What best describes the human respiratory system?

The respiratory system is composed of moist passageways that permit air to flow from the external environment to the lungs. In the lungs, blood vessels carrying CO2 on hemoglobin molecules releases the CO2 at the alveoli in exchange for O2. The CO2 is breathed out through the lungs, while the fresh O2 travels through the bloodstream to the cells in all parts of the body. (The first sentence is an extremely simplified and narrow description of the respiratory system, which is much more complicated.)