For oxygen to enter the RBC, it has to pass through squamous epithelium of alveoli, squamous epithelium of capillaries and finally it will reach the RBC. It will enter the squamous epithelium of alveolus through one end and will leave it through the other. Thus, for passing through the squamous epithelium of alveolus, it has to pass through plasma lemma or cell membrane twice. Similarly, to pass through the squamous epithelium of capillary it has to pass through cell membrane twice. Then it passes through the cell membrane of RBC to reach haemoglobin. Thus, it crosses the cell membrane 5 times.