A two-stage continuous bioreactor with cell recycle (TSCB) was developed in order to study the behaviour of Saccharomyces cerevisiae in high cell density. Residence time (RTD) and internal age (IAD) distributions of gas, liquid and cell biomass phases have been investigated according to a systemic approach (model identification, parametric study) in order to highlight the complex interactions between hydrodynamics (gas, liquid and solid phases) and biological activity. The noticeable differences between liquid and cell biomass could be explained by the competition between the internal recirculation and dilution rates and described by the dimensionless circulation index, K [3]. A parametric study is reported and discussed. Assumption based on these dimensionless parameters and reduced adaptation time enable to define acceptable and unacceptable working conditions in order to maintain the biological activity of the microorganism (specific rates and viability) as well as identifying optimal working conditions.