Pulmonary Compliance
The compliance of any system is defined as the change in volume per unit change in the pressure of the system. It is synonymous to ease with which an elastic structure can be stretched and hence is a measure of the elasticity of a system.
Pulmonary compliance (C) measures the total compliance of both the lungs. It is the increase in the volume of the lungs for each unit increase in the transpulmonary pressure (assuming that the system is in equilibrium). Lung compliance can be calculated as follows:
The total compliance of both lungs in a healthy adult human is about 200 mL/cm H2O. The reciprocal of compliance is elastance.
Table 7.1 Respiration rate of different species of animals under varied conditions
| Animal | Conditions | Respiration rate (range) |
| Dairy cow | At rest (standing position) | 26-35 |
| Dairy cow | Sterna recumbency | 24-50 |
| Sheep | Ruminating (standing position at 18 °C) | 20-34 |
| Sheep | Ruminating (standing position at 10 °C) | 16-22 |
| Pig | Lying down (23-27 kg bd. Wt) | 32-58 |
| Dog | Sleeping | 18-25 |
| Dog | Standing | 20-34 |
Adapted from Duke’s physiology of domestic animals, 12th edition
7.3.1 Compliance Diagram
On plotting the volume changes occurring in lungs with change in transpulmonary pressure changes, two pressurevolume curves are obtained; one is the inspiratory compliance curve, and the other is the expiratory compliance curve.
The whole image is called the compliance diagram (Fig. 7.2), and the characteristic curve obtained is due to lung elasticity and elastic forces contributed by the surfactants lining the alveoli. The lung volume varies even at a given transpulmonary pressure depending on the inspiration or expiration phases. From the curves, it is evident that lung volume is higher when the lung deflates during expiration at the same pressure. The inspiration curve thus lags behind the expiration curve, so these curves are also termed as hysteresis curve. Therefore, the compliance is high at low lung volume, both during inspiration and expiration, making the curve steeper. However, at high lung volume, the compliance falls, making the curve flatter.Lung compliance is measured by two different methods: static and dynamic compliance.
Fig. 7.2 Compliance of a healthy lung. [Even at the same transpulmonary pressure, the lung volume differs during inflating and deflating. The inspiration curve always lags behind the expiration curve, so these curves are also known as the hysteresis curve.]. (Adapted from Guyton and Hall Textbook of medical physiology, 12th edition)
1. Static compliance: Pulmonary compliance measured at a fixed specific volume of lungs under relaxed conditions of no airflow is called static compliance. Static compliance measures the elastic resistance of lungs that occurs when transpulmonary pressure equals the recoil pressure of the lungs.
2. Dynamic compliance: The compliance that is assessed during flow or when breathing. As frequency rises, dynamic lung compliance falls, implying that some airways and subtending alveoli are becoming more constricted.
Low lung compliance indicates that it needs to work more for inflating the lungs for inspiration. Bovine lungs have relatively low lung compliance due to more lobulation and greater interstitial tissues.
7.3.2 Factors Determining Lung Compliance
Two essential factors determine lung compliance. One is the presence of elastic forces, which is determined by elastin and collagen fibres present in lung parenchyma in an interwoven manner. In deflated lungs, these fibres are in a relaxed state being contracted together. With the expansion of the lungs, the fibres attain a stretched position from a relaxed state.
The second factor that determines lung compliance is the presence of pulmonary surfactant. According to Laplace law, i.e. pressure = 2 ? T (surface tension)∕R (radius), the Paiv within the smaller alveoli would be higher than the pressure within the large alveoli. The pressure can collapse the tiny alveoli, which, however, does not occur in a true sense, as the surfactants lower the surface tension preventing alveolar collapse.
A surfactant is a substance produced by type II alveolar epithelial comprising primarily of phospholipids, proteins and calcium ions. Smaller alveoli have a small surface area but have higher concentration of the surfactants, which eventually lowers the surface tension. So, by modulating surface tension, surfactants indirectly affect lung compliance.
The surfactant is rich in phospholipids and surfactant- associated proteins, viz. SP-A, SP-B, SP-C and SP-D. Among the proteins, SP-A and SP-D are hydrophilic and SP-B and SP-C
Fig. 7.3 Pulmonary volumes and capacities as recorded by spirometry. IRV inspiratory reserve volume, VT tidal volume, ERV expiratory reserve volume, RV residual volume, TLC total lung capacity, FRC functional residual capacity, VC vital capacity, IC inspiratory capacity. (Adapted from Duke’s Physiology of Domestic Animals. 12th edition)
are hydrophobic. The surface tension-lowering function is attributed to dipalmitoylphosphatidylcholine, a unique phospholipid, and two hydrophobic SP-B and SP-C proteins. The hydrophilic SP-A and SP-D proteins maintain the pulmonary immune defence by clearing invading pathogens and modifying the immune responses. Besides, the surfactant can aid in preventing plasma exudation, help the smooth muscles lining the airways to relax and prevent the respiratory surfaces from adhering. The lung compliance increases in the presence of surfactant and stabilises alveoli, thus reducing the respiratory workload. Surfactant constitutes about 10% of the alveolar surface. In humans, it begins to appear in the lung at the 26th week of gestation, and deep breathing and cortisol stimulate its production. The lack of surfactant in neonatal lungs results in a condition called respiratory distress syndrome or hyaline membrane disease.
7.3