**3. Reactive oxygen species and reactive nitrogen species and their effects on biological macromolecules and organs**

Reactive oxygen species / reactive nitrogen species (ROS/ RNS) are constantly being formed in living organisms (Ceconi *et al.*, 2003). In the course of oxygen metabolism, 1- 5% of all inhaled oxygen becomes ROS (Berk, 2007). Endogenously, ROS are produced from various sources such as mitochondria, activated macrophages and leucocytes, oxidase enzyme (NADPH), cyclo-oxygenase and lipoxygenase (Zalba *et al.,* 2006). Reactive oxygen species have oxidation ability and are classified either as free radicals (superoxide anion O2 **.**- , hydroxyl radical OH**•**, nitric oxide NO) or as non-free radicals (hydrogen peroxide H2O2, peroxynitrite ONOO- ) (Higashi *et al*., 2006). Previous studies have shown the involvement of ROS in physiological and pathophysiological conditions (Fortuño *et al*., 2005; Berk, 2007; Heistad *et al*., 2009). At low concentrations, ROS are involved in normal cell signaling pathways (smooth muscle and endothelial cell growth, apoptosis and survival) and in the remodeling of vessel walls (Fortuño *et al*., 2005; Heistad *et al.,* 2009). At high concentrations, ROS are identified as harmful compounds and constitute an important risk factor for the development of many diseases such as cardiovascular diseases (Maxwell & Lip, 1997; Heistad *et al*., 2009).
