Dialysis is a medical treatment that takes over the function of failing kidneys by cleaning the blood and balancing the body’s chemistry. This process involves two major tasks: removing waste products, such as urea and creatinine, and eliminating excess fluid that the kidneys can no longer excrete. The mechanism for fluid removal is distinct from waste removal, relying on precise physical forces to safely draw accumulated water out of the bloodstream.
The Principle of Ultrafiltration
The primary method for extracting fluid during hemodialysis is ultrafiltration, a physical process involving the bulk movement of water and small dissolved substances across a semipermeable membrane. This membrane, housed within the dialyzer (artificial kidney), acts as a highly selective filter, allowing water and small molecules to pass while blocking large blood components like red blood cells and plasma proteins.
This fluid movement is driven by a difference in pressure, known as a hydraulic pressure gradient, rather than a difference in concentration. In the dialyzer, the machine creates a higher pressure on the blood side of the membrane compared to the fluid (dialysate) side. This pressure differential physically pushes the excess water out of the blood and into the dialysate, where it is then drained away as waste.
The hydraulic pressure mechanism of ultrafiltration is fundamentally different from diffusion, which removes toxins. Diffusion moves solutes from high concentration (the patient’s blood) to low concentration (the dialysate) until equilibrium is reached. In contrast, ultrafiltration is the mechanical force that pulls the water solvent, and with it, some smaller solutes through a process called solvent drag.
Creating the Pressure Gradient (Transmembrane Pressure)
The dialysis machine precisely controls the hydraulic pressure gradient necessary for ultrafiltration. This pressure difference is quantified as the Transmembrane Pressure (TMP), which is the net pressure acting across the dialyzer membrane. TMP is calculated as the average pressure in the blood compartment minus the pressure in the dialysate compartment.
To achieve fluid removal, the machine creates a lower pressure on the dialysate side of the filter, often by applying negative pressure or suction to the dialysate compartment. This enhances the pressure differential, ensuring the net force pushes fluid from the higher-pressure blood side toward the lower-pressure dialysate side.
Modern dialysis machines automatically adjust TMP throughout the session using volumetric control systems to measure the exact amount of fluid being removed and match the required Ultrafiltration Goal. The higher the TMP, the greater the hydraulic force, and the faster the rate of fluid removal from the patient’s blood.
Fluid Removal in Peritoneal Dialysis (Osmotic Ultrafiltration)
For patients undergoing Peritoneal Dialysis (PD), the mechanism for fluid removal is entirely different, relying on osmosis instead of hydraulic pressure. In PD, the body’s own peritoneal lining, which surrounds the abdominal organs, serves as the semipermeable membrane. A sterile dialysate solution is introduced into the patient’s abdominal cavity, where it dwells for several hours.
Fluid removal is driven by the high concentration of an osmotic agent, typically dextrose, within the dialysate, which creates a significant concentration gradient between the dialysate fluid and the blood flowing through the capillaries lining the peritoneum. Because water naturally moves from low solute concentration to high solute concentration, the water in the patient’s blood is drawn across the peritoneal membrane and into the dextrose-rich dialysate.
The concentration of dextrose in the PD solution is varied to control the amount of fluid removed; solutions with higher dextrose concentrations generate a greater osmotic gradient. This osmotic gradient is strongest at the beginning of the dwell time but gradually decreases as the dextrose is absorbed into the blood. The resulting fluid, containing both the original dialysate and the pulled excess water, is then drained out of the abdomen.
Clinical Control of Fluid Volume
Before each dialysis session, the patient’s weight is measured and compared to their “dry weight,” the target weight representing normal fluid volume. The difference between the current weight and the dry weight determines the total Ultrafiltration Goal, or the exact amount of fluid that must be removed during the treatment. The goal also accounts for any fluid consumed or infused during the session.
The dialysis team then calculates the Ultrafiltration Rate (UFR), which is the speed at which the fluid must be removed to meet the goal within the prescribed treatment time. This rate is expressed in milliliters per kilogram of body weight per hour (mL/kg/hr). Clinicians must manage the UFR carefully, as removing fluid too quickly can lead to complications such as cramping and low blood pressure.
Medical evidence suggests that a UFR exceeding approximately 13 mL/kg/hr may be poorly tolerated and is associated with adverse health outcomes. If a patient has gained too much fluid between sessions, the treatment time may be extended or fluid restriction advice reinforced to ensure a gradual and safe removal rate.