The Lung Is Mechanically Connected to the Thoracic Cage by the Pleural Liquid
The lung is covered by the visceral pleura, and the thorax is lined by the parietal pleura. These two pleural surfaces are maintained in close apposition by a thin layer of pleural fluid.
It is useful to liken the pleural fluid to a thin layer of Waterbetween two pieces of glass (the pleural surfaces). The liquid allows the pieces of glass to slide over one another but makes them difficult to separate. In the thorax, therefore, the pleural fluid mechanically links the lungs to the thorax so that the respiratory system behaves as a single unit. W7Iien the thorax expands during inhalation, for example, the lungs must expand as well. Similarly, when an animal exhales below FRC, the stiff thorax increasingly resists deformation, so that residual volume, the volume of air in the lung at the end of a maximal exhalation, is determined by the limits to which the rib cage can be compressed.The thorax is generally stiffer—that is, it is less compliant— in large animals than small animals; the stiff chest wall of the horse and cow is in contrast to the very compliant chest wall of small rodents. Neonates need to have a compliant chest to pass through the birth canal. Lung collapse, also known as atelectasis, is more likely to occur in species with compliant thoraces because the thorax cannot adequately support the lung to prevent its collapse. This is one reason that atelectasis is more common in newborn animals than in adults.
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