Chapter 19

### RESPIRATORY TRACT

Pediatric pulmonary diseases account for almost 50% of deaths in children younger than age 1 year and about 20% of all hospitalizations of children younger than age 15 years. Approximately 7% of children have a chronic disorder of the lower respiratory system. Understanding the pathophysiology of many pediatric pulmonary diseases requires an appreciation of the normal growth and development of the lung.

### GROWTH & DEVELOPMENT

The lung has its origins from an outpouching of the foregut during the fourth week of gestation. The development of the lung is divided into five overlapping stages.

1. Embryonic stage (3–7 weeks' gestation): During this stage, the primitive lung bud undergoes asymmetrical branching and then subsequent dichotomous branching, leading to the development of the conducting airways. This stage of lung development is dependent on a complex interaction of various growth factors originating in both the pulmonary epithelium and the splanchnic mesenchyme. It also sees the development of the large pulmonary arteries from the sixth aortic arch and the pulmonary veins as outgrowths of the left atrium. Abnormalities during this stage result in congenital abnormalities such as lung aplasia, tracheoesophageal fistula, and congenital pulmonary cysts.

2. Pseudoglandular stage (5–17 weeks' gestation): During this stage, which overlaps with the embryonic stage, the lung has a glandular appearance. The conducting airways (bronchi and bronchioles) form over these 12 weeks. The respiratory epithelium of these airways begins to differentiate, and the presence of cartilage, smooth muscle cells, and mucus glands are first seen. In addition, the pleuroperitoneal cavity divides into two distinct compartments. Abnormalities during this stage lead to pulmonary sequestration, cystic adenomatoid malformation, and congenital diaphragmatic hernia.

3. Canalicular stage (16–26 weeks' gestation): This stage witnesses the delineation of the pulmonary acinus. The alveolar type II cells differentiate into type I cells, the pulmonary capillary network develops, and the alveolar type I cells closely approximate with the developing capillary network. Abnormalities of development during this stage include neonatal respiratory distress syndrome and lung hypoplasia.

4. Saccular stage (26–36 weeks' gestation): During this stage further branching of the terminal saccules takes place as well as a thinning of the interstitium and fusion of the type I cell and capillary basement membrane in preparation for the lungs' function as a gas-exchange organ. This stage sees the beginning of an exponential increase in the epithelial surface area for gas exchange. During this stage of lung development the lung is able to fulfill its function, in terms of a gas-exchange organ.

5. Alveolar stage (36 weeks' gestation to 3–8 years of age): Controversy surrounds the length of this stage of lung development. During this stage, secondary alveolar septa form to increase the surface area for gas exchange, the capillary network has a rapid phase of growth, and true alveoli develop. Abnormalities during this stage lead to lung hypoplasia and can result in the development of bronchopulmonary dysplasia (BPD).

At birth, the lung assumes the ...

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