Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 26

An inspired breath brings air into the lung. That air does not distribute itself evenly, however. Some parts of the lung are more compliant than others, and are therefore more accommodating. This variability in compliance occurs on a gross scale across the lungs (upper zones verses lower zones) and also on a very small scale in a more random pattern. At the gross level, the lungs can be imagined as ‘hanging’ inside the thorax and resting on the diaphragm; the effect of gravity means that the upper parts of the lungs are under considerable stretch, whilst the bases sit relatively compressed on the diaphragm. During inspiration (as the diaphragm descends) the upper parts of the lung, which were already stretched, cannot expand much more to accommodate the incoming air; the bases, on the other hand, are ripe for inflation. Therefore, far more of each inspired breath ends up in the lower zones than the upper zones.

On a small scale, adjacent lobules or even alveoli may not have the same compliance. Airway anatomy is not precisely uniform either, and airway resistance between individual lung units will vary. It can therefore be seen that ventilation will vary in an apparently random fashion on a small scale throughout the lung. This phenomenon may be rather modest in health, but is likely to be exaggerated in many lung diseases in which airway resistance or lung compliance is affected.