[Normoxia ?]

Under the aspect of tissue O (2)-supply anatomical and functional features within the range of the microcirculation are described. With regard to "Normoxia?" the term "critical pO (2)" is introduced, its functional role discussed and data from earlier studies are presented. Recent investigations on the dependence of the accumulation of HIF's (Hypoxia inducible factors) which increase tissue O (2)-supply and lower O (2)-demand, have shown that HIF-accumulation increases steeply below a tissue pO (2) of 20 mm Hg, thus in a pO (2) range where earlier investigators determined their pO (2)(crit.) data. Apparently, the the pO (2)(crit.) of a tissue marks the boundary between Hypoxia and Normoxia. Under normoxic conditions O (2)-consumption is limited by tissue O (2)-demand, under hypoxic conditions consumption is limited by O (2)-supply.