[The other face of oxygen (1)].

The other face this two-faced Janus, the oxygen, allows us, by now, to see dimly is the bad, aggressive one shown by its free radicals, should they escape the respiratory machine, or, rather, should they follow the respiratory explosion of leucocytes and macrophage, or the same compromission of the cellular structure. The antioxidative mechanisms, although articulate and quibbled, appear inadequate, also because the oxygen radicals are formed on the outer side of the cellular membrane, and, just in the intercellular space, the defensive protections are extremely poor. If lung deserves a particular status, being the usual point of aggression by the oxygen radicals: here, these active metabolites are produced by its macrophage, but also by its endothelial cells, especially in conditions of hyperoxy; all organs and segments are their targets. Besides the immediate results, which are also caused by oxidizers having a long half-life, there are mediate results, essentially rotating around the arachidonic acid, a sort of multiplicating pin, with its products of the cyclooxygenase line (thromboxane A2, endoperoxides, prostacyclins) and the lipoxygenase line (the wide range of leukotrienes). But not even this "free body", in a position to escape, provided it wants so, the enzymatic control, seems inclined to free itself, what attenuates its bad face, from a basic rule in animal biology, that is, "negative feedback".