The respiratory membrane is what separates the inside of the alveoli from the blood. It consists of six layers that gasses must cross to enter the blood from the alveoli or vice versa. Imagine molecules of oxygen and carbon dioxide that must pass through the following six layers.

1) fluid covering the inside of the alveoli (surfactant)

2) squamous cells (alveolar epithelium) lining the alveoli

3) basement membrane of alveolar epithelium

4) small space between two basement membranes (interstitial space)

5) basement membrane of capillary endothelium

6) squamous cells that make up the wall of the capillaries (capillary endothelium)