Kidney Loss, Dialysis and Transplantation

Explain asks for the mechanism twice over: what the kidney does that is lost, and how the machine substitutes for it. The second cannot be explained without the first.

Plan: name the principle, state what the kidney maintains, then explain how dialysis reproduces it and where it falls short.

The kidney maintains the composition of the blood by filtering it and then selectively returning what the body needs. Failure therefore does not disrupt one variable but several at once, because a single organ regulates water, ions, pH and the removal of nitrogenous waste together.

Four consequences follow. Urea and other nitrogenous wastes from protein metabolism accumulate in the blood and become toxic. Water is not excreted in proportion to intake, so fluid accumulates, raising blood pressure and causing oedema. Ion concentrations drift, and rising potassium is the most immediately dangerous because it disturbs the electrical activity of the heart. And pH is no longer corrected, so the blood becomes progressively acidic.

Dialysis substitutes a concentration gradient for the kidney's filtration. In haemodialysis blood is passed on one side of a partially permeable membrane while dialysate flows on the other. The dialysate is made with the concentrations a healthy blood plasma should have, so substances present in excess — urea, excess potassium — diffuse out down their gradients, while substances at correct levels, such as glucose and essential ions, have no gradient and are not lost. Flowing the dialysate counter-current to the blood maintains the gradient along the whole length of the exchanger.

Dialysis assists rather than replaces. It is intermittent, typically three sessions a week, whereas kidneys work continuously, so wastes and fluid accumulate between sessions and blood composition sawtooths instead of being held steady. It also performs only the excretory function: the kidney additionally secretes erythropoietin to stimulate red blood cell production and activates vitamin D for calcium absorption, so dialysis patients commonly require both to be supplied separately. And it is a continuing treatment rather than a repair — stopping it returns the patient to the failed state, which is why transplantation is the definitive option.

Evaluate is this dot point's own verb — evaluate the effectiveness of a technology that is used to manage and assist with the effects of a disorder. Note that the dot point says manage and assist, so effectiveness is judged against that purpose, not against cure.

Plan: state the criteria, judge what it achieves, judge where it falls short, then reach a verdict and qualify it.

Dialysis will be judged on whether it sustains life and how well, on how completely it reproduces kidney function, and on what it costs the patient in time, freedom and quality of life.

On the first criterion dialysis is decisively effective. Untreated kidney failure is fatal within weeks, and dialysis sustains patients for years and in some cases decades. It corrects the acute dangers — it removes urea, brings potassium down before it disturbs cardiac rhythm, removes excess fluid and corrects acidosis — and it does so reliably enough to be delivered on a routine schedule. It also functions as a bridge, keeping patients alive and well enough to receive a transplant when one becomes available.

On the second criterion it is partial. Three sessions a week cannot reproduce continuous regulation, so patients live through cycles of accumulation and clearance rather than in a steady state, and must restrict fluid, potassium and phosphate intake between sessions to stay within safe limits. The endocrine functions are absent altogether, so erythropoietin and activated vitamin D must be supplied separately or anaemia and bone disease follow. Long-term complications accumulate, including vascular access failure and cardiovascular disease.

The cost is substantial and unevenly distributed. Haemodialysis typically requires four to five hours three times weekly at a centre, which constrains employment, travel and daily life for years. Peritoneal dialysis can be done at home and overnight, giving more independence and a steadier blood composition, at the cost of daily self-management and a risk of peritonitis. Access is also geographically uneven — in remote areas patients may have to relocate to reach a centre, which is a cost the technology's clinical performance does not capture.

Dialysis is highly effective for the purpose the dot point names — managing and assisting — and it is not a replacement for kidney function. Judged against no treatment it converts a rapidly fatal condition into a chronic manageable one, which is a large effect; judged against a healthy kidney it reproduces one of several functions, intermittently. Both are true, and the second is not a criticism of the technology so much as a description of what a machine outside the body can do.

The qualification is that effectiveness should be judged against the available alternative rather than an ideal. Transplantation restores continuous function including the endocrine roles and gives better survival and quality of life, so it is the better option where a compatible organ and a suitable recipient coincide — but donor organs are scarce, so for most patients dialysis is not the inferior choice, it is the only one.