[Review of the Respiratory System]

19. How many times does an oxygen molecule cross a plasma membrane when moving from inside an alveolus to the hemoglobin of a red blood cell?

A) Two

B) Three

C) Four (The walls of the alveolus is one cell thick (2 membranes) while the oxygen must also cross one capillary cell (2 additional membranes) for a total of 4 membranes.

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Slichter