Clinical meaning
COVID-19 is the disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a positive-sense single-stranded RNA virus belonging to the Betacoronavirus genus. The virus emerged in late 2019 and was declared a pandemic by the WHO in March 2020. Understanding the pathophysiology remains essential for clinical nursing practice as COVID-19 has become an endemic respiratory illness.
SARS-CoV-2 enters human cells through the angiotensin-converting enzyme 2 (ACE2) receptor, which is widely expressed on type II alveolar pneumocytes, endothelial cells, cardiac myocytes, enterocytes, and renal tubular cells. The viral spike protein binds to ACE2, and the serine protease TMPRSS2 facilitates viral-cell membrane fusion, allowing viral RNA entry. ACE2 distribution explains the multi-organ tropism of SARS-CoV-2.
In the respiratory tract, the virus initially infects upper airway epithelial cells, causing symptoms similar to the common cold (rhinorrhea, sore throat, cough). In most individuals, the immune response contains the infection at this stage. However, in a subset of patients, the virus spreads to the lower respiratory tract, infecting type II alveolar pneumocytes. These cells are critical for surfactant production and alveolar repair. Their destruction triggers inflammatory cascades that recruit neutrophils and macrophages, causing diffuse alveolar damage (DAD) - the pathological hallmark of COVID-19 pneumonia and the basis for acute respiratory distress syndrome (ARDS).
The cytokine storm (hyperinflammatory response) is a key driver of severe COVID-19. Dysregulated release of pro-inflammatory cytokines (IL-6, IL-1beta, TNF-alpha, interferon-gamma) and chemokines causes systemic inflammation, endothelial activation, and capillary leak. This mirrors the inflammatory cascade seen in sepsis and contributes to multi-organ failure.