Dialysis
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Direct answer
Dialysis is the artificial clearance of nitrogenous wastes from blood when the kidneys fail, done either by haemodialysis or peritoneal dialysis. In haemodialysis, blood drained from a convenient artery has an anticoagulant (heparin) added and is pumped into a dialysing unit where it flows through a coiled cellophane tube bathed in dialysing fluid; because this fluid has the same composition as plasma except for the nitrogenous wastes, urea and other wastes diffuse out of the blood while its normal constituents are retained. The cleaned blood, after addition of anti-heparin, is returned to the body through a vein. In peritoneal dialysis, the peritoneum of the abdominal cavity serves as the dialysing membrane.
What you must remember
- Indication: uraemia and renal failure — the accumulation of urea and other nitrogenous wastes that functioning kidneys would remove.
- Access and anticoagulation: blood is drawn from a convenient artery; heparin is added before the blood enters the machine, and anti-heparin is mixed in before the blood returns through a vein.
- The membrane: a coiled cellophane tube inside the dialysing unit — semi-permeable, allowing small solutes to diffuse while cells and proteins stay in.
- Dialysing fluid composition: identical to plasma except that it lacks nitrogenous wastes — the exact NCERT phrase that fills blanks; this keeps glucose and electrolytes in blood while pulling urea out.
- Direction of diffusion: urea moves from blood to fluid down its concentration gradient; no active transport is involved.
- Peritoneal dialysis: dialysing fluid is introduced into the abdominal cavity and the peritoneum, richly vascular and semi-permeable, acts as the exchange membrane.
- The bigger picture: dialysis supports but does not cure; kidney transplantation from a close relative donor remains the definitive correction of renal failure, with the patient maintained on immunosuppression to prevent graft rejection.
A urea molecule's exit route
Follow one urea molecule from a uraemic patient's artery. It meets heparin first, so no clot forms in the extracorporeal tubing. Then the coiled cellophane tube: cellophane is porous enough for small dissolved molecules but not for blood cells or plasma proteins, so urea, creatinine and excess ions slip across while haemoglobin and albumin cannot. On the other side waits dialysing fluid deliberately brewed to mimic plasma — sodium, chloride, glucose at physiological concentrations, but no urea. Because the fluid is urea-free, the gradient across cellophane relentlessly favours exit; the molecule leaves the blood, and fresh fluid continuously sweeping the bath keeps the gradient steep. Minutes later the same blood, now clean, meets anti-heparin to restore clotting and re-enters a vein.
Peritoneal dialysis relocates the same physics inside the body. Warm fluid instilled into the abdominal cavity bathes the visceral organs; the peritoneum, a thin and richly vascular membrane, plays cellophane's role. Wastes diffuse from capillary blood across the peritoneum into the fluid, which is drained and replaced in cycles. It is slower than haemodialysis but needs no machine, which is why continuous ambulatory peritoneal dialysis suits many home settings. The exam's core insight is identical in both: dialysis is selective diffusion down concentration gradients across a semi-permeable membrane, with the dialysing fluid's composition doing the selecting — change the fluid's composition and you change what leaves the blood.
Where the marks leak
Three leaks dominate. First, composition: "dialysing fluid has the same composition as plasma" must be completed with "except nitrogenous wastes" — options that drop the exception are the commonest wrong pick, and an option claiming the fluid contains urea in higher concentration inverts the gradient logic. Second, the anticoagulant choreography: heparin in, anti-heparin out, with artery as exit and vein as return — sequence questions quote this verbatim. Third, membrane identity: cellophane tube in haemodialysis versus peritoneum in peritoneal dialysis; students who swap them lose an easy mark. A final true/false favourite: "dialysis cures renal failure" — false; it substitutes for the kidney, and transplantation is the definitive correction.
Frequently asked questions
What is the composition of dialysing fluid?
The same as plasma except for the nitrogenous wastes, so urea and similar solutes diffuse out while normal plasma constituents are conserved.
Which membrane is used in the haemodialysis unit?
A coiled cellophane tube, semi-permeable to small solutes but impermeable to blood cells and proteins.
What is added to blood before and after it passes through the artificial kidney?
Heparin is added as the blood leaves the artery to prevent clotting, and anti-heparin is added before the cleaned blood returns through the vein.
What serves as the membrane in peritoneal dialysis?
The peritoneum lining the abdominal cavity, across which wastes diffuse from blood into dialysing fluid placed in the abdomen.
Why is dialysis not a cure for renal failure?
It only substitutes mechanically for the kidney's filtration function; the definitive treatment is kidney transplantation, commonly from a close relative.