The partial pressure of $\mathrm{O}_{2}$ in the atmosphere is $160 \mathrm{mm} \mathrm{Hg}$, but when $\mathrm{O}_{2}$ moves into the blood its partial pressure is one third lower than the atmospheric partial pressure. Which structure is it moving through at this partial pressure?
a. alveoli
b. bronchi
c. bronchioles
d. tracheae
e. pharynges