Dialysis: How Kidney Replacement Therapy Works and Who Needs It

When Kidneys Can No Longer Clean the Blood

The kidneys perform one of the most demanding filtration tasks in the human body. Each day, they filter approximately 200 liters of blood, excreting waste products, regulating electrolytes, controlling fluid balance, maintaining acid-base homeostasis, and producing hormones including erythropoietin (stimulating red blood cell production) and the active form of vitamin D. When chronic kidney disease progresses to end-stage renal disease (ESRD) — kidney function below approximately 10 to 15 percent of normal — these life-sustaining functions must be replaced artificially or through transplantation.

Approximately 808,000 Americans lived with ESRD as of 2020, according to the U.S. Renal Data System. Of these, about 69 percent relied on dialysis as their primary kidney replacement therapy. Without dialysis or transplantation, ESRD is fatal — uremic toxins accumulate, potassium rises to lethal levels, and fluid overload produces pulmonary edema and respiratory failure within days to weeks.

Principles Common to All Dialysis Modalities

All dialysis modalities share two fundamental processes for removing waste and excess fluid from the blood:

Diffusion

Solutes move across a semipermeable membrane from an area of higher concentration to lower concentration. In dialysis, blood on one side of the membrane contains high concentrations of uremic toxins (urea, creatinine, potassium, phosphorus). The dialysate (the dialysis fluid) on the other side contains little or none of these substances. Toxins diffuse from blood into dialysate and are discarded.

Ultrafiltration

Water is removed from the blood by applying a pressure gradient across the semipermeable membrane. In hemodialysis, the dialysis machine creates transmembrane pressure to drive water out of blood and into dialysate. This removes the excess fluid that accumulates between sessions (typically 1 to 3 liters in standard three-times-per-week hemodialysis).

Hemodialysis

Hemodialysis (HD) is the most common form of dialysis worldwide. Blood is withdrawn from the patient's body, pumped through an extracorporeal circuit, filtered through an artificial kidney (dialyzer), and returned to the body — typically over 3 to 5 hours, performed three times per week.

Vascular Access

Hemodialysis requires reliable, high-flow vascular access capable of sustaining blood flow rates of 300 to 500 mL/min. Three types of access are used:

Arteriovenous fistula (AVF): A surgically created connection between an artery and vein in the forearm or upper arm; allows the vein to "arterialize" and develop high flow; takes 6 to 12 weeks to mature; the preferred long-term access due to lowest complication rates
Arteriovenous graft (AVG): A synthetic tube connecting artery and vein; available sooner than a fistula (2–3 weeks) but higher rates of thrombosis and infection
Central venous catheter (CVC): A large-bore catheter placed in the jugular, subclavian, or femoral vein; immediately available but associated with high infection risk (bacteremia); used for urgent initiation or as a bridge to permanent access

The Dialyzer

The dialyzer — often called an artificial kidney — contains thousands of hollow fibers made of semipermeable membranes (typically polysulfone, polyamide, or cellulose derivatives). Blood flows inside the fibers; dialysate flows countercurrent outside them. This countercurrent design maximizes the concentration gradient for diffusion throughout the entire length of the dialyzer.

Dialysate Composition

Dialysate is a carefully formulated electrolyte solution. Its composition is adjusted to correct each patient's specific abnormalities:

Component	Typical Dialysate Level	Purpose
Potassium	2–3 mEq/L	Draw excess potassium from blood (normal serum: 3.5–5.0 mEq/L)
Sodium	138–145 mEq/L	Maintain osmolar balance; modeled to prevent cramping
Bicarbonate	35–40 mEq/L	Correct metabolic acidosis from acid accumulation
Calcium	2.5 mEq/L	Maintain calcium balance; prevent bone disease
Glucose	0–200 mg/dL	Prevent hypoglycemia in diabetic patients; modest caloric intake

Peritoneal Dialysis

Peritoneal dialysis (PD) uses the patient's own peritoneal membrane — the tissue lining the abdominal cavity — as the dialysis membrane. A catheter is surgically placed through the abdominal wall into the peritoneal cavity. Dialysate (2 to 3 liters) is instilled, dwells for 4 to 8 hours, and then drained — taking accumulated toxins and fluid with it. This cycle is called an exchange.

Peritoneal dialysis offers two main formats:

Continuous ambulatory peritoneal dialysis (CAPD): 4 manual exchanges per day, each requiring 20 to 40 minutes; no machine required
Automated peritoneal dialysis (APD): A cycler machine performs 3 to 5 exchanges overnight while the patient sleeps; daytime freedom from exchanges

PD provides continuous, gentle dialysis — more physiologically similar to native kidney function than intermittent hemodialysis. It is associated with better preservation of residual kidney function in the first 1 to 2 years. It requires the patient to perform or supervise exchanges daily and maintains the peritoneal membrane health.

Hemodialysis vs. Peritoneal Dialysis — Key Comparisons

Feature	Hemodialysis	Peritoneal Dialysis
Location	Dialysis center (3x/week) or home	Home (daily)
Duration	3–5 hours per session	Continuous or overnight
Vascular access required	Yes (AVF, AVG, or CVC)	No (peritoneal catheter)
Fluid removal	Rapid (during session)	Continuous and gradual
Infection risk	Bloodstream infection from access	Peritonitis (intraabdominal)
Dietary restrictions	Strict (between sessions)	Somewhat more liberal
Travel flexibility	Limited (requires dialysis center)	Greater (supplies can be shipped)

When Dialysis Begins and What Patients Experience

The decision to initiate dialysis is made when the estimated glomerular filtration rate (eGFR) falls below 10 to 15 mL/min/1.73m² and the patient has symptoms of uremia (nausea, vomiting, confusion, seizures, pericarditis, fluid overload) or unmanageable metabolic derangements.

Common side effects during and after hemodialysis sessions include hypotension (occurring in 10 to 30 percent of sessions), muscle cramps, headache, and fatigue. Between sessions, patients must restrict fluid intake (typically 1,000 to 1,500 mL per day), dietary potassium (limit bananas, oranges, potatoes), phosphorus (limit dairy, nuts, processed foods), and sodium.

Kidney Transplantation as an Alternative

Kidney transplantation — receiving a functioning kidney from a living or deceased donor — offers superior survival and quality of life compared with long-term dialysis for most patients. The median waiting time for a deceased-donor kidney in the U.S. is approximately 3 to 5 years, highly variable by blood type and geographic location. Living donor transplantation eliminates the waiting period and produces better long-term outcomes. Not all dialysis patients are suitable transplant candidates due to age, cardiovascular risk, active malignancy, or other comorbidities.

This article is for informational purposes only. Consult a qualified healthcare professional before making any health decisions.