Oxidative Stress and Free Radicals: Biochemical Mechanisms and Clinical Implications

Background: Free radicals, including reactive oxygen and nitrogen species (ROS/RNS), are chemically reactive molecules with unpaired electrons. They are generated endogenously through metabolic processes like mitochondrial respiration and exogenously from sources such as UV radiation and pollution. At low concentrations, they act as crucial signaling molecules in physiology; however, an imbalance between their production and the body's antioxidant defenses leads to oxidative stress. Aim: This article aims to comprehensively review the biochemical mechanisms of free radical formation, their dual roles in physiological signaling and pathological damage, and their clinical implications in a wide spectrum of human diseases. Methods: The review synthesizes existing scientific literature to detail the sources of radicals (e.g., mitochondria, NADPH oxidase), their chemical reactions (e.g., lipid peroxidation, protein carbonylation, DNA oxidation), and the multi-layered antioxidant defense systems that counteract them. Results: Oxidative stress is a key pathogenic driver in cardiovascular diseases (atherosclerosis, hypertension), neurodegenerative disorders (Alzheimer's, Parkinson's), cancer, diabetes, and chronic inflammatory conditions. It contributes to disease by damaging lipids, proteins, and DNA, and by dysregulating cellular signaling pathways. Conclusion: The biological impact of free radicals is context-dependent, balancing essential signaling against destructive potential. Therapeutic strategies must therefore be nuanced, aiming to mitigate pathological oxidative damage without disrupting beneficial redox signaling, moving beyond indiscriminate antioxidant supplementation.