  Cellular cortex, which is a highly viscous thin cytoplasmic layer just below
the cell membrane, controls the cell's mechanical properties, which can be
characterized by a hydrodynamic length scale $\ell$. Cells actively regulate
$\ell$ via the activity of force generating molecules, such as myosin II. Here
we develop a general theory for such systems through coarse-grained
hydrodynamic approach including activity in the static description of the
system providing an experimentally accessible parameter and elucidate the
detailed mechanism of how a living system can actively self-regulate its
hydrodynamic length scale, controlling the rigidity of the system. Remarkably,
we find that $\ell$, as a function of activity, behaves universally and roughly
inversely proportional to the activity of the system. Our theory rationalizes a
number of experimental findings on diverse systems and comparison of our theory
with existing experimental data show good agreement.
