He said covid patients are having respiratory distress and hypoxia (low oxygen levels) before they experience and loss of lung function or lung damage. The current theory being researched is that covid attacks red blood cells and damages hemoglobin which prevents oxygen transport in blood. The lung damage and decreased function doesn't happen until a few days to a few weeks later.

This could explain why symptoms take so long to develop, and why there's such a high mortality rate among patients on ventilators. Push all the oxygen you want into lungs, if hemoglobin doesn't work it's not going anywhere.