A thorough understanding of the differences in normal pulmonary function, respiratory mechanics, and gas exchange at different stages of life is essential to the practice of anesthesiology.
FETAL DEVELOPMENT OF THE PULMONARY SYSTEM
The embryologic development of the respiratory system is briefly summarized in this section as background for the morphology, physiology, and pathophysiology of prematurely born infants and many congenital and perinatal problems. See Chapter 21 for the embryologic development of the cardiovascular system.
Unlike the heart, the lungs are not ready for their main adult function until late in fetal development. Lung development is usually divided into three stages based on histological appearance: the pseudoglandular stage, the canalicular stage, and the alveolar stage.
The Pseudoglandular Stage (5–16 Weeks)
The airways of the respiratory system are derived from the laryngotracheal groove of the embryo, which begins as an outpouching of the primitive pharynx during the fourth week of gestation. This outpouching, or diverticulum, grows away from the pharynx, forming a separate lung bud (Figure 35-1A). The folds in the tracheoesophageal structure grow toward each other, ultimately fusing to form the tracheoesophageal septum (also shown in Figure 35-1A). The septum thus separates the developing larynx and tracheobronchial tree from the esophagus. The lung splits into the two bronchial buds (Figure 35-1B and C) during the end of the fourth week of development. These buds extend into the pleural cavities and become the two mainstem bronchi. During the fifth week after conception, the right bronchopulmonary bud gives rise to three secondary buds which ultimately develop into the right upper, middle, and lower lobes. The left bronchopulmonary bud gives rise to the two secondary buds, which develop into left upper and lower lobes (see Figure 35-1D and E.) The tertiary buds shown in Figure 35-1F develop into the bronchopulmonary segments. As the lungs continue to grow they are covered by the visceral pleura; at the same time the thoracic cavity is lined with the parietal pleura.
Illustration of ventral views of the development of the fetal tracheobronchial tree at about 4 weeks of development. (A) Lateral view of the pharynx. (B) At 4 weeks. (C) At about 32 days. (D) At about 33 days. (E) At the end of the fifth week. (F) Early in the seventh week.
The histology of the tracheobronchial tree develops during this same period generally beginning with the trachea and progressing peripherally. Mesenchymal cells surrounding the endodermal lining of the laryngotracheal tube give rise to the connective tissue, cartilage, and smooth muscle of the larynx and the trachea; the columnar epithelium and glands of the tracheobronchial tree are derived from the endoderm. ...