We have proposed a wearable, indirect dialysis system that is permanently connected to the circulation. It employs direct (membraneless) diffusive contact between blood and a miscible “sheathing” fluid to extract water and toxins from the blood. The most important factor in such a system is preventing blood cells from entering the sheath fluid. Past research shows the tendency for such cells to migrate to the center of a flowing stream, and it is believed that blood cells can be retained with proper system design and operation. We have used a combination of computational fluid dynamic simulations (Fluent) and experimental analysis to show that red blood cells should remain in the blood stream in high proportion. However a full understanding of the conditions to minimize cell migration from the center stream is unclear. The purpose of this research is to quantify and model the major conditions that affect erythrocyte migration when blood flows in direct contact with 2 miscible blood-free streams. Also, obtaining basic knowledge of erythrocyte behavior is vital to microfluidic devices designed around blood flows.