When the products are formed on the surface they desorb back into the gaseous phase. Transition metals and their compounds are often heterogenous catalysts. Practically every reaction in the human body is carried out through catalysts known as enzymes . From this point there is a large number of ways in which the enzyme mediates the reaction. This is an very active field of research.
Enzymes are fantastically efficient catalysts.
Furthermore, the rate of the reaction in the presence of an enzymes can be modified by inhibitors , depending on the biochemical need of the organism.