Blue arrows indicate the actions of SARS-COV-2. ACE2 is key for SARS-CoV-2 to enter into cells in human organs such as the lung and heart. The injuries of organs and cells are caused by (1) direct SARS-CoV-2 viral infection of cells; (2) systemic inflammation and immune overreactions, including cytokine storms and imbalances in T cells, helper T cells and regulatory T cells; (3) acute respiratory and hypoxia stress, including an altered myocardial demand-supply ratio, plaque rupture and coronary thrombosis due to respiratory failure and hypoxemia; and (4) endothelial cell involvement across vascular beds of different organs in COVID-19 and direct viral infection of endothelial cells and diffuse endothelial inflammation (endotheliitis), which could explain the systemic impaired microcirculatory function in different vascular beds and the resulting clinical sequelae in patients with COVID-19. Red dashed lines indicate the potential mechanisms of the therapeutic and toxic effects of hydroxychloroquine on SARS-CoV-2 and the organs in COVID-19 patients.