The answer: 1.covid infects lungs after entry into human body 2.it decreases ventilation perfusion ratio (0.8 normal) 3.after infection alveoli get fibrosis ,as a result elasticity or stretchbility may decreased 4.breathlessness results All the above factors directly interferes with oxigenation of RBCs in blood So oxygen therapy (ventilation 100%pure oxygey) is treatment for covid19.