I"m at the mouth where there are others just like me. 21% of the air is filled with other oxygen molecules. I first travel through the conducting zone where there is no diffusion, only conduction of air. I see that at the trachea there is much connective tissue such as collagen, but not as much smooth muscle. There is much turbulence in the first six bifurcations due to the high velocity that i and other gas molecules are traveling at. As i continue through the bifurcations, i notice that the bronchioles decrease in connective tissue and increase in smooth muscle. There has been humidification on the journey so far. Now there is a greater concentration of myself and other o2 molecules: midway through inspiration phase, i enter an alveolus at bifurcation 20. The po2 has decreased from 150 mm hg to around 100 mm hg here in the alveolus.