Exchanges on the bulge scale. The bulge is considered as a passive component not fed by fresh accreted gas. A bulge (ℬ) is formed during a merger or by clumps migrations from the disc (). An SMBH can evolve in the bulge. It is fed by a gas reservoir (the torus, ) with an accretion rate Ṁ_acc. The torus is fed during a merger or by the clump migration process. The activity of the SMBH generates ejecta (Ṁ_•,out).