In order for oxygen to diffuse from the alveolar air spaces to the site of its binding to hemoglobin, it must diffuse across how many plasma membranes?

The minimum gas diffusion barrier in the alveolar septum includes the highly flattened cytoplasm of a Type I pneumocyte, the fused basement membrane, the highly flattened cytoplasm of an endothelial cell making up the capillary wall, the blood plasma and finally the plasma membrane of the erythrocyte. There is a plasma membrane on each side of the flattened Type I pneumocyte and each side of the flattened endothelial cell. Therefore, there are a total of five (5) plasma membranes that must be crossed in order for the oxygen molecule to diffuse from the alveolar air space to the hemoglobin inside the cytoplasm of the erythrocyte (RBC). Two contributed by the Type I cell, 2 contributed by the endothelial cell and one by the erythrocyte.