A numerical dosimetry procedure is applied to a biological tissue in order to evaluate the temperature increase produced by RF exposure. The originality of the proposed method regards the application of the bio-heat equation in a vascularized region considering a discrete blood vessel structure which operates as a cooling system in the biological region. First, the blood structure is assumed to be a hydraulic system where a laminate incompressible fluid flows. By this approach the blood velocity in all vessels are computed. Then, the diffusion-convection heat equation is analyzed in a three dimensional domain by a differential numerical method based on the finite difference method. A simple test configuration is finally analysed.