AQA · GCSE · Biology

Course version: 8461 · Version from Sept 2018

People who suffer from kidney failure may be treated by organ transplant…

Q: What mistakes do students make with People who suffer from kidney failure may be treated by…?

Thinking that dialysis fluid contains urea. It contains no urea so that urea continues to diffuse out of the blood. Believing that dialysis fluid is pure water. It contains glucose and mineral ions at normal blood concentrations to prevent their loss. Assuming a transplant always works immediately and permanently. Rejection is a risk and immunosuppressant drugs are usually needed.

Full specification wording

Maintaining water and nitrogen balance in the body (biology only): People who suffer from kidney failure may be treated by organ transplant or by using kidney dialysis. Students should know the basic principles of dialysis.

When kidneys fail, waste products such as urea and excess water and ions are not removed effectively.

Checked against the AQA 8461 specification (opens in a new tab) on 21 September 2026.

Point 17 of 17
2 key terms

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

Dialysis uses a partially permeable membrane to separate blood from dialysis fluid.
Dialysis fluid contains glucose and mineral ions at normal blood concentrations but no urea, so useful substances are not lost.
Urea diffuses from the blood into the dialysis fluid down its concentration gradient.
Excess water and ions are also removed from the blood during dialysis.
A kidney transplant can restore normal kidney function but the immune system may reject the donor organ.
Patients with a transplant usually need immunosuppressant drugs, whereas dialysis patients do not.

Common mistakes and how to put them right

Thinking that dialysis fluid contains urea.
Put it right: It contains no urea so that urea continues to diffuse out of the blood.
Believing that dialysis fluid is pure water.
Put it right: It contains glucose and mineral ions at normal blood concentrations to prevent their loss.
Assuming a transplant always works immediately and permanently.
Put it right: Rejection is a risk and immunosuppressant drugs are usually needed.

Exam tips

Compare dialysis and transplant by referring to lifestyle, risk and availability of donors.
Explain diffusion in dialysis using the concentration gradient of urea between blood and dialysis fluid.
State clearly that the membrane is partially permeable, not fully permeable.

Try the lesson

The start of the Maintaining water and nitrogen balance in the body (biology only): People who suffer from kidney failure may be treated by organ transplant or by using kidney dialysis. Students should know the basic principles of dialysis. lesson, made from checked teaching notes for this specification point.

Have a go first

Which treatment for kidney failure uses a machine to filter the blood outside the body?

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Listen

When kidneys fail, waste products such as urea and excess water and ions are not removed effectively. Dialysis is a machine treatment that filters the blood outside the body. Blood flows through tubing made of a partially permeable membrane surrounded by dialysis fluid. The fluid contains glucose and mineral ions at the same concentration as healthy blood, but no urea. This means useful substances do not diffuse out, while urea diffuses from the blood into the fluid down its concentration gradient. Excess water and ions also move out. The cleaned blood returns to the body. A kidney transplant replaces the failed kidney with a donor organ, which can restore normal kidney function but carries risks of rejection and requires immunosuppressant drugs. A strong answer earns marks for these points. Dialysis uses a partially permeable membrane to separate blood from dialysis fluid. Dialysis fluid contains glucose and mineral ions at normal blood concentrations but no urea, so useful substances are not lost. Urea diffuses from the blood into the dialysis fluid down its concentration gradient. Excess water and ions are also removed from the blood during dialysis. Exam tip: Compare dialysis and transplant by referring to lifestyle, risk and availability of donors.

Key words

Say what each word means before you turn the card.

Dialysis

Try answering aloud or in your head before turning the card.

Kidney transplant

Try answering aloud or in your head before turning the card.

See every definition

Dialysis: A machine treatment that filters the blood outside the body when the kidneys fail.
Kidney transplant: Replacement of the failed kidney with a donor organ, which can restore normal kidney function but carries risks of rejection.

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