Observations of transition region emission lines reveal the presence of redshifts in lines formed from the top of the chromosphere up to temperatures of about 250000 K and blueshifts for temperatures above that. However, it is doubtful that the apparent large downward flows in the lower transition region represent an emptying of the corona, so some mechanism must be responsible for maintaining the mass balance between the lower atmospheric layers and the corona. We use a 3D-MHD code, Bifrost, which includes non-LTE radiative transfer in the photosphere and lower corona, optically and effectively thin radiation in the upper chromosphere, transition region and corona, conduction along the magnetic field lines and heating through the ohmic dissipation of tangled magnetic field lines to simulate the solar atmosphere. The motion of material originally in the transition region is tracked. We report on the coronal mass balance and the mass flow mechanism for conditions typical of the quiet sun.