The Problem of Assessing Individual Sensitivity and Tolerance to Hypoxia in Animals and Humans

Abstract

The problems of individual sensitivity and tolerance of an organism to environmental exposures refer to most fundamental challenges of physiology being tackled for a long time. Meanwhile, different authors may be at variance in interpreting the terms “sensitivity” and “tolerance”. This review summarizes literature data on the issues of interpreting and assessing sensitivity and tolerance to hypoxia. A special focus is on those studies which compare the initial individual response of an organism to hypoxia and its viability at subsequent stages of hypoxic and other extreme exposures. When resolving these issues, it is necessary to allow for the existence of two opposite life support strategies, active and passive. The first strategy takes advantage of early detecting detrimental exposures and mobilizing energy resources aimed at their avoidance. The second strategy implies saving (instead of mobilizing) energy resources due to hierarchical functional activity limitation (downregulation) which allows an organism to survive exposure as long as possible at relatively low sensitivity and reactivity. The existence of the second strategy disallows unambiguous interpretation of results of hypoxic tolerance assessment based on analyzing functional disorders, such as mental and physical performance decrement and locomotor incoordination, which arise under hypoxic exposures.