Pressure-Volume Curves (Lung Compliance)

Pressure-Volume Curves

Lung Compliance Changes in Disease Compliance refers to the distensibility of the lungs, chest wall, or both,* and is a measure of how much volume changes as a result of changes in pressure. – Compliance is inversely related to elastance.

First Diagram - Review Compliance in Healthy Systems.

First we show the compliance curve for healthy systems*. – Indicate the volume at which functional residual capacity is achieved; this is the volume of air in the lungs after tidal expiration. – At this point, the pulmonary system is in equilibrium, because, at this volume, the collapsing force of the lungs is equal to the expanding force of the chest wall. – When a person expires and volume is lower than Functional Residual Capacity, the system "wants" to expand (the expanding force of the chest wall is greater than the collapsing force of the lungs at this point). – When a person inspires and volume is above Functional Residual Capacity, the system "wants" to collapse (the collapsing force of the lungs is greater than the expanding force of the chest wall at this point).

Second diagram - Respiratory Diseases

Diseases that alter lung compliance alter the slopes of the lines for the lungs and combined system (not for the chest wall, alone).

– So, we show that, in emphysema, which is an obstructive disorder, the slope of the line representing compliance is increased. Thus, at any volume, the collapsing force of the lungs is reduced, so more volume must be added to establish equilibrium; a new, higher functional residual capacity is established. – In fibrosis, which is a restrictive disorder, the slope of the line is reduced, and a new, lower functional residual capacity is established.

Review Lung Volumes and Capacities

Lung Volumes and Capacities

Obstructive and restrictive lung diseases alter lung volumes and capacities.
We show a of couple rounds of normal, quiet breathing, followed by maximal inspiration, maximal expiration, and, then, a couple more rounds of normal breathing.
Tidal Volume: Quiet breathing comprises inspiration and expiration of so-called "tidal volume" (TV), approximately 500 mL of air.
Inspiratory Reserve Volume: The volume of air above tidal volume that was inspired during maximal inspiration comprises Inspiratory Reserve Volume (IRV) (approximately 3000 mL).
Inspiratory Capacity: Tidal volume plus the inspiratory reserve volume comprise the Inspiratory Capacity – this is how much the individual can inspire in one intake.
Expiratory Reserve Volume: The volume of air below tidal volume that was expired during maximal expiration is the Expiratory Reserve Volume.
Residual Volume: Even at maximal expiration, we do not expire all of the air in our respiratory system; indicate that the remaining air volume is the Residual Volume.
Functional Residual Capacity: The expiratory reserve volume plus the residual volume comprises the Functional Residual Capacity.

– As we'll soon see, changes in functional residual capacity differentiate obstructive and restrictive lung diseases.

Vital Capacity: Vital capacity comprises the inspiratory reserve volume, tidal volume, and the expiratory reserve volume (in other words, all volumes except residual volume).
Total Lung Capacity: Total lung capacity comprises all air in the respiratory system.
Forced Expiratory Volume: Forced expiratory volume (FEV) is the volume of air that can be forcibly expired following maximal inspiration; we measure how much air can be forcibly expired in 1, 2, and 3 seconds to determine expiratory rate.

– For example, patients with obstructive lung disease have lower values of forced expiration volume in 1 second because expiration is impaired.

For references, please see our Respiratory Pathologies Overview Tutorial.

Pressure-Volume Curves (Lung Compliance)

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