Lung Compliance and Elastance
Lung Compliance and Elastance
The ability of the lungs to expand is expressed using a measure known as the lung compliance. Lung compliance is the volume change that could be achieved in the lungs per unit pressure change. Elastance, also known as the elastic resistance is the reciprocal of compliance, i.e. the pressure change that is required to elicit a unit volume change. This is a measure of the resistance of a system to expand.
Elastance = 1/Compliance = Pressure change / Volume change
Elastance is a measure of the work that has to be exerted by the muscles of inspiration to expand the lungs. An increased elastance needs to be counteracted by an increased power of the muscles of inspiration, leading to an increased work of breathing (work of breathing is the physical work that have to be carried out by the muscles of respiration to overcome the elastic resistance of the respiratory system and the non-elastic resistance of the airways).
Factors Affecting Elastance of the Respiratory System...
The elastance of the whole respiratory system depends on the elastance of the chest wall and that of the lungs. Since the chest wall and the lungs have a serial relationship, in forming the respiratory system, the elastance of the whole respiratory system can be calculated by the addition of the elastance of the chest wall and the lungs. Since the elastance in each of the lungs and the chest wall is approximately 5 cmH2O, the elastance of the respiratory system is approximately 10 cmH2O.
Elastance of the Respiratory System Depends on the Elastance of the Lungs...
Changes in the elastance (and therefore the compliance) of the chest wall are uncommon. In contrast, the elastance of the lungs is affected by many respiratory diseases. Thus, variations in the elastance of the respiratory system are mainly due to alterations of the elastance of the lungs, which is governed by two main factors:
- Elastic recoil forces of the lung tissue
- Forces exerted by surface tension at the air-alveolar interface
1. Elastic Recoil Forces of the Lung Tissue
The elastin fibers forming the pulmonary interstitium resist stretching and exhibit the property of returning to its original length, when stretched (in accordance with the Hook’s Law). This accounts for approximately one fourth to one third of the elastic resistance of the lungs and holds the responsibility of generating the recoil forces necessary to increase the intra-alveolar pressure during expiration, which is a passive process.
2. Forces Exerted by Surface Tension at the Air-Alveolar Interface
This is responsible for the remaining two-thirds to three-fourths of the elastance of the lungs. Since the alveoli are globular structures, having a thin lining of fluid, which comes into contact with air, the net surface tension force acts inwards. Therefore, going by the Laplace’s Law, to prevent the alveoli from collapsing, a transmural pressure should be acting across the alveolar wall. This pressure, for a single alveolus, is equal to 2 X surface tension / radius of an alveolus (2T/r). When a whole lung is considered, the transmural pressure is the transpulmonary pressure (intra-alveolar pressure – intra-pleural pressure).
Importance of Surface Tension on Lung Compliance and Elastance
The contribution of elastic recoil and the surface tension on the total elastance can be demonstrated by pressure-volume curves, determined in vitro, of lungs which are either gas-filled or liquid filled. The elastance of the gas-filled lungs can be assumed to have the same elastance as that, which is attached to the thoracic wall. Since, the surface tension forces are eliminated in the liquid-filled lungs as there is no air-liquid interface, the elastance becomes much lower (approximately one-fourth) compared to a normal lung as the elastance is entirely due to the elastin fibers.
Surfactant and Reduction in Surface Tension
Reduction in the surface tension would lead to a reduction in the trasnpulmonary pressure that is required to keep the alveoli expanded. Thus, this decreases the power that needs to be generated by the muscles of inspiration and hence, the work of breathing. The surface tension in the lungs is reduced by a chemical agent, known as surfactant, secreted by the type II alveolar cells in the lungs. Details regarding the secretion of surfactant and the functions of surfactant will be described in a separate hub.
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