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Lung development is separated into several phases starting at the embryonic stage. This state begins several weeks into conception and is the formation of the lung bud and initial branches of the airways. The lung is a foregut derivative and is recognizable at 25 days of conception as the laryngotracheal groove. If this groove fails to close, it can lead to tracheoesophageal fistulas. The lung divides into the five primordial divisions (two on the left, three on the right) during this stage.

The pseudoglandular stage is the division of the lung into the large conducting airways, the trachea, the segmental and the subsegmental bronchi. This bronchial tree down to the terminal bronchioles is complete by the seventeenth week of gestation. The canalicular stage is the completion of the small conducting airways and the branching of the terminal bronchioles as well as vascularization, which occurs in the second trimester. The saccular stage starts at 24 weeks and is the phase at which the alveolocapillary membrane is created and the terminal air sacs form. This stage goes until prior to term gestation. Overlapping with this stage is the alveolar phase, which begins at 36 weeks of gestation and completed postnatally. During this phase, the capillary networks proliferate allowing for establishment of the large surface area for gas exchange. The arterial smooth muscle formation continues until late adolescence. Alveolar formation continues after birth (0–50 million alveoli) to age 8–10 years (>300 million alveoli) until growth of chest wall ceases.

Other anatomical features develop in conjunction with the lung, such as the chest wall and diaphragm. By the fourth week of gestation the ventral body wall forms from the mesoderm and the overlying ectoderm that close and meet midline. Failure of closure results in thoracic midline defects (ectopia cordis, sterna cleft) or abdominal wall defects (omphalocele, bladder exstrophy). The sternum is formed by the mesenchyme in the sixth week and fuses by the 10th week. Ossification of the ribs begins in the seventh week; the ribs are initially horizontal and then slope down until taking the adult position at 10 years of age. Therefore, in infancy, the horizontally placed ribs reduce the effectiveness of the bucket handle movement in increasing intrathoracic volume.

The diaphragm is complete by closure of the pleuroperitoneal canal closure at 8–10 weeks of gestational age. Failure of closure results in herniation of abdominal contents (stomach, intestine and/or liver) into the thoracic cavity resulting in ipsilateral pulmonary hypoplasia and to some degree also affects development of the lung on the contralateral side secondary to a mediastinal shift.

The intercostal and diaphragmatic muscles reach adult configuration at approximately 2 years of age. In infancy, any respiratory obstruction leads to early fatigue and increased work of breathing (WOB) secondary to the paucity of type 1 (marathon) slow twitch—highly oxidative ventilatory muscles used for prolonged activity and repeated exercise. ...

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