The lower respiratory system begins its development during the 4th week as an outgrowth of the ventral wall of the foregut (respiratory diverticulum). The endodermal lining of the respiratory diverticulum gives rise to the epithelial lining of the larynx, trachea, bronchi and alveoli. The cartilaginous and muscular components of the trachea and lungs are derived from the surrounding splanchnic mesoderm. The diverticulum elongates in the caudal direction and soon becomes separated from the foregut by the esophagotracheal septum. The ventral portion forms the trachea and lung buds, while the dorsal portion forms the esophagus. Initially wide communication between the respiratory diverticulum and foregut is transformed into a T-shaped slit. The cartilaginous (thyroid, cricoid, and arytenoid cartilages) and muscular components of the larynx are derived from the mesoderm of the fourth and sixth branchial arches.

1. Stomodeum
2. Pharyngeal gut
3. Thyroglossal duct
4. Tracheobronchial diverticulum

1. Lung bud
2. Heart
3. Common cardinal vein
4. Aorta
5. Pleuropericardial membrane
6. Pericardial cavity

1. Pleural cavity
2. Lung
3. Visceral pleura
4. Aorta
5. Pericardial cavity
6. Parietal pleura
7. Superior vena cava
8. Fibrous pericardium
9. Heart

The proliferation of the laryngeal epithelium results in a temporary occlusion of the lumen, while the subsequent recanalization forms a pair of lateral recesses, the laryngeal ventricles. During the 4th week, the respiratory diverticulum divides into left and right lung buds, which subsequently divide into two and three main bronchi, respectively. The lung buds expand throughout a part of coelomic cavity called the pericardioperitoneal canal. This canal will be separated from the peritoneal and pericardial cavity by the pleuroperitoneal and pleuropericardial membranes, respectively, thus forming the primitive pleural cavities. During further development the main bronchi divide approximately 17 times, forming smaller canals (canalicular phase). Additional 6 divisions happen after the birth.

1. Foregut
2. Esophagotracheal septum
3. Respiratory diverticulum

1. Pharynx
2. Lung buds
3. Trachea
4. Esophagus

1. Lung buds
2. Trachea

1. Right upper lobe
2. Left upper lobe
3. Right lower lobe
4. Left lower lobe
5. Right middle lobe
6. Splanchnic mesoderm
7. Bronchial buds
8. Visceral pleura