THE AIRWAY STRUCTURE



The inspired air travels through a complex pathway on its way to the alveoli. Most people (85 per cent) breathe through the nose, although the mouth is an additional route of air passage when ventilatory requirements are high (in excess of 35 to 40 L/min). The nose and pharynx serve to heat, humidify, and filter the air. Air then passes through the larynx, a complex group of muscles and cartilages, which remains patent dur­ing inspiration and closes during swallowing and maneuvers that require an increase in intrathor­acic pressure, such as defecation and vomiting.

Beyond the larynx is the trachea, a 10- to 12-cm tube supported by U-shaped cartilages and a fibrous posterior membrane. The trachea divides into two mainstem bronchi at the carina. The right mainstem bronchus is a more direct continuation of the trachea, and thus aspirated foreign bodies tend to lodge on the right side. Moving further out, the airways branch in an irregular dichotomous pattern. In the smaller airways, the cartilage be­comes less complete and disappears when the air­ways are 1 to 2 mm in diameter. The first 19 branches, ending at the terminal bronchioles, pro­vide a rapid and enormous expansion of the total airway cross-sectional area, increasing from 2.5 sq cm in the trachea to approximately 900 sq cm (Fig. 17-1). Beyond this are three generations of res­piratory bronchioles with their walls made up of increasing proportions of alveoli. This progresses into the alveolar ducts and culminates in the al­veolar sacs. At this point, the cross-sectional area of the alveolar capillary membrane has increased to an incredible 50 to 100 sq meters.

The epithelial surface of the alveolar capillary membrane, the site of gas exchange, consists of Type I and Type II pneumocytes, the latter thought to be the progenitor of the former. The Type II cell is responsible for the production of surfactant and possibly for other metabolic activ­ities within the lung. Following a significant in­jury to the alveolar-capillary membrane, this cell proliferates and is probably responsible for the re­pair.