Airways.docx

AIRWAYS The airways, forming the connection between the outside world and the terminal respiratory units, are of central importance to our understanding of lung function in health and disease. Intrapulmonary airways are divided into three major groups: bronchi (Fig. 1-7), membranous bronchioles (Fig. 1-8) and respiratory bronchioles/ gas exchange ducts (Fig. 1-9). Bronchi, by definition, have cartilage in their wall. Respiratory bronchioles serve a dual function as airways and as part of the alveolar volume (gas exchange). The anatomic dead space, as measured by the singlebreath nitrogen dilution technique, measures principally the volume of the extrapulmonary (upper) airways (see Chapter 43) and the intrapulmonary bronchi.27 The trachea and bronchi are cartilaginous. The membranous bronchioles (noncartilaginous airways of ~1 mm diameter or less), although exceedingly numerous, are short. They consist of about five branching generations and end at the terminal bronchioles. In contrast to the bronchi, the membranous bronchioles are tightly embedded in the connective tissue framework of the lung and therefore enlarge passively as lung volume increases.28 Histologically, the bronchioles down to and including the terminal bronchioles ought to contribute about 25% to the anatomic dead space. In life, however, they contribute little because of gas phase diffusion and mechanical mixing in the distal airways resulting from the cardiac impulse. By definition, the respiratory bronchioles and alveolar ducts do not contribute to the anatomic dead space. The volume of the respiratory bronchiole-alveolar duct system is approximately one third of the total alveolar volume, and it is into this space that the fresh air ventilation enters during inspiration. Most airway resistance resides in the upper airways and bronchi. Normally, smooth muscle tone in the large airways maintains partial constriction. Minimal airway diameter in the human lung, about 0.5 mm, is reached

at the level of the terminal bronchioles; succeeding generations of exchange ducts (respiratory bronchioles and alveolar ducts) are of constant diameter (see Fig. 1-9).29,30 The functional significance of centralized resistance is

that the terminal respiratory units (the functional alveoli) within a lung subsegment are ventilated chiefly in proportion to their individual distensibilities (compliances) because most of their airway resistance is common. This is demonstrated normally by the finding that regional lung ventilation is dependent upon the initial volumes of the alveoli. Terminal respiratory units toward the top of the lung, which are more expanded at FRC, do not receive as great a share of the inspirate as do the terminal respiratory units near the bottom of the lung. The balance between anatomic dead space volume, which ought to be as small as possible for efficient alveolar ventilation (dead space–to–tidal volume ratio), and airflow resistance, which means the airway diameter ought to be as large as possible for low work of breathing, requires a compromise. Normally, anatomic dead space is not maximal, nor is resistance minimal.