A general review of the principal parameters influencing the helium and hydrogen distributions is presented for several physical conditions which are responsible for the upper atmospheric structure. The main differences between the helium and the hydrogen problems are pointed out. In particular, the effect of vertical transport is not similar for the two components. Helium is mainly dominated by the eddy diffusion coefficient whereas, in the case of hydrogen, the vertical flow is the lead ng factor. It is also shown how eddy diffusion can play a role in the structure of the helium bulge which is now very well confirmed by satellite observations.