Renal Dialysis

Compare requires similarities and differences on shared criteria, and the question sets a minimum of two each. The comparison must be expressed as a comparison — connected with "whereas" or "in contrast" — rather than as two separate accounts.

6 marks. Explain briefly how dialysis works, then give two similarities and at least two differences, each stated for both sides in the same sentence.

In haemodialysis, blood is passed through tubes made of a selectively permeable membrane, bathed in a fluid called dialysate. The dialysate is made to match the concentrations of glucose and useful salts found in healthy blood, so there is no concentration gradient for those substances and they are not lost, but it contains no urea, so urea diffuses out of the blood down a steep gradient and is carried away. Excess water and salts also diffuse out, and the cleaned blood is returned to the patient.

Both the kidney and dialysis remove urea and excess water and salts from the blood while retaining what the body needs. In both cases blood cells and large proteins are too big to cross the barrier and stay in the blood, and in both cases glucose is retained — by reabsorption in the kidney and by matching the dialysate concentration in the machine.

Both work by passing blood across a selectively permeable surface, so that substances move according to size and concentration. The nephron uses the capillary wall and tubule membranes; the machine uses an artificial membrane.

The kidney works continuously inside the body, so blood composition is corrected minute by minute, whereas dialysis is intermittent, typically around four hours, three times a week, and outside the body, so wastes accumulate between sessions. The kidney also uses active transport to reabsorb substances precisely, adjusting exactly how much water and salt is reclaimed according to the body's current needs, whereas dialysis relies on diffusion alone into a fixed dialysate and cannot fine-tune in the same way.

A further difference is that a healthy kidney is also an endocrine organ, producing hormones that regulate blood pressure and stimulate red blood cell production, and dialysis does not replace this function at all. Taken together, dialysis reproduces the kidney's waste-removal outcome well enough to sustain life, but it does not replicate the kidney's continuous, precisely regulated control of blood composition or its hormonal role, which is why a transplant, being a real kidney with all its functions, is a substantially better outcome than long-term dialysis.

Evaluate asks for a judgement supported by criteria. Judging sources means applying consistent tests — who wrote it and why, how current it is, and whether independent sources agree — rather than ranking them by how authoritative they sound.

5 marks. State the criteria, judge each of the three sources against them, then explain what cross-checking adds. Every source needs a judgement, including the good one.

Three questions can be asked of any source. Who wrote it, and for what purpose — does the author have expertise, and do they have an interest in the conclusion? How current is it, given that medical technology changes? And does it agree with independent sources?

The peer-reviewed review article is the most reliable of the three. It has been assessed by independent experts before publication, which checks its claims against the evidence, and a review article in particular synthesises many primary studies rather than resting on one. Its limitations are that it may be technical enough to be misread, and that a review can date quickly in a field where treatment changes, so its publication year should be checked.

The national kidney foundation webpage is generally reliable: it is published by a recognised non-commercial body with relevant expertise and no financial interest in promoting dialysis over transplantation. Its limitation is one of purpose rather than honesty — it is written to inform patients and the public, so it may simplify mechanisms, omit qualifications, and lack the detail needed for a biological comparison.

The manufacturer's leaflet is the least reliable, because the publisher has a commercial interest in the product being evaluated. It may emphasise what dialysis achieves and understate its limitations — particularly that it does not replace the kidney's hormonal function or its continuous regulation — since those points do not favour the product. It is not worthless, as it may carry accurate technical detail about the equipment, but any evaluative claim in it should be treated as potentially biased and confirmed elsewhere.

A conclusion drawn from all three is stronger because the sources are independent of one another and have different interests, so where they agree it is unlikely that any single bias explains the agreement. If the foundation, the review article and even the manufacturer all report that dialysis removes urea effectively, that claim is well supported; where they diverge — for example on how completely dialysis substitutes for a kidney — the divergence itself is informative and points to where the manufacturer's interest lies. Cross-checking also exposes out-of-date claims, since an older source disagreeing with two newer ones is a signal to check publication dates.