TRACHEA, BRONCHI AND BRONCHIOLES
The trachea is shorter and narrower in children and is also angled posteriorly. Due to the higher position of the paediatric larynx, the cervical trachea segment appears to be composed of more tracheal rings in children than in adults, with 10 countable rings above the sternal notch in newborns, 8 in adolescents, and 6 or less in adults (12). Moreover, in infancy, tracheal size is approximately 50%, 36% and 15% of the length, diameter and cross-sectional area of an adult trachea, respectively (12). The pattern of growth of the trachea is still a matter of debate: in 1986, Griscom et al. analysed the trachea of 130 children aged <6 years by CT, describing how the length, diameter and cross-sectional area grow from birth to adolescence and showed that by the end of adolescence, the length of the trachea doubles and that the mean transversal diameter is wider than the mean antero-posterior diameter in children up to the age of 6 years. These diameters increase afterwards, becoming nearly identical, so that the cross-section of the trachea becomes rounder, and at age 18, the anteroposterior diameters usually become slightly larger (12, 31). Increases in tracheal length and diameter were believed to occur with a direct linear relationship from approximately 18 weeks gestation (32, 33) to age 14 in girls, while in males, tracheas continue to enlarge (but not lengthen) for a time after growth in height ceases. As previously mentioned, this pattern of growth has recently been questioned by Luscan et al., who showed a pattern of development similar to that of height and confirmed that the trachea is not round in shape, with a greater transverse diameter than anteroposterior diameter (26).
The bronchial tree originates from the trachea, forming an asymmetric and dichotomic system of bronchi and bronchioles that are characterized by a progressive reduction of cartilage in their walls. The smaller bronchioles (diameter ≤ 1 mm) are not supported by cartilage in their walls and are called terminal bronchioles , representing the last generation of the airway conduction system. Their distal continuations are called respiratory bronchioles , and they show some alveoli in their walls and open in the alveolar ducts. Respiratory bronchioles, together with alveolar ducts and alveolar sacs, represent the basic functional unit of the lung, commonly known as an acinus(approximately 1 cm in diameter) (Figure 3) (34, 35). The “pre-acinar” or conducting airways are considered complete at birth, having been described as a miniature version of an adult’s (33, 36), and only enlarge and elongate during growth, doubling or tripling their dimension up to adulthood (37). There is still some debate on the fact that the peripheral airways beyond the eighteenth generation may be disproportionately narrower than larger airways in infants and young children (38, 39), since a number of physiologic studies have not confirmed this pattern (33, 40-43).