A membraneless separation device can be applied to a variety of treatments, such as the ultrafiltration of blood for a patient with end stage renal disease. An ultrafiltration device can include a membraneless separation device, which separates an incoming blood flow into a substantially cytoplasmic body-free plasma flow and remaining fraction, and a dialysate-free second stage, which selectively removes excess fluid, toxins and other substances from the plasma flow and returns the processed plasma to the membraneless separation device. A treatment protocol can include ultrafiltering blood of a patient using the ultrafiltration device and performing a secondary treatment on the blood of the patient at a reduced frequency compared to the ultrafiltering. The membraneless separation device can also be applied to treatment, analysis, and/or exchange of plasma from blood, or combined with conventional dialyzers to perform dialysis on a cytoplasmic body-free plasma fraction.
