Pathophysiology of reactive oxygen species (ROS)

Reactive oxygen species (ROS) are context-dependent mediators that function as second messengers at low, localized flux and as drivers of damage when production overruns buffering capacity. Outcomes are dictated by source identity, subcellular compartment and pulse kinetics—the “where–when–how much” rule. We synthesize advances (2015–2025) across principal generators—mitochondrial electron transport, NADPH oxidases, xanthine oxidoreductase and ER/peroxisomal oxidoreductases—to show how compartmental H2O2 microgradients encode reversible cysteine signaling, while iron-rich niches pivot chemistry toward peroxynitrite, Fenton-derived ·OH, lipid peroxidation and regulated cell death (apoptosis, ferroptosis, parthanatos). We integrate these mechanisms with endothelial dysfunction, innate immune priming, ECM remodeling and barrier failure across cardiovascular, metabolic, neurodegenerative, oncologic, pulmonary, renal and critical-illness contexts, emphasizing crosstalk with RNS/RSS and iron metabolism as key modulators. Methodologically, we advocate species-resolved, compartment-aware assessment—e.g., DHE → 2-OH-E⁺ HPLC for O2·−, targeted HyPer/roGFP-Orp for H2O2 and peroxiredoxin redox state—embedded in composite panels that pair flux with damage footprints and iron/ferroptosis metrics for attribution and trial guidance. Therapeutically, we argue against indiscriminate antioxidant loading in favor of node-specific, compartment-targeted modulation (NOX/NOS tuning, mitochondrial QC/RET tempering, ER redox control, iron/ferroptosis management, calibrated sulfur-axis support), implemented as time-staged sequences and titrated to biomarkers. Clarifying which species arise, where and when, reframes ROS from generic toxicity to precision redox modulation with translational impact.