The respiratory tract is a passageway for air between the external environment and the alveoli (air sacs) of the lungs.There are two gases that are exchanged during respiration. Carbon dioxide diffuses from the blood into the alveoli while the other gas, oxygen, diffuses from the alveoli into the blood.Receptors located in the superior regions of the nasal cavity called olfactoryreceptors detect odors as air moves across them.The vocal cords of the larynx (voicebox) vibrate as air moves across them to produce sound. Sounds then resonate in upper respiratory structures.