The present examination of the relation between the global parameters of spiral galaxies and their morphological type represents rotational velocity V at 25 mag/sq arcsec radius against this radius R for a 56-galaxy sample. Isolines of total mass, angular momentum, and mean surface density, plotted above the V-R diagram for isolated galaxies with circular motion in centrifugal equilibrium, indicate that morphological type is represented by mean surface density, V-squared/R. Taking V-squared/R as representative of morphological type, the empirical Tully-Fisher relation and the dependence of the slope and the zero point on the morphological type can be theoretically deduced. For every galaxy morphological type, there is a critical mass above which a dynamical behavior change occurs.