Is formed from “C‐shaped” cartilaginous rings whose incomplete dorsal aspects are bridged by the trachealis muscle.The trachealis muscle is composed of smooth muscle fibers.

       The tracheal cartilages prevent collapse of the trachea.

       The trachea terminates near the hilus of the lungs as it bifurcates into the left and right principal bronchi.

       The esophagus is located dorsal to the trachea in the cranial and caudal portions of the cervical region and to the left of the trachea in the middle portion of the cervical region.

       The lungs are a paired organ found within the thoracic cavity.

       The left and right lungs are each divided into cranial and caudal lobes by a large fissure called the cardiac notch.

       The right lung also possesses an accessory lobe.

       The lungs receive two forms of blood supply (pulmonary, bronchial).

       The pulmonary circulation involves blood that is delivered to the lungs for the purpose of oxygenation.

       The bronchial circulation involves blood that supplies O2 and nutrients to the lung parenchyma.

       The lungs receive sympathetic nerve fibers from the sympathetic trunks.

       The lungs receive parasympathetic nerve fibers from the vagus nerves.

       These autonomic nerve fibers control smooth muscle and glands within the lung.

       Primary bronchi branch within the lung into lobar (secondary) bronchi, which correspond to the lobation of the lungs.

       Secondary bronchi branch into tertiary bronchi.

       The branching of tertiary bronchi gives rise to the bronchiolar system.

       The bronchiolar system branches into primary, secondary and tertiary components in the same fashion as did the bronchi.

       Tertiary bronchioles give rise to terminal bronchioles, which are the final segment of the conducting components of the respiratory system.

       Lumen diameter of the airway decreases with each incidence of branching.

       Is the primary epithelial‐type lining the conducting components of the respiratory system.

       Classified as ciliated, pseudostratified columnar epithelium with goblet cells.

       The goblet cells secrete mucus, which serves to trap inhaled particles.

       Cilia on columnar cells move this mucus layer out of the respiratory tract.

       This epithelium changes to simple columnar and eventually to simple cuboidal epithelium as the conduction system branches toward the gas exchange components.

       Terminal bronchioles divide into several alveolar ducts.

       Alveolar ducts end in alveolar sacs.

       Alveolar sacs are dilatations of the airway lined with hemispherical chambers called alveoli.

       Gas exchange occurs between the blood and inspired air across the wall of the alveolus.

       Alveoli are lined with two cell types classified as type I and type II pneumocytes.

       Type I pneumocytes are simple squamous cells with extremely thin processes.

       They are more abundant than type II pneumocytes.

       They are a component of the blood–air barrier.

       Type II pneumocytes are cuboidal cells.

       They function to secrete pulmonary surfactant.

       Pulmonary surfactant stabilizes alveoli during inflation by keeping them uniform in size thus preventing collapse.

       The partition between inspired air and the pulmonary capillary system.

       Formed by the processes of type I pneumocytes, a small amount of connective tissue and capillary endothelial cells.

       Gas molecules must cross this barrier to enter or exit the blood vascular system.

       The diaphragm is the primary muscle of respiration.It is innervated by the phrenic nerves, which originate from the 4th, 5th, and 6th cervical spinal nerves pairs.Contraction produces the negative pressure within the thoracic cavity involved with inspiration.

       Scalenus, serratus dorsalis cranialis, and intercostal muscles are also involved with inspiration.

       Serratus dorsalis caudalis muscles are involved with expiration.

      1 Froydenlund, T.J., Dixon, P.M., Smith, S.H. et al. (2015). Anatomical and histological study of the dorsal and ventral nasal conchal bullae in normal horses. Vet. Rec. 177: 542.

      2 Johnson, L., Montgomery, J.B., Schneider, J.P. et al. (2014). Morphometric examination of the equine adult and foal lung. Anat. Rec. 297: 1950–1962.

      3 Pekarkova, M., Kircher, P.R., and Konar, M. (2009). Magnetic resonance imaging anatomy of the normal equine larynx and pharynx. Vet. Radiol. Ultrasound. 50: 392–397.

       The primary function of the respiratory system is the transport of O2 from the environment to