  We investigate the atomic (HI) and molecular (H_2) Hydrogen content of normal
galaxies by combining observational studies linking galaxy stellar and gas
budgets to their host dark matter (DM) properties, with a physically grounded
galaxy formation model. This enables us to analyse empirical relationships
between the virial, stellar, and gaseous masses of galaxies and explore their
physical origins. Utilising a semi-analytic model (SAM) to study the evolution
of baryonic material within evolving DM halos, we study the effects of baryonic
infall and various star formation and feedback mechanisms on the properties of
formed galaxies using the most up-to-date physical recipes. We find that in
order to significantly improve agreement with observations of low-mass galaxies
we must suppress the infall of baryonic material and exploit a two-phase
interstellar medium (ISM), where the ratio of HI to H_2 is determined by the
galactic disk structure. Modifying the standard Schmidt-Kennicutt star
formation law, which acts upon the total cold gas in galaxy discs and includes
a critical density threshold, and employing a star formation law which
correlates with the H_2 gas mass results in a lower overall star formation
rate. This in turn, allows us to simultaneously reproduce stellar, HI and H_2
mass functions of normal galaxies.
