Kidney failure and dialysis: compare a nephron with a dialysis machine as kidney function drops
Dialysis copies the kidney's waste removal, not the rest of its job
The kidney does not "clean" blood — it does membrane exchange, solute by solute, across a million tiny filters. When kidney function drops, a dialysis machine takes over part of that exchange. What they share is a selectively permeable membrane that lets small solutes through. A dialysis machine moves waste by diffusion (and pulls off water by ultrafiltration); a nephron filters blood under pressure, then reabsorbs what the body needs and secretes extra waste, and the kidney also makes hormones. Dialysis only starts once kidney function falls below about 15%. Lower kidney function and compare where the two match and where they differ.
Nephron vs dialysis
Normal range
Left: a nephron filters under pressure, reabsorbs and secretes. Right: waste only diffuses out across the dialyser membrane.
Kidneys coping · no dialysis
0%15%50%100%
Kidney function
100%
Haemodialysis a week
None
Haemodialysis routes blood out of the body through a machine containing thousands of hollow fibres bathed in dialysate fluid. Blood flows inside the fibres, dialysate flows outside in the opposite direction. Waste molecules like urea and creatinine diffuse from blood to dialysate across the semi-permeable fibre walls, following their concentration gradient. The machine also uses pressure to pull excess water from the blood (ultrafiltration). It cannot filter under blood pressure the way a glomerulus does, reabsorb what the body needs, secrete extra waste or make hormones. In a dialysis centre a session usually runs four to five hours, three times a week, and patients must limit potassium, phosphate and fluid because the machine cannot regulate these as precisely as a living kidney. The technology manages kidney failure but does not cure it: the moment dialysis stops, waste begins accumulating again.
Know This
A nephron filters under pressure, then reabsorbs and secretes; dialysis only lets waste diffuse across a membrane, cannot reabsorb selectively or make hormones, and needs about 12 to 15 hours a week to do what healthy kidneys do all the time, which is why it manages but never replaces kidney function.