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Pathophysiology is the study of the functional changes that accompany disease, describing how normal physiological processes are disrupted and lead to signs, symptoms, and complications. It links mechanisms at molecular, cellular, organ, and systemic levels to clinical presentation, helping to explain why abnormalities arise and persist.

The field integrates concepts from anatomy, physiology, biochemistry, immunology, genetics, and pharmacology. It distinguishes etiology (the cause) from pathogenesis (the process by which disease develops) and emphasizes the functional consequences of disease rather than just structural alterations. Common themes include inflammation and immune dysregulation, metabolic disturbances, hormonal and electrolyte imbalances, tissue injury, and failures of homeostasis.

Examples of pathophysiology include diabetes mellitus, where insulin resistance and beta-cell dysfunction produce hyperglycemia and complications; heart failure, in which reduced cardiac output triggers neurohumoral responses and fluid shifts; acute kidney injury, involving impaired filtration and tubular injury; and sepsis, characterized by a dysregulated host response leading to systemic inflammation and organ dysfunction.

Clinical relevance lies in guiding diagnosis, prognosis, and treatment. By understanding the mechanisms that produce symptoms, clinicians select appropriate tests, interpret results, and target therapies to interrupt or compensate for dysfunctional pathways. Pathophysiology informs medical education and research, aiding hypothesis generation and the development of new interventions.

Overall, pathophysiology serves as a bridge between basic science and patient care, translating what goes wrong in disease into actionable clinical reasoning.