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).