ACE2 and acute respiratory distress syndrome: In addition to pneumonia and ARDS, severe COVID-19 mainly involves multiple extrapulmonary organs and systems such as cardiovascular, renal, gastrointestinal, and hepatobiliary systems, as well as hematological, neurological, endocrine and metabolic systems, etc. SARS-CoV-2 may directly invade the host cells of multiple organs through the ACE2 that is widely distributed in various human tissues and TMPRSS2 or other possible entry routes.