I saw that with final kidney failure, the seric creatinin curve had usually an exponential feature, is there some statical analysis proving that? thnx

Don't need: Many of our organs have a huge reserve and the consequence would not show until over 70% or more of the organ has shut down. For example, many can live with just one kidney or one lung with little restriction in lifestyle. When the system fails, it is near the end run. Let's use your bank account as an example. If you overspends regularly, you can still buy things until the last dollar has gone.

Here you go: An old kidney international classic from 1985 http://www.Nature.Com/ki/journal/v27/n6/abs/ki1985101a.Html.