Dialysis is a form of kidney replacement therapy that removes selected waste products and excess fluid from the blood when the kidneys cannot perform these functions adequately. It can replace some, but not all, functions of healthy kidneys. The two principal forms are haemodialysis and peritoneal dialysis. Haemodialysis circulates blood through an extracorporeal filter called a dialyser, where diffusion and ultrafiltration allow waste and excess water to move across a semipermeable membrane.
Peritoneal dialysis uses the patient's peritoneal membrane as the dialysis membrane, with dialysis solution introduced into and drained from the abdominal cavity. Dialysis may be temporary, such as during severe acute kidney injury, or long term in advanced kidney failure when recovery is unlikely without transplantation or continued kidney replacement therapy.
Dialysis works through several physical principles. Diffusion allows solutes such as urea and certain electrolytes to move down concentration gradients across a semipermeable membrane. Ultrafiltration removes water by applying a pressure gradient, while convection can contribute to solute removal as water moves across the membrane. In haemodialysis, dialysate is formulated so that unwanted substances can move from blood into the dialysis fluid while selected electrolytes and buffers are controlled.
Peritoneal dialysis relies on the vascularised peritoneal membrane and repeated exchanges of dialysis solution. Neither method reproduces the full endocrine and metabolic functions of the kidney, including normal erythropoietin production, vitamin D activation, and the complex hormonal regulation performed by healthy renal tissue. Consequently, dialysis is one component of comprehensive kidney-failure management rather than a complete biological replacement for the kidneys.
Haemodialysis generally requires reliable vascular access, most commonly an arteriovenous fistula, an arteriovenous graft, or a central venous catheter. A fistula connects an artery and vein and, after maturation, can provide durable high-flow access with relatively lower infection risk than a catheter. During treatment, blood is pumped through the dialyser and returned to the patient, while anticoagulation is commonly used to reduce clotting within the extracorporeal circuit.
Conventional haemodialysis is often delivered several times per week, although schedules vary according to residual kidney function, treatment modality, clinical circumstances, and prescription. Home haemodialysis can provide greater flexibility and, in selected patients, increased treatment frequency. Treatment prescriptions are individualised according to fluid balance, solute control, nutritional status, vascular access, residual kidney function, and tolerance.
Peritoneal dialysis uses a catheter placed into the abdominal cavity and repeated exchanges of dialysis fluid. In continuous ambulatory peritoneal dialysis, exchanges are performed manually at intervals during the day, whereas automated peritoneal dialysis uses a machine, commonly during sleep, to perform multiple exchanges. The dialysis solution contains an osmotic agent that helps draw water from the bloodstream into the peritoneal cavity, while waste products diffuse into the fluid and are removed when the fluid is drained.
Peritoneal dialysis can offer independence from a dialysis centre and may preserve residual kidney function for some time, but it requires careful technique and a suitable home environment. The major infectious complication is peritonitis, while mechanical problems can include catheter dysfunction, leaks, hernias, and changes in membrane performance.
Dialysis is initiated because kidney function is insufficient to maintain physiological stability, not solely because a particular creatinine or estimated glomerular filtration rate has been reached. Indications include refractory fluid overload, severe or persistent hyperkalaemia, significant metabolic acidosis that cannot otherwise be controlled, certain uraemic complications, and some toxin exposures. In advanced chronic kidney disease, symptoms, biochemical abnormalities, nutritional status, fluid balance, and overall clinical trajectory are considered when deciding when kidney replacement therapy is appropriate.
Starting dialysis too early solely on the basis of a laboratory number does not necessarily improve outcomes. Conversely, delaying dialysis in a patient with dangerous complications can be life-threatening. Decisions should therefore be based on the whole clinical picture and should incorporate preparation for access and modality selection before dialysis becomes urgently necessary when possible.
Dialysis has important physiological and practical complications. Haemodialysis can cause intradialytic hypotension when fluid is removed faster than the circulation can refill, as well as muscle cramps, headache, fatigue, access complications, infection, and less commonly air or clot-related problems. Long-term vascular access can develop stenosis or thrombosis and requires monitoring. Peritoneal dialysis can cause peritonitis, catheter infection, protein loss, hernias, and metabolic complications associated with dialysis solutions.
Both modalities can be associated with anaemia, mineral and bone abnormalities, cardiovascular disease, malnutrition, and reduced quality of life because these problems reflect the underlying kidney failure as well as the limitations of replacement therapy. Care therefore involves dietary and medication management, anaemia treatment, mineral-bone management, cardiovascular risk reduction, vaccination, and monitoring for modality-specific complications.
Dialysis can be used as a bridge to kidney transplantation, as long-term kidney replacement therapy when transplantation is unavailable or unsuitable, or temporarily during acute kidney injury while the kidneys recover. Kidney transplantation generally provides more complete restoration of renal function than dialysis but requires an appropriate donor organ and lifelong immunosuppression.
Conservative kidney management without dialysis is another pathway for some people with advanced kidney disease, particularly when the burdens and risks of dialysis outweigh its expected benefits according to the individual's clinical circumstances and goals. The choice between haemodialysis and peritoneal dialysis involves medical suitability, residual kidney function, vascular access, abdominal factors, lifestyle, support requirements, and personal preferences. Modality choice can sometimes change over time as circumstances and medical needs evolve.
A common misconception is that dialysis completely replaces healthy kidney function or permanently cures kidney failure. Dialysis performs essential filtration and fluid-removal functions but does not fully reproduce renal endocrine, metabolic, and regulatory activity, and it does not by itself reverse irreversible chronic kidney disease. Another misconception is that every person with severe chronic kidney disease must automatically start dialysis immediately; timing depends on symptoms and complications as well as kidney function.
Dialysis can also be temporary in acute kidney injury when renal recovery occurs. Understanding the term therefore requires distinguishing kidney replacement from kidney recovery, haemodialysis from peritoneal dialysis, acute from chronic indications, and dialysis from transplantation or conservative management. The central purpose is to maintain physiological stability and remove substances and fluid that failing kidneys can no longer adequately control.
