  The spherically symmetric steady accretion of polytropic perfect fluids onto
a black hole is the simplest flow model that can demonstrate the effects of
backreaction. Backreaction keeps intact most of the characteristics of the
sonic point. For any such system the mass accretion rate achieves maximal value
when the mass of the fluid is 1/3 of the total mass. Fixing the total mass of
the system, one observes the existence of two weakly accreting regimes, one
overabundant and the other poor in fluid content.
