Chapter 19 In mammals, the respiratory system consists of a gaseous conducting portion and a site where exchange of respiratory gases takes place. The conducting portion comprises the nostrils, nasal cavities, paranasal sinuses, pharynx, larynx, trachea, bronchi and bronchioles. The structures involved in gaseous exchange include the respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli. Development of the nostrils, nasal cavities and paranasal sinuses are discussed in association with the development of the head. The respiratory primordium develops as a ventral groove in the floor of the foregut at the level of the fourth pharyngeal arch. The groove, referred to as the laryngo‐tracheal groove, deepens and forms an elongated outgrowth, which extends in a caudal direction and becomes separated from the foregut proper by the formation of two tracheo‐oesophageal grooves, one on the left side and one on the right side (Fig 19.1). When these grooves meet and fuse they form a septum, the tracheo‐oesophageal septum. The septum separates the dorsal portion of the foregut, the primordium of the oesophagus, from the ventral portion, the primordium of the laryngo‐tracheal tube. The part of the foregut cranial to the tracheo‐oesophageal septum becomes the primordial pharynx. Figure 19.1 Lateral views, A, B, C and D and a ventral view, E, of sequential stages in the formation of the respiratory diverticulum from the foregut. The larynx, which develops from the cranial region of the laryngo‐tracheal tube, communicates with the primordial pharynx. The epithelium of the larynx is derived from foregut endoderm, while the cartilages and muscles of the larynx develop from pharyngeal arch mesenchyme. The mesenchyme of the left and right fourth pharyngeal arches gives rise to two swellings which develop lateral to the laryngo‐tracheal groove, the primordia of the arytenoid, thyroid and cricoid cartilages. As these arytenoid swellings develop, they convert the cranial end of the slit‐like laryngo‐tracheal groove into a T‐shaped aperture, the glottis. A single swelling which develops from the mesenchyme of the left and right third and fourth pharyngeal arches cranial to the developing glottis, referred to as the epiglottic swelling, gives rise to the epiglottic cartilage. The intrinsic laryngeal muscles, which develop from myoblasts in the fourth and sixth pharyngeal arches, are innervated by branches of cranial nerves X and XI. The crico‐thyroid muscles, which are derived from the fourth pharyngeal arches, are innervated by the cranial laryngeal branches of cranial nerve X. The other intrinsic laryngeal muscles, which develop from the sixth pharyngeal arches, are innervated by the recurrent laryngeal nerves, which contain fibres from cranial nerves X and XI. As the laryngeal cartilages develop, the epithelial lining of the larynx forms a left and a right diverticulum in the lateral walls of the larynx. Cranial vestibular and caudal vocal folds of the larynx, composed of mucosal, connective and muscular tissue, form the boundaries through which the diverticula project laterally. These diverticula, referred to as laryngeal ventricles, are present in humans, horses, dogs and pigs but are not present in ruminants and cats. The laryngo‐tracheal tube, which consists of an inner endodermal lining and an outer layer of splanchnic mesoderm, elongates. Bifurcation of the blind end of this tube results in the formation of two bronchial buds, the primordia of the left and right lungs. The portion of the laryngo‐tracheal tube from the larynx to the bifurcation gives rise to the trachea. The endodermal lining of the tube gives rise to respiratory epithelium and the mucosal and submucosal glands of the trachea. The connective tissue of the tracheal lamina propria, cartilaginous rings, smooth muscle, blood vessels and lymphatic vessels of the tracheal wall are all of mesenchymal origin. Each bronchial bud enlarges, forming a left and a right principal bronchus. These bronchi elongate caudally, between the developing oesophagus dorsally and the developing heart ventrally. Unlike the left bronchus, which deviates from the midline in a lateral direction, the right bronchus deviates to a lesser extent, and consequently the right lung is more prone to inhalation pneumonia than the left lung. In domestic animal species, with the exception of horses, the right principal bronchus gives off four secondary or lobar bronchi which later give rise to the cranial, middle, accessory and caudal lobes of the right lung. A middle lobe is not present in the equine right lung. The right human bronchus gives off three branches and so an accessory lobe is not present. The left principal bronchus in domestic animals gives off two lobar bronchi, which in turn give rise to the cranial and caudal lobes. In ruminants and pigs, the right cranial lobar bronchus, which branches off the trachea, is referred to as the tracheal bronchus. During further development, the lobar bronchi give off tertiary or segmental bronchi which supply large areas within the lobes known as bronchopulmonary segments. The number of bronchopulmonary segments within a particular pulmonary lobe in a given species is usually constant but the number of bronchopulmonary segments within a given pulmonary lobe is subject to significant variation among species. Stages in lung development, showing the formation of principal and lobar bronchi and their branches, are illustrated in Figure 19.2. Figure 19.2 Ventral views of sequential stages in lung development showing the formation of the principal bronchi and the origins of lobar bronchi and their branches (A to C). The segmental bronchi undergo 14 to 18 bifurcations with the diameter of each succeeding branch becoming progressively smaller until a diameter approaching 0.5 mm is reached; structures of this size are referred to as bronchioles. The final bronchiolar branches, which represent the termination of the exclusively conducting portion of the respiratory system, are referred to as terminal bronchioles. Each terminal bronchiole subdivides into two or more respiratory bronchioles which are structurally similar to terminal bronchioles except that their walls give off numerous saccular alveoli where gaseous exchange takes place. The respiratory bronchioles are transitional zones between the conducting and respiratory regions of the respiratory system. These respiratory bronchioles give off a number of alveolar ducts from which alveolar sacs and alveoli arise. Respiratory bronchioles are present in humans and carnivores. In horses, cattle, sheep and pigs, respiratory bronchioles are either absent or poorly developed and the alveolar ducts arise directly from the terminal bronchioles. Based on histological features, the development of the lungs may be divided into five arbitrary stages, namely the embryonic, pseudoglandular, canalicular, terminal sac and alveolar stages (Fig 19.3
Respiratory system
Formation of the larynx
Trachea, bronchi and lungs
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