4.1: Radial Traction
- Page ID
- 34436
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Taking a breath in one might imagine that each part of the lung gets an equal share of the inspired air. But regions of the lung are not equally ventilated. The unequal distribution is caused by (1) gravity and (2) lung volume.
Lung Parenchyma and Radial Traction
Before we look at ventilation distribution we need to understand a little more about lung structure. Although the lung is composed of individual components, such as the alveoli within distinct ascini, the airways, and the blood vessels, the parenchymal tissue between these components helps form the mechanical structure of the lung.
In brief, collagenous and elastic fibers run the length of the large airways and into the lobes forming the axial network. Fibers beneath the pleura and within the septal spaces between the lobes form a peripheral network, and finally thin fibers surrounding the alveoli within the lobes form the septal network. Together these networks form a fibrous "web" of the lung (figure 4.1).
Figure 4.1: The fiber networks of the lung.
Functionally what this means is that movement of one lung structure is transferred to others. As the lung inflates these fibrous connections have a significant impact on lung function and the pulmonary vasculature. Expanding alveoli pull on fibers that are attached to neighboring airways and blood vessels and, indeed, other alveoli. The expanding lung volume tends to pull open airways and blood vessels, lowering the resistance of both as inspiration continues, as is illustrated below (figure 4.2).
Figure 4.2: The action of radial traction.
Radial traction (sometimes called parenchymal traction) is an important component of the lung’s mechanical behavior, and it means that lung volume has an effect on airway and vascular resistance.
More important for us now though is the understanding that the lung is highly connected within itself. And it is a good thing that it is these fiber networks transfer changes in pleural pressure from the lung periphery to its center; without the networks, only the alveoli at the periphery of the lung would expand when pleural pressure became negative during inspiration.
It also means that the effects of gravity are transferred to the lung as a single unit, and we will look at that now.