you should be able to explain how dialysis and transplants treat kidney failure and evaluate the two options.
Key terms and definitions
Kidney — An organ that filters the blood, removing urea and adjusting water and ion levels.
Urea — A toxic waste product from the breakdown of excess amino acids, removed by the kidneys.
Kidney failure — When the kidneys stop working properly and cannot clean the blood.
Dialysis — A treatment that uses a machine to filter the blood artificially.
Dialysis fluid — The fluid used in dialysis, with controlled concentrations of substances.
Transplant — Replacing a failed organ with a healthy donor organ.
Rejection — When the immune system attacks a transplanted organ as foreign.
Immunosuppressant — A drug that reduces the activity of the immune system.
Core concepts
What the kidneys do
The kidneys clean the blood. They filter it, removing the toxic waste urea (from the breakdown of excess amino acids), and they adjust the water and ion levels of the blood, keeping the internal environment stable. Useful substances such as glucose are reabsorbed, while urea and excess water and ions are removed in the urine. This role is vital, because a build-up of urea and imbalances of water and ions would be harmful.
Kidney failure
In kidney failure, the kidneys stop working properly, so they can no longer clean the blood. This causes toxic urea to build up, and the water and ion balance of the blood is disturbed. If untreated, this is fatal, because the body is poisoned by its own waste. The two main treatments are dialysis and a kidney transplant.
Dialysis
Dialysis uses a machine to filter the blood artificially, doing the job the kidneys can no longer do. In a dialysis machine:
- The patient's blood flows on one side of a partially permeable membrane.
- Dialysis fluid flows on the other side.
- The dialysis fluid contains a normal blood concentration of glucose and ions, but no urea.
Because of these concentration differences, urea diffuses out of the blood into the dialysis fluid (as there is none in the fluid), and excess ions also diffuse out. But glucose and the ions the body needs do not leave the blood, because their concentrations are the same on both sides, so there is no net movement. This cleans the blood while keeping the useful substances at the right level.
Dialysis has to be done regularly — typically several times a week, for several hours each time — because urea builds up between sessions.
Kidney transplants
A kidney transplant replaces a failed kidney with a healthy one from a donor. A single healthy kidney is enough to clean the blood, so only one is needed. A transplant is a long-term solution that frees the patient from regular dialysis and allows a more normal life.
However, there is a risk of rejection: the patient's immune system may recognise the donor kidney as foreign and attack it. To reduce this risk:
- The donor kidney is chosen to have a tissue type as similar as possible to the patient's.
- The patient takes immunosuppressant drugs to reduce the activity of their immune system so it does not reject the kidney.
The drawback is that immunosuppressants make the patient more vulnerable to infections.
Comparing dialysis and transplants
The two treatments have different advantages and disadvantages:
Dialysis:
- Available to anyone (no need for a donor), but must be done regularly and is time-consuming.
- The patient must control their diet between sessions, and urea builds up in between.
- It is expensive over time and can affect quality of life.
Transplant:
- Allows a more normal life and is cheaper in the long term.
- But donor kidneys are in short supply, there is a risk of rejection, and the patient must take immunosuppressants (with side effects) and undergo surgery.
Because there are strong points on both sides, this is often set as an "evaluate" question.
How the healthy kidney works (context)
To understand the treatments, it helps to recall how a healthy kidney cleans the blood. Blood is filtered under high pressure, so small molecules — water, glucose, ions and urea — are forced out of the blood, while large molecules such as proteins and blood cells stay behind. Then selective reabsorption takes back the useful substances: all the glucose, and the water and ions the body needs, are reabsorbed into the blood, while urea and any excess water and ions are left to leave as urine. Dialysis is designed to imitate this: it removes urea and excess ions while keeping glucose and needed substances in the blood, which is why the dialysis fluid is made up so carefully.
Why the water balance is controlled too
Kidney treatment must manage water balance as well as removing urea. In healthy people, the kidneys adjust how much water is lost in urine to keep the blood at the right concentration. When the kidneys fail, water can build up in the body, which is dangerous. Dialysis therefore also removes excess water from the blood, and patients on dialysis often have to limit how much fluid they drink between sessions. A transplant restores normal water control because the new kidney works like a healthy one. Mentioning water balance, not just urea, shows a fuller understanding of what the kidneys — and their treatments — actually do.
Worked examples
Example 1: Why urea leaves the blood in dialysis
Explain why urea moves from the blood into the dialysis fluid. The dialysis fluid contains no urea, while the blood has a high concentration of urea. So urea diffuses from the blood (high concentration) into the fluid (low concentration) across the partially permeable membrane, removing it from the blood.
Example 2: Why glucose is not lost
Explain why glucose does not leave the blood during dialysis. The dialysis fluid contains a normal blood concentration of glucose, so there is no concentration difference between the blood and the fluid. Therefore there is no net movement of glucose, and it stays in the blood.
Example 3: Reducing rejection
Explain two ways the risk of a transplanted kidney being rejected is reduced. The donor kidney is chosen to have a tissue type as similar as possible to the patient's, so the immune system is less likely to see it as foreign. The patient also takes immunosuppressant drugs, which reduce the immune system's activity so it does not attack the kidney.
Example 4: Evaluating the treatments
Give one advantage of a transplant over dialysis, and one disadvantage. An advantage is that a transplant allows a more normal life without regular dialysis sessions. A disadvantage is that donor kidneys are in short supply and there is a risk of rejection, requiring immunosuppressant drugs.
Common mistakes and how to avoid them
A common error is saying dialysis fluid contains no glucose or ions. It contains a normal concentration of glucose and useful ions (but no urea), so that these useful substances are not lost from the blood while urea is removed.
Students often forget why useful substances stay in the blood. It is because their concentrations are the same on both sides of the membrane, so there is no net diffusion. Only substances with a concentration difference (urea, excess ions) move out.
Another mistake is thinking a patient needs two donor kidneys. A single healthy kidney is enough to clean the blood, so only one is transplanted.
When explaining rejection, remember it is the immune system attacking the kidney as foreign, reduced by tissue matching and immunosuppressant drugs — which in turn increase the risk of infection.
Finally, for evaluation questions, give both treatments' advantages and disadvantages. Dialysis is available to all but is regular and restrictive; transplants allow a normal life but depend on scarce donors and carry rejection risk.
Exam technique for "Kidney failure and treatment"
Be ready to explain how dialysis works in terms of diffusion across a partially permeable membrane, with dialysis fluid containing normal glucose and ion concentrations but no urea — so urea and excess ions leave but useful substances do not.
For transplants, explain the risk of rejection and the two ways it is reduced (tissue matching and immunosuppressants), and the drawback of increased infection risk.
Most questions are "explain" or "evaluate", so compare dialysis and transplants with balanced advantages and disadvantages, and reach a judgement where asked. Use precise terms — urea, dialysis, partially permeable membrane, rejection, immunosuppressant — throughout.
Quick revision summary
- The kidneys remove toxic urea and adjust water and ion balance; kidney failure lets urea build up and disturbs the blood, and is fatal if untreated.
- Dialysis filters the blood using a machine: blood and dialysis fluid are separated by a partially permeable membrane.
- Dialysis fluid has a normal concentration of glucose and ions but no urea, so urea and excess ions diffuse out while useful substances stay in.
- Dialysis must be done regularly and is time-consuming and restrictive.
- A transplant replaces the kidney with a donor one (only one needed); risk of rejection is reduced by tissue matching and immunosuppressant drugs (which increase infection risk).
- Evaluate the two: dialysis is available to all but restrictive; transplants allow a normal life but depend on scarce donors and carry rejection risk.