Question
Why is dialysis needed, and how does it function as an artificial kidney? Explain the process in detail.
Solution — Step by Step
The kidneys filter about 180 litres of blood plasma per day, removing urea, creatinine, excess salts, and water while retaining useful molecules like glucose and plasma proteins. When both kidneys fail — due to chronic kidney disease, diabetes, or hypertension — this filtration stops, and toxic wastes accumulate in the blood. This condition is called renal failure, and without intervention it is fatal.
Dialysis uses a semi-permeable membrane — a membrane with pores large enough to let small molecules (urea, creatinine, K⁺, Na⁺) pass through, but too small for large proteins and blood cells. The patient’s blood flows on one side of this membrane. On the other side flows a specially prepared fluid called the dialysate.
Because the dialysate contains no urea or creatinine, these waste molecules diffuse from the blood (high concentration) into the dialysate (zero concentration) down their concentration gradient. This is simple diffusion — the same principle as osmosis, applied to dissolved solutes.
The dialysate is carefully formulated to match the normal plasma concentrations of glucose, Na⁺, Ca²⁺, and HCO₃⁻. This means these useful substances have no concentration gradient across the membrane — they do not diffuse out. Only waste molecules that are absent in the dialysate leave the blood.
This selectivity is what makes dialysis mimic a healthy kidney: it removes what should not be there while keeping what should.
Haemodialysis: Blood is drawn out of the body, pumped through a machine containing the semi-permeable membrane (called the dialyser or artificial kidney), cleaned, and returned. It typically takes 3–4 hours, three times a week.
Peritoneal dialysis: The patient’s own peritoneum (the membrane lining the abdominal cavity) acts as the semi-permeable membrane. Dialysate is infused into the abdomen, wastes diffuse across the peritoneum into the fluid, and the fluid is drained out. This can be done at home.
The kidney does far more than filter — it also produces erythropoietin (hormone for RBC production), activates vitamin D, regulates blood pressure via the renin-angiotensin system, and maintains acid-base balance continuously (24/7). Dialysis only addresses filtration and does so intermittently. This is why kidney transplantation, when possible, is always preferred over long-term dialysis.
Why This Works
The physics underlying dialysis is straightforward: diffusion across a concentration gradient. What is clever is the engineering — the dialysate composition is adjusted so that the blood emerges from dialysis with near-normal electrolyte levels.
Blood clotting inside the machine is prevented by adding heparin (an anticoagulant) to the blood as it enters the machine. The semi-permeable membrane used industrially is usually cellulose acetate or a synthetic polymer — it is essentially many thousands of tiny hollow fibres bundled together, giving a large surface area (about 1.5 m²) in a compact device.
In NEET and board exams, questions often ask you to distinguish between the two types of dialysis and to link the semi-permeable membrane principle to osmoregulation topics you studied earlier (Malpighian tubules in insects, flame cells in platyhelminthes).
Alternative Method — Kidney Transplant
A functioning kidney from a compatible donor is surgically placed, usually in the pelvic region. The recipient’s original kidneys are typically left in place unless they are causing problems. The transplanted kidney connects to the recipient’s blood supply and ureter. The major challenge is immune rejection — the patient must take immunosuppressant drugs for life. A successful transplant eliminates the need for dialysis entirely.
Common Mistake
Many students confuse dialysis with filtration by a physical filter (like a sieve) and think large molecules pass and small ones are retained — the exact reverse of reality. Remember: the semi-permeable membrane retains large molecules (proteins, blood cells) and lets small waste molecules (urea, creatinine) pass through. Also, students sometimes incorrectly state that dialysis “produces urine” — it does not. It cleans blood externally; the patient’s body still has no urine output if kidneys are completely non-functional.
In NEET, dialysis appears under Chapter 19 (Excretory Products and Their Elimination). Common question types: (1) principle of haemodialysis — always mention semi-permeable membrane + concentration gradient; (2) distinguish haemodialysis vs peritoneal dialysis; (3) why dialysis does not fully replace the kidney — mention hormonal and regulatory functions.