The lower respiratory system includes the trachea, lungs, bronchi and bronchioles. Of these structures, the bronchioles are of special importance since they contain the alveoli, structures surrounded by veins and arteries of small thickness that allow the passage of gases between the blood of the circulatory system and the cavity of the alveoli. Specifically, oxygen (O2) and carbon dioxide (CO2) gases are exchanged, O2 enters the body and is transported by hemoglobin to other parts of the body, while CO2 is exhaled out of the body. Therefore, the answer is D.