Hemodialysis Explained

Hemodialysis is the most widely used form of kidney replacement therapy for people with end-stage renal disease worldwide — removing waste products, excess fluid, and electrolytes from the blood by circulating it through an external filter (dialyzer) that mimics the filtration function of healthy kidneys. In the United States, approximately 500,000 people receive dialysis each year, and hemodialysis accounts for approximately 90% of all dialysis treatments. Understanding exactly how hemodialysis works — the mechanism of filtration, the session structure, the types of vascular access used to connect patients to the machine, what happens during a session, and what determines treatment adequacy — gives patients and families the knowledge to participate actively in their own dialysis care, recognize problems when they occur, and make informed choices about whether in-center or home hemodialysis is most suitable for their lifestyle. This guide covers hemodialysis from first principles — how it cleans the blood, what the access options are, what a typical session involves, how adequacy is measured, and what the most important complications to understand and watch for are.

Hemodialysis is one of two primary dialysis modalities — the other being peritoneal dialysis, which uses the patient’s own peritoneal membrane as the filtration surface rather than an external dialyzer. For the detailed comparison of hemodialysis versus peritoneal dialysis — including which patients are better suited to each modality, the relative quality of life impacts, and the residual kidney function considerations — the dialysis guide for patients covers the modality decision framework. For people who are newly approaching dialysis and want to understand the preparation steps — access surgery timing, the education process, and what to arrange before the first session — the preparing for dialysis guide provides the complete pre-dialysis roadmap.

How Hemodialysis Cleans the Blood: Diffusion and Ultrafiltration

The hemodialysis machine cleans the blood through two physical processes working simultaneously: diffusion and ultrafiltration. Understanding these mechanisms explains why hemodialysis effectively removes small molecules like urea and creatinine but is less efficient at removing larger molecules (middle molecules) — a limitation that influences dialysis prescription choices. Diffusion is the movement of dissolved substances across a semi-permeable membrane from areas of higher concentration to areas of lower concentration — the fundamental driving force for waste removal in hemodialysis. The dialyzer contains thousands of hollow fiber membranes — thin tubes (typically 200 micrometers in diameter) with semi-permeable walls through which small molecules can pass. Blood flows through the inside of these fibers in one direction, while dialysate solution (a carefully formulated electrolyte solution with zero urea, creatinine, potassium, and phosphorus) flows around the outside in the opposite direction (countercurrent flow, which maximizes the concentration gradient across the membrane). Because blood entering the dialyzer has high concentrations of urea, creatinine, potassium, and phosphorus, and the dialysate has zero concentration of these solutes, they diffuse rapidly across the membrane from blood into dialysate and are removed. Conversely, bicarbonate (which is low in the blood of uremic patients who are acidotic) diffuses from the dialysate into the blood, correcting acidosis. Calcium and magnesium in the dialysate are set at target concentrations that allow correction of blood levels depending on whether the patient tends toward low or high values. Ultrafiltration is the removal of fluid — the process that removes the excess water and sodium the patient has accumulated between sessions (the interdialytic weight gain, which is primarily water). The dialysis machine applies a negative pressure to the dialysate side of the membrane, drawing fluid from the blood side through the membrane into the dialysate — physically removing the excess water. The ultrafiltration rate is calculated based on the patient’s dry weight (target weight without excess fluid) and the actual weight at session start: the difference (interdialytic weight gain) must be removed during the session. Typical interdialytic weight gains of 1.5–3 kg are removed over a 3–4 hour session; gains above 4 kg are associated with higher rates of intradialytic hypotension (low blood pressure during the session) because the ultrafiltration rate exceeds the body’s capacity to refill the vascular space from tissues as fluid is removed. The NIDDK’s comprehensive hemodialysis patient resources are at the NIDDK hemodialysis page.

Vascular Access for Hemodialysis: Fistulas, Grafts, and Catheters

Vascular access is the lifeline of hemodialysis — the means by which large volumes of blood (typically 300–500 mL/min) are delivered to the dialyzer and returned to the body. Without adequate, functioning vascular access, hemodialysis cannot be performed. The three types of vascular access — arteriovenous fistula, arteriovenous graft, and central venous catheter — differ dramatically in durability, infection risk, blood flow adequacy, and time to usability, and the choice and management of vascular access is one of the most clinically consequential aspects of hemodialysis care. Arteriovenous fistula (AVF) is created by a vascular surgeon who connects an artery (usually the radial artery at the wrist, or the brachial artery at the elbow) directly to an adjacent vein. The high-pressure arterial blood flowing through the vein causes it to dilate and its walls to thicken over 3–6 months — a process called maturation — resulting in a vessel large enough to be cannulated with two large-bore (15–17 gauge) needles for each dialysis session. The AVF has the lowest infection rate of any access type (because it is entirely inside the patient’s body with no external components between sessions), the longest average patency, and the best association with improved survival and fewer hospitalizations. The “Fistula First Catheter Last” initiative has successfully increased AVF prevalence in dialysis patients in most countries. The key disadvantage is the 3–6 month maturation requirement, which mandates that AVF creation occur 4–6 months before anticipated dialysis start to avoid catheter-dependent initiation. Arteriovenous graft (AVG) uses a synthetic prosthetic tube (typically polytetrafluoroethylene, PTFE) to bridge an artery to a vein when the patient’s own veins are too small, damaged, or previously used. Grafts can be cannulated within 2–4 weeks of creation (or as early as 24–72 hours for “early cannulation” grafts), and provide reliable blood flow. The main disadvantages compared to AVF are a higher infection rate, a tendency to develop stenosis at the venous anastomosis requiring repeated angioplasty, and shorter overall patency. Central venous catheter (CVC) — a dual-lumen tunneled catheter placed in the internal jugular vein — can be used immediately upon insertion, making it the only access option for patients who need urgent dialysis without a mature AVF or AVG. However, CVC use is associated with the highest infection rate (catheter-related bloodstream infections, a leading cause of dialysis mortality), inadequate blood flow rates (limiting dialysis adequacy), and central vein stenosis from repeated instrumentation. CVCs should be regarded as temporary access to be replaced by AVF or AVG as quickly as possible. The StatPearls hemodialysis review covering access management is at the StatPearls hemodialysis clinical review.

