  We address the question of the relations between the black hole's mass, the
accretion rate, the bolometric luminosity, the optical luminosity and the size
of the Broad Line Region (BLR) in Active Galactic Nuclei, using recent
observational data obtained from monitoring campaigns. We show that a standard
accretion disc cannot account for the observed optical luminosity, unless it
radiates at super-Eddington rates. This implies the existence of another,
dominant emission mechanism in the optical range, or a non standard disc (non
stationary, ADAF and/or strong outflows). Narrow Line Seyfert 1 galaxies
(NLS1s) are most extreme in this context: they have larger bolometric to
Eddington luminosity ratios than Broad Line Seyfert 1 (BLS1s), and most likely
a larger "non disc" component in the optical range. From realistic simulations
of self-gravitating $\alpha$-discs, we have systematically localized the
gravitationally unstable disc and shown that, given uncertainties on both the
model and observations, it coincides quite well with the size of the BLR. We
therefore suggest that the gravitationally unstable disc is the source which
releases BLR clouds in the medium. However the influence of the ionization
parameter is also required to explain the correlation found between the size of
the BLR and the luminosity. In this picture the size of the BLR in NLS1s
(relative to the black hole size) is larger (and the emission line width
smaller) than in BLS1s simply because their Eddington ratio is larger.
