How Effluent Fluid is Removed from Blood During Dialysis
Effluent fluid is removed from the blood during dialysis through ultrafiltration, a pressure-driven process where water and solutes are forced across a semipermeable membrane by hydrostatic pressure gradients, with the fluid being removed almost exclusively from the extracellular fluid space. 1
Fundamental Mechanisms of Fluid Removal
The removal of fluid during dialysis operates through two primary physical principles that work simultaneously:
Ultrafiltration Process
Ultrafiltration is the core mechanism for fluid removal, functioning as a pressure-driven process where the hydrostatic pressure difference across the dialyzer membrane forces water (the solvent) to move from blood into the dialysate compartment 1, 2. This process simultaneously carries dissolved solutes across the membrane through a phenomenon called "solvent drag" or convection 1.
Membrane-Specific Mechanisms
The type of membrane determines how fluid removal occurs:
For Hemodialysis (Extracorporeal):
- Blood flows through an artificial semipermeable membrane in an external filter
- A dialysis machine controls the transmembrane pressure to achieve the desired ultrafiltration rate
- The hydraulic permeability of the membrane determines how efficiently fluid can be removed 3
- Modern high-flux dialyzers create internal diafiltration through variable hydrostatic pressure as blood enters the dialyzer 3
For Peritoneal Dialysis (Intracorporeal):
- The peritoneal membrane serves as the natural semipermeable barrier
- Fluid removal occurs by creating an osmotic gradient using hypertonic solutions (glucose or icodextrin) instilled into the peritoneal cavity
- The osmotic gradient draws fluid from peritoneal capillaries into the dialysate 1
Source of Removed Fluid
Critical finding: Ultrafiltration removes fluid almost exclusively from the extracellular fluid compartment, not from intracellular spaces. A prospective study of 40 hemodialysis patients demonstrated that with an average ultrafiltration of 2.38 liters, extracellular fluid volume decreased from 16.84 to 14.89 liters (P < 0.0001), while intracellular fluid volume remained essentially unchanged (16.88 to 16.55 liters, P = 0.45) 4. This means the source of net fluid loss is the extracellular compartment.
Rate and Volume Considerations
The amount of fluid removed depends on the total ultrafiltrate volume, not the rate of removal. Research shows that blood volume decrease during dialysis is dependent only on the amount of ultrafiltered fluid, not whether it's removed slowly or rapidly 5. Patients generally tolerate up to a 20% decrease in blood volume, and fluid can be safely removed even at rates up to 2,000 mL/hour in stable patients 5.
However, typical ultrafiltration rates are more conservative:
- Standard hemodialysis typically removes fluid at controlled rates (often around 900 mL/hour) 6
- The rate must be balanced against vascular refilling capacity to avoid hypotension and cramping 2
Clinical Monitoring
Effluent collection and measurement varies by modality:
For peritoneal dialysis, two methods are used 7:
- Batch method: All effluent bags from a 24-hour period are collected and measured
- Aliquot method: A proportional sample (1/1000th) is drawn from each bag after measuring its volume, with samples pooled for analysis
For automated peritoneal dialysis, the cycler automatically quantifies effluent volume as it pools in collection containers 7.
Important Caveats
Backfiltration risk: In hemodialysis, the ultrafiltration control process can inadvertently cause backfiltration, where dialysate moves back into the blood, potentially transporting bacterial products from dialysate to blood 2. This is a recognized complication of the fluid dynamics involved in ultrafiltration control.
Long-duration dwells: In peritoneal dialysis, extended dwell times (such as overnight in CAPD or daytime in APD) can result in net peritoneal fluid absorption rather than removal, which must be accounted for in the prescription 8.
Membrane performance factors: Ultrafiltration volume control is complicated by plasma protein adsorption, hematocrit changes, and coagulation parameters affecting membrane performance 2. Previously developed equations for ultrafiltration rates don't apply well to modern high-flux dialyzers and high blood flow rates 2.