Exchanges on the disc scale. The disc is the main component. It is fed by the fresh gas coming from the large scales (Ṁ_ff and Ṁ_cool). SNe associated with the stellar population of the disc generate ejecta Ṁ_wind,sn. In a disc, dynamics and gravity are in balance. Under some conditions, giant clumps can be formed () and/or disrupted (). At any given time, a given fraction of the total disc mass is in those clumps. The clumps migrate () and feed a pseudo-bulge component (ℬ).