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Biological fluid dynamics studies fluid flows generated by and acting on organisms, biological cells, tissues and biological transport networks. Examples include blood flow through microcirculation, cytoplasmic streaming within a biological cell, flagellum propulsion and bioconvection in swimming suspensions.

The Reynolds number $\mathrm{Re}=\rho UL/\mu$ compares inertia with viscous forces, using density $\rho$, dynamic viscosity $\mu$, speed $U$ and length $L$. A microscopic swimmer can operate in Stokes flow, where viscous forces dominate and motion responds to the instantaneous force balance. Larger swimming animals can instead transfer appreciable momentum to the surrounding fluid, as modeled by Lighthill elongated-body theory. ^[ref]

Transport introduces another comparison: the Péclet number $\mathrm{Pe}=UL/D$ measures advection against diffusion with diffusion coefficient $D$. Thus the same flow can move slowly enough to have negligible inertia, yet carry a slowly diffusing substance effectively. Choosing the appropriate dimensionless quantities connects biological scale to the governing mechanical and transport models.