What term describes the stacking of red blood cells at low velocities?

• A. Rayleigh scattering
• B. Rouleaux formation
• C. Huygens principle
• D. Newtonian fluid

Answer: (B)

Rouleaux formation describes red blood cells stacking together when flow is slow. At low velocities, shear forces are weak, so cells can come into contact and plasma proteins like fibrinogen promote adhesion between them. The result is coin-shaped stacks of red cells that resemble rouleaux. This stacking is a classic sign of low-shear conditions and helps explain why the erythrocyte sedimentation rate tends to rise in such situations. Other terms don’t describe this stacking behavior: Rayleigh scattering is about light interaction with tiny particles, Huygens principle relates to wave propagation, and Newtonian fluid refers to a constant-viscosity fluid, while blood behavior under low shear involves cell aggregation rather than simple Newtonian flow.