The primary functions of the respiratory system are 1) smell, 2) air conduction, and 3) the exchange of oxygen and carbon dioxide between the animal and the environment, referred to as respiration. Respiration occurs at the air-blood interface, which lies at the most distal part of the respiratory tract. Although this exchange of oxygen and carbon dioxide is the ultimate functional goal of the respiratory tract, air must first be transported to this air-blood interface. Therefore, the two crucial functions of the respiratory tract are air conduction and respiration. The importance of considering these two roles is apparent during respiratory diseases in which interference with either air conduction (such as obstructive respiratory disease) or the air-blood interface (such as bacterial bronchopneumonia) result in life-threatening hypoxia.
Clinical discussion of the respiratory tract is generally divided into the upper respiratory tract and lower respiratory tract. The upper respiratory tract is comprised of the nasal cavity and nasopharynx, whereas the lower respiratory tract includes the larynx, trachea, bronchi, bronchioles, and alveoli. Histologically, there is much overlap between the upper and lower respiratory tract, and therefore, the distinction of upper vs lower respiratory tract is not reinforced in the discussion to follow. Instead, regions of air conduction and gas exchange have important functional and histologic distinctions, and this categorization will be utilized for the purpose of the discussion to follow.