Critical care nursing demands understanding of multi-organ pathophysiology and the physiological cascades that drive organ failure. Sepsis, the most common ICU admission diagnosis, begins with a dysregulated host immune response to infection: pathogen-associated molecular patterns activate toll-like receptors on macrophages, triggering a cytokine storm (TNF-alpha, IL-1, IL-6) that causes widespread endothelial dysfunction, capillary leak, vasodilation, and microvascular thrombosis leading to tissue hypoperfusion and lactic acidosis. Acute respiratory distress syndrome (ARDS) involves diffuse alveolar damage with protein-rich edema flooding the alveoli, inactivating surfactant, and causing refractory hypoxemia quantified by the P/F ratio. The nurse in critical care interprets hemodynamic parameters (CVP, MAP, cardiac output, SvO2), manages vasoactive infusions titrated to physiological targets, monitors ventilator waveforms for patient-ventilator dyssynchrony, and coordinates complex interdisciplinary care to prevent secondary organ injury.