Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 20

It is generally understood that resistance to flow in a tube increases sharply as luminal radius (r) decreases (with laminar flow, resistance is inversely proportional to r⁴). It seems rather contradictory, therefore, to learn that in a healthy individual, the greater part of total airway resistance is situated in the large airways (larynx, trachea and main bronchi) rather than in the small airways. This is in part due to the fact that the flow velocity is greatest and flow most turbulent in the central airways, but also due to the much greater total cross‐sectional area in the later generations of airway (Fig. 1.6). Remember, we only have one trachea, but by the 10th division we have very many small airways, which effectively function in parallel.

Figure 1.6 Diagrammatic representation of the increase in total cross‐sectional area of the airways at successive divisions.

Conditions may be different in disease states. Asthma and COPD – diseases that affect airway calibre – tend to have a greater proportionate effect on smaller generations of airway. The reduced calibre of the smaller airways then becomes overwhelmingly important and the site of principal resistance moves distally.

Figure 1.7 Model of the lung, demonstrating the flow‐limiting mechanism (see text). The chest is represented as a bellows. The airways of the lungs are represented collectively as having a distal resistive segment (Res) and a more proximal collapsible or ‘floppy’ segment. The walls of the floppy segment are kept apart by the retractile force of lung recoil (Rec). EXP, expiration; INSP inspiration.

Consider the model of the lung represented in Fig. 1.7. Here, the tube represents a route through generations of airways from the alveoli to the mouth. The smaller generations, without cartilaginous support, are represented by the ‘floppy’ segment (B). Airways are embedded within the lung and are attached externally to lung tissue whose elastic recoil and ultimate connection to the chest wall supports the floppy segments. This recoil force is represented by the springs.

During expiration, a positive pressure is generated in the alveolar space (A). Air flows from A along the airway, past B, where the pressure is lower (it must be, otherwise the air would not have flowed in this direction), and on to the mouth, where the pressure is nominally ‘zero’.

The pressure difference across the walls of the floppy segment (A minus B) would tend to cause this part of the airway to collapse. It is prevented from doing so by the retractile force of lung recoil (tension within the springs).