By adopting the thin flux tube model as representative of spatially unresolved magnetic elements in the photosphere of the Sun, we identify an adequate set of parameters for the model and derive the corresponding response functions (RFs) which inform about the variation of the emergent Stokes spectrum when such parameters are perturbed. We numerically compute these RFs for reasonable values of the parameters and explore their main properties. It turns out that the RFs at a given height often depend on the state of layers above as a result of the constraints imposed by flux tube geometry. As a whole, RFs can be classified into two well defined groups: one containing RFs dominated by local effects and another which contains RFs governed by non-local effects (i.e., contributions coming from layers other than that where the perturbation takes place). In particular, the RFs to the temperature, line of sight velocity and microturbulence of both the internal and the external atmospheres belong to the first group, while the RFs to the magnetic field strength, external gas pressure and radius of the tube at the base of the atmosphere must be ascribed to the second group. The RFs presented in this paper constitute a first step for the inversion of Stokes spectra from faculae and the network.