With regards to the transportation of oxygen and carbon dioxide, the goal is to move oxygen in and carbon dioxide out of the bloodstream through the steady breathing pattern of inhalation and exhalation.
The inhale on all upward movements is when the air passes through the nasal passages – where particulate impurities are removed and air is humidified. This then passes into the alveoli through the trachea that branches into bronchi that further split into bronchioles.
The oxygen is diffused into the bloodstream through the capillaries present in the alveolar and the carbon dioxide diffused out, which is expelled through the same path air takes into the lungs when an exhale occurs on a downward movement.