What Happens During a Hemodialysis Session

A standard in-center hemodialysis session follows a consistent structure that most patients learn to navigate within the first few weeks of treatment. Understanding what happens during each step reduces anxiety for new patients and helps experienced patients recognize when something is not proceeding as expected. Pre-session assessment: before each session, the patient is weighed on a calibrated scale — the weight is compared to the target dry weight to determine the ultrafiltration goal for the session. Blood pressure and pulse are measured; significantly elevated pre-dialysis blood pressure may prompt adjustment of ultrafiltration or antihypertensive medication timing. Patients are asked about symptoms since the last session (breathing difficulty, ankle swelling, missed medications, changes in urine output) that may affect the session prescription. Access cannulation or catheter connection: for AVF or AVG patients, two needles are placed — one (arterial needle) to withdraw blood to the dialyzer, one (venous needle) to return cleaned blood. Access sites are cleaned with antiseptic solution and may be numbed with lidocaine cream or ice. Catheter patients have their CVC caps removed and the lines connected to the dialysis tubing after disinfection. The dialysis session: the pump circulates blood through the dialyzer at the prescribed blood flow rate (typically 300–400 mL/min) while the dialysate flows in the opposite direction at 500–800 mL/min. The session lasts 3–4 hours for standard three-weekly schedules, during which patients typically read, watch television, sleep, use mobile devices, or receive medications by IV or during the session (iron sucrose, erythropoiesis-stimulating agents). Nurses monitor blood pressure, heart rate, and access function regularly throughout the session, typically every 30 minutes. Post-session: needles are removed and pressure is held until bleeding stops (typically 10–20 minutes for mature AVFs). Weight is measured again to confirm adequate fluid removal. Post-session blood pressure is checked. Patients often report fatigue in the first 1–3 hours after sessions — from the hemodynamic stress of fluid removal and the physiological adjustment — that typically resolves by the following morning. The NKF’s hemodialysis patient resources including session guides are at the NKF hemodialysis page. The practical guide to life on dialysis — covering sessions, schedule management, employment, and emotional wellbeing — is the life on dialysis guide.

Measuring Hemodialysis Adequacy: Kt/V and What It Means

Dialysis adequacy — the measure of how effectively each session removes waste products — is quantified by a formula called Kt/V (pronounced “kay-tee over vee”), which represents the ratio of urea clearance (K × t) to the body’s water volume (V). In this formula, K is the urea clearance rate of the dialyzer (in mL/min), t is the dialysis session duration (in minutes), and V is the total body water (in liters — proportional to body weight). A Kt/V of 1.2 per session (for three-weekly hemodialysis) is the minimum target recommended by KDIGO and other guidelines; most centers target Kt/V of 1.3–1.4 to provide a safety margin. Kt/V below 1.2 is associated with increased mortality, hospitalizations, and malnutrition — the consequences of inadequate waste clearance. Kt/V is measured monthly by collecting pre- and post-dialysis blood samples for urea (BUN) and using the Daugirdas formula or an online clearance monitor built into the dialysis machine. When Kt/V falls below target, the response is to increase dialysis dose — by extending session duration, increasing blood flow rate, increasing dialysate flow rate, or switching to a higher-flux dialyzer. Patient non-compliance with prescribed session duration (leaving early) is one of the most common reasons for inadequate Kt/V. Every hemodialysis patient should know their current Kt/V value from their monthly labs and ask their dialysis team if it meets target. Home hemodialysis — performed either daily (short daily home HD, typically 2–2.5 hours per day, 5–6 days per week) or nocturnal (slow nocturnal HD, typically 6–8 hours per night, 3–6 nights per week) — achieves dramatically higher dialysis adequacy and removes middle molecules more effectively than conventional three-weekly in-center HD. Home HD patients have lower blood pressure, fewer dietary restrictions (less fluid and phosphorus accumulation due to more frequent sessions), better quality of life scores, and potentially better survival outcomes than in-center HD patients in observational studies. The KDIGO CKD guidelines covering dialysis adequacy targets are available at the KDIGO CKD guidelines page. For people considering whether home hemodialysis might offer a better quality of life and more flexibility than in-center HD, the dialysis overview guide covers the comparative advantages of home versus center-based HD.

Sources: NIDDK — Hemodialysis · NKF — Hemodialysis · KDIGO CKD Guidelines · StatPearls — Hemodialysis