Many clinicians become comfortable ordering CRRT long before they fully understand how it actually removes solutes. While modern CRRT machines automate much of the process, understanding the underlying physiology can improve prescription design and troubleshooting at the bedside.
At its core, CRRT removes solutes through diffusion, convection, or a combination of both.
Diffusion refers to movement of solutes down a concentration gradient. Small molecules naturally move from areas of higher concentration to areas of lower concentration across a semipermeable membrane.
This is the primary mechanism used in continuous venovenous hemodialysis. Because diffusion depends on concentration gradients, it is highly effective at clearing small molecules such as urea, creatinine, potassium, and other electrolytes.