5.19 Kidney failure: dialysis and transplants
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Explain how a dialysis machine removes urea from the blood without removing glucose.
Mark scheme
- blood flows past / through a partially permeable membrane (1)
- dialysis fluid contains no urea (1)
- so urea diffuses out of the blood (into the fluid) (1)
- down its concentration gradient (1)
- fluid has the same glucose concentration as (normal) blood (1)
- so there is no net movement / no concentration gradient for glucose (1)
Max 5
Model answer
Blood flows through tubes made of a partially permeable membrane, surrounded by dialysis fluid. The fluid contains no urea, so urea diffuses out of the blood into the fluid, down its concentration gradient. The fluid has the same concentration of glucose as normal blood, so there is no concentration gradient for glucose and none is lost.
Explain why the dialysis fluid flows in the opposite direction to the blood.
Mark scheme
- keeps / maintains a (steep) concentration gradient (1)
- along the whole length of the membrane / so more urea diffuses out (1)
Model answer
It keeps a steep concentration gradient along the whole length of the membrane, so urea keeps diffusing out of the blood.
Explain why a patient must take drugs for the rest of their life after a kidney transplant.
Mark scheme
- the immune system may attack the new kidney (1)
- because it has foreign / different antigens (1)
- immunosuppressant drugs reduce the risk of rejection (1)
Model answer
The new kidney has antigens that are different from the patient's, so their immune system may attack it as foreign and reject it. Immunosuppressant drugs suppress the immune system to stop this.
Evaluate the use of a kidney transplant rather than dialysis to treat kidney failure.
Mark scheme
Level 3 (5-6): advantages and disadvantages of both treatments compared, with a justified conclusion. Level 2 (3-4): several relevant points for both treatments, or detail on one, with limited comparison. Level 1 (1-2): simple points about one treatment.
Indicative content: Transplant advantages: normal life / no hours on a machine; free diet; cheaper over several years; long-term solution Transplant disadvantages: shortage of donors / waiting list; tissue matching needed; risk of rejection; immunosuppressants for life, so more infections; surgery risks; may eventually fail Dialysis advantages: available straight away; no rejection; no surgery Dialysis disadvantages: 3-4 hours, three times a week; strict diet; infection / clotting risk; expensive each year; not a cure Conclusion: e.g. a transplant is better for quality of life, but dialysis is needed while waiting
Model answer
A transplant lets the patient live a normal life without spending three sessions of 3 to 4 hours a week on a machine or following a strict diet, and over several years it costs less than dialysis. However, there are not enough donors, so there are long waiting lists; the kidney must be a close tissue match, and the patient must take immunosuppressant drugs for life, which increase the risk of infections, and the kidney may still be rejected. Dialysis is available immediately and has no risk of rejection, but it is time-consuming, needs a restricted diet, has risks of infection and clots, and is expensive every year. Overall a transplant is the better long-term treatment for most patients, but dialysis is essential to keep them alive while they wait for a matching kidney.
Give two substances that are present in the blood but cannot pass through the dialysis membrane.
Mark scheme
Any two from: proteins / blood cells / red blood cells / white blood cells / platelets (1 each)
Model answer
Proteins and red blood cells.
What is in fresh dialysis fluid?
Choose one answer.
Correct Not quite: the answer is Glucose and ions at the same concentration as normal blood, and no urea
Mark scheme
B
Model answer
B: glucose and useful ions at the same concentration as normal blood, and no urea.
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