  In order to close the set of relativistic hydrodynamic equations,
constitutive relations for the dissipative fluxes are required. In this work we
outline the calculation of the corresponding closure for the heat flux in terms
of the gradients of the independent scalar state variables: number density and
temperature. The results are compared with the ones obtained using Marle's
approximation in which a relativistic correction factor is included for the
relaxation time parameter. It is shown how including such correction this
BGK-like model yields good results in the relativistic case and not only in the
non-relativistic limit as was previously found by other authors.
