Ultrafiltration in dialysis replaces the kidney’s ability to manage the body’s fluid balance. Dialysis performs two primary functions: removing waste products and toxins, and safely extracting excess water and small molecules that accumulate between treatments. Ultrafiltration is the technique used to remove this excess fluid from the patient’s blood during the session.
The Purpose of Ultrafiltration
Kidney failure causes patients to retain fluid, leading to accumulation between dialysis sessions known as volume overload. Excess fluid can cause swelling (edema) and high blood pressure (hypertension). Fluid overload places significant strain on the cardiovascular system, increasing the heart’s workload and potentially causing breathing difficulties. The primary goal of ultrafiltration is to restore the patient’s fluid balance to a healthy state, referred to as achieving “dry weight.” Dry weight is the patient’s weight when all excess fluid has been removed, without causing low blood pressure or other complications.
How Transmembrane Pressure Drives Fluid Removal
Ultrafiltration is a physical process that relies on pressure to move fluid through a specialized filter called a dialyzer. The dialyzer contains a semipermeable membrane, which allows water and small molecules to pass through while keeping larger components like blood cells and proteins in the bloodstream.
The force driving the fluid across this membrane is the Transmembrane Pressure (TMP). TMP represents the difference in hydrostatic pressure between the blood and dialysate compartments within the dialyzer. The dialysis machine creates this pressure gradient by making the pressure on the blood side higher than the dialysate side. This difference physically pushes the plasma water out of the blood and into the dialysate, taking small dissolved solutes with it. The higher the TMP is set, the faster the rate of ultrafiltration.
Managing the Ultrafiltration Rate
The rate at which fluid is removed, known as the ultrafiltration rate (UFR), must be carefully controlled for patient safety and comfort. Before each session, the healthcare team calculates the exact amount of fluid to be removed based on the patient’s weight gain, setting the UFR over the prescribed treatment time.
Modern dialysis machines use volumetric control systems to manage this rate precisely. These systems continuously monitor fluid flow to ensure the exact prescribed volume is removed over the entire duration of the treatment. A controlled rate prevents complications such as intradialytic hypotension, a sudden drop in blood pressure that can cause muscle cramps and organ injury. Clinicians strive to keep the UFR below 13 milliliters of fluid per hour per kilogram of body weight, as higher rates are associated with increased risk of poor outcomes. The goal is a steady, gentle removal of fluid that allows the body’s vascular system time to refill the fluid lost from the bloodstream.
Ultrafiltration Versus Solute Clearance
Ultrafiltration must be distinguished from solute clearance, the other main function of dialysis (the removal of metabolic waste products). Ultrafiltration is driven by hydrostatic pressure and primarily removes volume, or water, along with any small dissolved molecules. The removal of these small waste molecules, such as urea, is minimal compared to the other main cleansing mechanism.
The bulk of toxin removal in dialysis is achieved through diffusion, a process where solutes move across the membrane from an area of high concentration (the blood) to an area of low concentration (the dialysate). Another process, convection, is the movement of solutes along with the fluid, which is referred to as “solvent drag.” While ultrafiltration contributes to convection, diffusion remains the dominant force for cleaning the blood of accumulated toxins.