\bibitem{Grebenkov05}   D.S.~Grebenkov, M.~Filoche, B.~Sapoval, M.~Felici,
                        {\it Diffusion-Reaction in Branched Structures: Theory and Application to the Lung Acinus},
                        Phys.~Rev.~Lett. {\bf 94}, 050602, 2005.