We propose a unified model for viscous and kinetic energy losses in a discrete tube model of the vocal system including the glottis. In this model, a lossless Bernoulli flow is assumed at each transition between two tube sections if the downstream section has a smaller diameter than the upstream section, and otherwise the recovery of a fixed fraction of the dynamic pressure. For viscous losses, we propose a general equation according to which the pressure drop within a tube section is inversely proportional to a certain power of its cross-sectional area. The parameters of the model were adjusted to reproduce the results of measurements with physical replicas of the glottis and the vocal tract. The best agreement with the experimental data was achieved when 29% of the dynamic pressure were recovered at tube expansions, and when the viscous losses were proportional to the tube area to the power of -2.9. These results may improve articulatory speech synthesis.