How Do Lung Compliance and Elasticity Work?

TL;DR

Lung compliance measures how easily the lungs or chest wall expand, expressed as the change in volume divided by the change in pressure. Elasticity describes resistance to stretching and the tendency to recoil. Normal breathing depends on a balance among lung elasticity, alveolar surface tension, and chest wall elasticity, while fibrosis reduces compliance and elastic-tissue breakdown impairs passive expiration.

Transcript

all engineers in this video we're going to talk about compliance so first off how we Define compliance well we have a mathematical definition of compliance and I'm just going to give you a kind of like a a a simple term of compliance let's write compliance right down here so what is compliance compliance is defined as the change and volume in this ... Read More

Key Insights

• Compliance is the change in lung or chest wall volume divided by the corresponding change in pressure. It represents stretchability or distensibility, meaning that a highly compliant structure expands easily when pressure is applied, while a less compliant structure resists expansion.
• Elasticity is the change in pressure divided by the change in volume. It describes resistance to stretch and the tendency of the lungs or chest wall to recoil toward the smallest possible size after they have been expanded.
• Compliance and elasticity are opposing mechanical properties. Increasing elasticity makes a structure resist expansion and lowers compliance, while losing elastic tissue makes the lungs easier to inflate but reduces their ability to recoil and expel air passively.
• Lung compliance depends on transpulmonary pressure, while chest wall compliance depends on transthoracic pressure. These pressure differences are used with changes in volume to describe how easily each structure expands under the relevant mechanical conditions.
• Normal respiratory compliance is influenced by three principal factors: lung elasticity, surface tension at the alveolar air-water interface, and chest wall elasticity. Their combined effects determine how easily the respiratory system expands and how effectively it recoils.
• Alveolar surface tension is generated at the interface between air and water. This tension tends to shrink the alveoli and promote their collapse, making it one of the normal forces that affects overall lung compliance.
• Pulmonary fibrosis decreases lung compliance because accumulated fibrous scar tissue is not readily distensible. The resulting increase in elasticity makes the lungs resist inflation, produces a restrictive pulmonary pattern, and is associated in the lecture with a low forced vital capacity.
• Elastic-tissue breakdown increases compliance but decreases recoil. The lungs become floppy and can accept a large amount of air relatively easily, yet passive expiration becomes difficult because normal elastic collapse can no longer push air outward effectively.

Questions & Answers

Q: What is lung compliance and how is it calculated?

Lung compliance is a measure of how easily the lungs can be stretched or distended. It is calculated by dividing the change in lung volume by the change in pressure. A larger increase in volume for a given pressure change indicates greater compliance. The same general relationship can also describe chest wall compliance, although the relevant pressure differs.

Q: What is the difference between compliance and elasticity?

Compliance describes the ease with which the lungs or chest wall can be stretched, whereas elasticity describes resistance to that stretch and the tendency to recoil toward the smallest possible size. The properties oppose one another: a structure with increased elasticity generally has reduced compliance, while a structure that loses elasticity generally becomes more compliant but recoils less effectively.

Q: How do pressure and volume changes affect compliance?

Compliance is directly related to the change in volume and inversely related to the change in pressure. A greater volume change increases the calculated compliance when the pressure change is otherwise unchanged. A greater pressure change decreases compliance when the volume change is unchanged. This relationship shows why easily expanded lungs are considered compliant and stiff lungs are not.

Q: Which pressures are used to measure lung and chest wall compliance?

The relevant pressure depends on the structure being evaluated. Lung compliance uses the change in transpulmonary pressure, while chest wall compliance uses the change in transthoracic pressure. In both cases, the pressure change is compared with the resulting change in volume to determine how easily the lungs or chest wall can be stretched.

Q: What factors normally determine respiratory compliance?

Three major factors normally influence compliance: the elasticity of the lungs, surface tension within the alveoli, and the elasticity of the chest wall. Lung and chest wall elasticity contribute resistance and recoil, while alveolar surface tension tends to shrink the alveoli. Their interaction determines how easily the respiratory system expands and returns toward its resting size.

Q: How does alveolar surface tension affect lung compliance?

Alveolar surface tension develops at the interface between air and water inside the alveoli. It creates a force that tends to shrink the alveoli and encourage collapse. Because compliance measures the ease of expansion, this inward force is an important determinant of lung compliance and must be considered alongside lung elasticity and the mechanical properties of the chest wall.

Q: Why does pulmonary fibrosis reduce lung compliance?

Pulmonary fibrosis produces an accumulation of fibrous scar tissue within the lungs. This tissue is not readily distensible, so the lungs resist stretching and become harder to inflate. Consequently, compliance decreases while elasticity increases. The lungs tend to recoil and resist expansion, creating the restrictive pulmonary pattern described in the lecture and contributing to a low forced vital capacity.

Q: Why does loss of lung elasticity make expiration difficult?

Normal lung elasticity supports passive expiration because the expanded lungs recoil and help push air outward. When elastic tissue is broken down, the lungs become floppy and more compliant, making inhalation relatively easy. However, their recoil is weakened, so they cannot collapse effectively during expiration and cannot move air outward as efficiently without additional effort.

Summary & Key Takeaways

• Compliance describes the stretchability or distensibility of the lungs and chest wall. It is calculated as the change in volume divided by the change in pressure. A structure that gains substantial volume with relatively little pressure is highly compliant, while one requiring greater pressure for expansion has lower compliance and resists inflation.

• Elasticity is the resistance to stretching and the tendency of a structure to recoil toward its smallest possible size. It is expressed as the change in pressure divided by the change in volume. Compliance and elasticity therefore describe opposing mechanical properties, although both must remain appropriately balanced for normal lung expansion and expiration.

• Normal compliance is influenced by lung elasticity, surface tension at the alveolar air-water interface, and chest wall elasticity. Pulmonary fibrosis adds nondistensible scar tissue, reducing compliance and increasing elasticity. Breakdown of elastic tissue produces floppy, highly compliant lungs that accept air easily but cannot recoil effectively during passive expiration.