Biomedical Engineering Reference
In-Depth Information
If an average tidal volume of 500 ml is inhaled, at the end of inspiration only 350 ml
will have entered the alveoli and 150 ml will remain in the airways. The ventilation used for
gas exchange will be only 350/500 ths, or 70%, of the total ventilation. The remaining 30%
dead space is sometimes called wasted ventilation but does perform the vital function of
conditioning the air before it reaches the alveoli. If inspired air reached the alveoli directly,
if it were cold it would cool the tissue, if hot it would heat it, and if dry it would parch
and destroy the alveolar walls. As cold, dry air is inspired, it is raised to body temperature
and 100% humidity after passing through the first few generations of bronchi. In this
process the airway lining itself (the mucosa) is cooled and dried, but it has performed
its function of protecting the alveoli. On breathing out, the mucosa take up some of the
heat and water vapor from the expired air, restoring it to normal. In this way, not only
are the alveoli protected, but also loss of heat and water from the body as a whole is
reduced.
The walls of the trachea and bronchi, shown in Figure 9-3, consist of several layers.
On the inner lining surface, the
luminal
side, there is an epithelial layer the cells of which
are covered with microscopic cilia (hairs) that continuously sweep any surface material
toward the larynx, where it is coughed up or swallowed. Other cells secrete mucus. Just
under the epithelium is a dense blood capillary network that provides nutrition for the
epithelium and glands and may be the site of uptake of inhaled pollutants and drugs.
Deeper in the wall are the submucosal glands, the main source of the mucus that lines
the airways. The glands are stimulated to secrete by many factors, the most important
being pollutants (e.g., cigarette smoke) and viral or bacterial infections of the airways.
The secreted mucus creates a barrier and takes up soluble pollutants and smoke particles,
slowing down their entry into the body and protecting the epithelium from their harmful
effects. It can also stimulate coughing as an even more rapid means of their removal.
FIGURE 9-3
The trachea and
bronchi.
