12.4 Dialysis – an Artificial Kidney

A free GCSE Biology lesson for AQA (8461) from OBA School: kidney failure, how a dialysis machine uses diffusion, and the pros and cons of dialysis. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain why kidney failure is life-threatening
• Describe how a dialysis machine removes urea and excess ions from the blood by diffusion
• Explain why the dialysis fluid contains normal levels of glucose and ions but no urea
• Evaluate dialysis as a treatment for kidney failure

Key words

• Kidney failure: When the kidneys can no longer filter the blood well enough to keep a person healthy.
• Dialysis: A treatment in which a machine removes urea, excess ions and excess water from the blood.
• Dialysis fluid: The liquid on the other side of the membrane. It has normal blood levels of glucose and ions, and no urea.
• Partially permeable membrane: A barrier that lets small molecules (water, urea, ions, glucose) through, but not large ones such as proteins.
• Diffusion: The net movement of particles from a higher to a lower concentration.
• Concentration gradient: The difference in concentration between two areas. A steeper gradient gives faster diffusion.

When the kidneys fail

Kidneys can be damaged by infection, diabetes, high blood pressure, injury or inherited conditions. If they fail:

• urea builds up in the blood and poisons cells;
• the ion and water balance is lost, so water moves into or out of cells by osmosis and damages them;
• without treatment, a person dies within days or weeks.

Key fact: The two treatments for kidney failure are dialysis and a kidney transplant (see 12.5).

How a dialysis machine works

Blood is taken from a vein (usually in the arm) and pumped through the machine. Inside, it flows past dialysis fluid, separated by a partially permeable membrane.

• Urea; Blood of patient high; Dialysis fluid none; What happens Diffuses out of the blood down its concentration gradient.
• Sodium ions; Blood of patient too high; Dialysis fluid normal level; What happens Excess diffuses out until the levels are equal.
• Glucose; Blood of patient normal; Dialysis fluid same as normal blood; What happens No net movement, so glucose is not lost.
• Proteins, blood cells; Blood of patient present; Dialysis fluid none; What happens Too large to cross the membrane, so they stay in the blood.

The fluid flows the opposite way to the blood and is constantly replaced, so a steep concentration gradient for urea is kept along the whole membrane. A drug (an anticoagulant) is added to stop the blood clotting in the machine.

Exam tip: Every substance crosses the dialysis membrane by diffusion – not active transport and not osmosis.

Living with dialysis

Most patients need about 3 sessions a week, each lasting about 4 hours, usually at a hospital. Between sessions, urea and ions build up again, so patients must eat little salt and control their protein intake (protein → amino acids → urea) and how much they drink.

Worked example: reading Figure 2

By what percentage did blood urea fall in Monday’s session?
Fall = 30 − 10 = 20 mmol/dm³. 20 ÷ 30 × 100 = 66.7%
Why is the level highest on the second Monday? The gap from Friday to Monday is the longest, so the most urea builds up.

Answer: 66.7% (1 d.p.)

Worked example: time on a machine

3 sessions × 4 hours × 52 weeks = 624 hours a year. 624 ÷ 24 = 26 whole days.

Answer: 624 hours (26 days) a year

Evaluating dialysis

• Keeps a patient alive while waiting for a transplant.; Disadvantages Takes about 12 hours a week, plus travel; hard to work, study or travel.
• Available to everyone who needs it – no donor is needed.; Disadvantages Strict diet: little salt, controlled protein and fluid.
• No risk of rejection, so no immunosuppressant drugs.; Disadvantages Expensive for the NHS, every year, for as long as it is needed.
• Each session returns blood urea and ions to near-normal.; Disadvantages Risk of infection and blood clots; levels swing between sessions; it does not cure the disease.

Exam tip: In an "evaluate" question, give points for and against, then finish with a conclusion backed by a reason.

Common mistakes

• Saying urea leaves the blood by osmosis or active transport. It is diffusion.
• Saying dialysis fluid contains no glucose or no ions. It contains normal levels, so only the excess ions diffuse out and no glucose is lost.
• Saying the membrane lets blood cells or proteins through. They are too large.
• Saying dialysis cures kidney failure. It must be repeated several times a week.

Revision questions

Try these, then check your answers below each one.

Question 1. By which process does urea pass from the blood into the dialysis fluid? (Choose from: Filtration, Diffusion, Osmosis, Active transport.)

Answer: Diffusion

Working: Urea diffuses down its concentration gradient across the partially permeable membrane.

Question 2. What does fresh dialysis fluid contain? (Choose from: Normal levels of urea, glucose and ions, High glucose and no ions, Normal blood levels of glucose and ions, and no urea, No glucose, no ions and no urea.)

Answer: Normal blood levels of glucose and ions, and no urea

Working: Normal glucose and ion levels mean no glucose is lost and only excess ions diffuse out; no urea keeps a gradient so urea diffuses out.

Question 3. True or false? Red blood cells pass through the dialysis membrane into the dialysis fluid.

Answer: False

Working: False. Blood cells and proteins are too large to cross the partially permeable membrane.

Related lessons

• Previous lesson: 12.3 The Human Kidney
• Next lesson: 12.5 Kidney Transplants
• In this chapter: 12.1 Controlling Body Temperature
• In this chapter: 12.2 Removing Waste Products

Continue in the full course

This lesson is section 12.4 (AGCBH1204) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.