Introduction: Physicochemical Properties and Physiological Roles of ROS/Oxidative Eustress and Distress

Reactive species (RS) have long been known as undesired compounds to be avoided at all costs. This view has changed with the realization that RS, which are constantly produced in all cells, are important actors in physiology. RS can regulate several cellular pathways, having a key role in the homeostasis of all organisms. Cells contain multiple signal transduction pathways coupling the redox status to specific regulations of gene expression. Excessive RS production is considered to cause multiple changes in cells that can evolve into pathological conditions. In turn, these changes accelerate the generation of RS and increase oxidative modifications of lipids, DNA, and proteins in various tissues. This chapter covers medical terminology and definitions needed for the comprehension of the processes involving RS. It provides a comprehensive review on the basic functions of RS and their effects on biological molecules and physiological events. In addition, the chapter deals with general reactions involving RS. The chemistry of free radicals (FR), a matter of contemporary interest, is also addressed. The origin of these species and their connection with the dysfunction of cell constituents (oxidative eustress and distress) is described. Other aspects covered in this chapter are the physicochemical properties of RS and FR and their toxic effects.