A patient's kidneys have stopped functioning. Doctors decide to treat the patient using a dialysis machine. Describe how a dialysis machine works to remove waste products and maintain the correct composition of the blood.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. Blood is drawn from the patient and passed through the dialysis machine via tubing.
2. Inside the machine, the blood is separated from the dialysis fluid by a semi‑permeable membrane.
3. Waste products such as urea diffuse from the blood into the dialysis fluid along a concentration gradient.
4. The dialysis fluid is formulated to contain the normal concentrations of useful substances (e.g. glucose, mineral ions) so these are not lost from the blood.
5. The cleaned blood is then returned to the patient’s circulation.
2. Inside the machine, the blood is separated from the dialysis fluid by a semi‑permeable membrane.
3. Waste products such as urea diffuse from the blood into the dialysis fluid along a concentration gradient.
4. The dialysis fluid is formulated to contain the normal concentrations of useful substances (e.g. glucose, mineral ions) so these are not lost from the blood.
5. The cleaned blood is then returned to the patient’s circulation.
Examiner tips
- Use the exact sequence of steps – 5 points. Include the membrane and concentration gradient. Mention the composition of the dialysis fluid. Show the return of blood to the body.
Common mistakes
- Omitting the semi‑permeable membrane. Failing to state that the dialysis fluid contains normal concentrations of useful substances. Not mentioning that the cleaned blood is returned to the patient.
Mark scheme (5 marks)
- Blood is passed from the patient through the dialysis machine / tubing
- Blood is separated from the dialysis fluid by a partially permeable membrane
- Urea / waste products diffuse from the blood into the dialysis fluid down a concentration gradient
- The dialysis fluid contains the correct / normal concentration of useful substances (e.g. glucose, mineral ions) so these are not lost from the blood
- Cleaned blood is returned to the patient's body
Key terms in this question
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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