ACE2 and COVID-19: Since ACE2 expression is very high in the lungs, and since diffuse alveolar damage, bronchopneumonia, and alveolar hemorrhage are common in COVID-19 [40], it seems reasonable to hypothesize that the decrease in gas exchange across the alveolar membranes within the lung can alter not just the O2 carrying capacity but also increase metabolic demand for perfusion of the damaged lung.