Haemodialysis is a renal replacement therapy that removes metabolic waste products, excess water and certain dissolved substances from the blood when the kidneys cannot perform these functions adequately. The process uses an extracorporeal circuit in which blood leaves the body, passes through a dialyser containing a semipermeable membrane, and returns to the circulation.
Dialysis fluid, or dialysate, flows on the opposite side of the membrane, allowing substances such as urea and excess electrolytes to move according to concentration gradients while largely retaining blood cells and larger proteins. Water removal occurs principally through ultrafiltration, driven by a transmembrane pressure difference.
Haemodialysis can provide part of the functions normally performed by healthy kidneys, including clearance of small solutes, regulation of extracellular fluid volume and correction of some electrolyte and acid-base disturbances, but it does not reproduce the full endocrine, metabolic and regulatory activity of functioning kidneys.
The word haemodialysis combines “haemo-”, referring to blood, with “dialysis”, derived from Greek roots meaning separation or dissolution through a membrane. Modern dialysis developed from experimental membrane-based separation techniques and early artificial-kidney work, particularly during the twentieth century, eventually becoming a practical long-term treatment for kidney failure. Willem Kolff developed an influential early artificial kidney during the Second World War, while later advances in vascular access, membrane technology, anticoagulation, dialysis machines and treatment protocols made chronic haemodialysis possible.
Historically, dialysis was initially associated primarily with acute kidney failure, but the development of reliable repeated vascular access transformed it into a long-term treatment for end-stage kidney disease. Today, haemodialysis may be used for acute kidney injury, advanced chronic kidney disease and kidney failure, with the specific indication depending on clinical circumstances rather than a single blood-test threshold alone.
Conventional maintenance haemodialysis is commonly performed several times per week, with each treatment lasting several hours, although schedules vary according to residual kidney function, body size, treatment goals, modality and clinical circumstances. Treatment may take place in a dialysis centre or, for appropriately selected and trained patients, at home. The major determinants of dialysis adequacy include blood-flow rate, dialysate-flow rate, membrane characteristics, treatment duration, frequency, distribution of body water and the patient's residual renal function.
Measures such as Kt/V and urea reduction ratio are used to assess clearance of urea, although adequacy is broader than a single numerical target and also involves fluid management, electrolyte control, nutritional status, symptoms and quality of life. More frequent or longer treatments can alter fluid and solute removal patterns and may reduce the physiological fluctuations associated with conventional intermittent schedules.
Effective haemodialysis requires vascular access capable of delivering sufficient blood flow repeatedly while minimising complications. The principal forms are arteriovenous fistulas, arteriovenous grafts and central venous catheters. An arteriovenous fistula connects an artery to a vein and is generally preferred for durable long-term access because it can provide good blood flow and has comparatively lower infection and thrombosis risks than catheters. A graft uses synthetic or biological conduit material when native vessels are unsuitable.
Central venous catheters provide immediate access and are therefore useful in urgent circumstances, but prolonged catheter use is associated with substantial risks including bloodstream infection, thrombosis and central venous stenosis. Access preservation, regular assessment, appropriate needle placement and prompt investigation of dysfunction are fundamental components of long-term haemodialysis care.
Haemodialysis removes substances principally by diffusion and water principally by ultrafiltration, with convection also contributing to solute removal in some techniques. Dialysate composition can be adjusted to influence concentrations of sodium, potassium, calcium, bicarbonate and other components, allowing treatment to correct particular biochemical abnormalities. Anticoagulation is frequently used within the extracorporeal circuit to prevent clotting, although the approach may be modified when bleeding risk is high.
Modern dialysers use membranes with defined permeability and surface characteristics, and high-flux membranes permit greater clearance of certain middle-molecular-weight substances than conventional low-flux systems. Haemodiafiltration combines diffusion with substantial convection and can increase removal of selected larger solutes, although the clinical role of different modalities depends on patient factors, equipment, local practice and evidence.
Haemodialysis has important physiological effects because treatment rapidly changes circulating fluid and solute concentrations rather than providing continuous kidney function. Rapid fluid removal can contribute to intradialytic hypotension, cramps, dizziness, nausea, headache or fatigue, particularly when large interdialytic fluid gains must be removed over a limited session. Shifts in electrolytes and osmotic balance can produce neurological or cardiovascular symptoms in unusual circumstances, while repeated vascular access can cause infection, stenosis, thrombosis or aneurysmal changes.
Long-term kidney failure itself remains associated with anaemia, mineral and bone abnormalities, cardiovascular disease, malnutrition and other complications that dialysis may improve but does not necessarily eliminate. Medication dosing also requires careful consideration because some drugs are removed by haemodialysis whereas others are not, and dialysis timing can therefore affect pharmacotherapy.
Patients receiving haemodialysis often need coordinated management of fluid intake, dietary intake, medications, vascular access and other aspects of kidney failure. Dietary requirements vary according to residual kidney function, laboratory results and dialysis prescription, but attention commonly focuses on sodium, fluid, potassium, phosphate and protein intake rather than imposing one universal diet. Anaemia management may involve iron and agents that stimulate erythropoiesis, while disturbances of mineral metabolism may require phosphate-lowering treatment, vitamin-D-related therapies or other approaches.
Blood pressure and diabetes management remain important, and cardiovascular risk is particularly significant in people with kidney failure. Haemodialysis can permit substantial improvements in symptoms caused by uraemia and fluid overload, but the treatment schedule can impose considerable practical burdens involving travel, time, employment, sleep, family life and psychological adaptation.
Haemodialysis is not synonymous with curing kidney disease or restoring normal kidney function. It is a replacement therapy that supports physiological stability while an underlying kidney condition may remain present; in appropriate patients, kidney transplantation can provide a different form of renal replacement with potentially greater physiological independence from dialysis. Dialysis may be temporary in acute kidney injury when renal function later recovers, or long-term when irreversible kidney failure has occurred.
Decisions concerning dialysis initiation and modality involve kidney function, symptoms, biochemical and fluid abnormalities, comorbidities, functional status, prognosis, patient preferences and available support rather than kidney filtration measurements alone. Common misconceptions include the idea that dialysis removes every toxin from the body, that all patients must receive identical treatment schedules, or that dialysis automatically prevents all complications of kidney failure. In reality, haemodialysis is a sophisticated supportive treatment whose effectiveness depends on prescription, vascular access, adherence, residual kidney function, monitoring and the broader management of kidney failure.
