Lung Compliance

  • The compliance of the lung describes the relationship between the transmural pressure across the lung compared with organ's volume. By transmural pressure we mean the relative pressure between the alveoli compared with that in the intrapleural space. Positive transmural pressures mean greater alveolar pressures than intrapleural pressures.
Lung Compliance Relationship
  • Several qualitative features of the lung compliance relationship are important to appreciate. The first is to appreciate that the lung is most contracted when there is no transmural pressure (i.e. the transmural pressure is zero). As the lung encounters positive transmural pressure, meaning greater pressure in the alveolar air compared with that in the intrapleural space, the lung expands. These properties stem from the lung's inherent elastic recoil which renders the organ similar to a rubber balloon that expands when progressively blown in to. Consequently, progressively greater transmural pressures must be achieved in order to generate increasing lung volumes.
  • However, it is important to note the non-linear nature of the compliance curve which displays the greatest slope at smaller volumes and largely plateaus at large volumes. This indicates that the lung is most compliant, that is easiest to expand, at lower volumes as compared to larger volumes where greater changes in transmural pressure are required to achieve the same value of expansion. It should be pointed out that the compliance curve of the lung changes depending upon whether the lung is expanding or contracting during inspiration or expiration. We have left out this feature, termed "Hysterisis", in our discussion for the sake of simplicity as it is not critical for understanding the mechanics of breathing. However, the compliance curve displayed here is most similar to that of expiration.

The Lung Compliance Relationship and its Physical Basis
Pulmonary compliance refers to the relationship between the volume of the lungs and the transmural pressure across the lungs. At low lung volumes the pulmonary compliance is high; however, as the lungs expand their compliance progressively decreases. This reflects progressive stretching of elastin fibers to their physical limits as well as increasing surface tension as alveoli expand.

Transmural Pressures
  • Although positive transmural pressures are required to expand the lung, they are not achieved by generating positive alveolar air pressures. Rather, lung expansion is achieved by generating progressively negative pressures within the intrapleural space. Conceptually, one can think of this as inflating a balloon not by breathing into it, and thus increasing the internal pressure above that of atmospheric pressure, but rather by placing it within a chamber and generating a progressive vacuum around the balloon. Appreciation of this important feature of lung expansion is critical for understanding how lung expansion can be achieved given the fact that the lung is not physically attached to the chest wall and is instead separated by the intrapleural space.

Lung Expansion by Generation of Negative Intrapleural Pressures
Similar to a balloon, the lungs expand by generating positive transmural pressures. However, unlike a balloon these positive transmural pressures are generated by creating negative surrounding pressure within the intrapleural space. As shown, the pressure of air within the lungs remains equivalent with that of the atmosphere at the peak inspiration or expiration.

Physical Basis
  • Overview
    • The physical basis of the lung's elastic recoil and the shape of its compliance curve are the result of two basic components of pulmonary tissue. The first is the protein elastin which is a major component of the pulmonary interstitial connective tissue and the second is surface tension of alveolar fluid.
  • Elastin
    • Elastin is a highly stretchable protein which is found widely throughout the pulmonary interstitial connective tissue. Molecularly, elastin proteins naturally tend toward a globular structure but can be stretched if a force is applied; however, once the protein is fully extended the molecule is highly resistant to further stretching, similar to a rubberband. The elastic recoil of the lung and its tendency to have a higher compliance at lower lung volumes is in large part explained by the combined action of the elastin fibers spread throughout the pulmonary interstitium. These fibers serve to powerfully recoil the lung and only stretch when a force is applied; however, once the lung is stretched to large volumes, these proteins become highly resistant to further stretching.
  • Surface Tension
    • Surface Tension is a physical property of water which causes surfaces of water to achieve the smallest possible area. As one might recall from general chemistry, surface tension arises from the energetic preference of water molecules to interact with one another through hydrogen-bonding rather than with air, with which hydrogen bonds cannot be achieved. Individual alveoli all contain a highly thin inner lining of water-based fluid whose surface tension exerts a collapsing force on the alveolus. This is because smaller alveolar volumes would naturally reduce the surface area of the thin fluid lining.
    • The combined force of surface tension throughout the lung's alveoli serve as a powerful contributor to the elastic recoil of the lung. Again, increased lung volumes would require expansion of individual alveoli which is highly resisted by the surface tension of the alveolar fluid inner lining. In fact, the recoiling force of alveolar surface tension is so powerful that special surfactant chemicals must be synthesized and secreted into the inner lining fluid to reduce the surface tension. The most important surfactant is Dipalmitoyl Phosphatidyl Choline (DPPC) that is synthesized and released into the inner lining of alveoli by Type II Pneumocytes. Failure to produce these surfactants is an important contributor to major pulmonary pathologies including Neonatal Respiratory Distress Syndrome.