2.5 Compliance and airway resistance

The ease with which the lungs and pleura expand and contract based on changes in pressure is called compliance. Low lung compliance means that the lungs and alveoli are ‘stiff’, so a higher-than-normal pressure gradient is needed to get the lungs to expand and contract. It can result from insufficient amounts of surfactant or fibrosis of the lungs due to prolonged inhalation of small particles such as asbestos or coal (e.g. black lung) (Figure 8).

Figure 8 (a) Cross-section through a healthy lung showing the air-filled alveoli. (b) Cross-section of a diseased lung showing deposits of coal particles (arrow).

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Coloured light micrographs of lung sections.

(a) Alveoli and capillaries in cross-section, with extensive white spaces – alveolar cavities.

(b) Cross-section of a diseased lung, with relatively dense pink tissue and with blue-staining nuclei. Extensive accretions of black carbon (coal) particles present throughout the section.

Figure 8 (a) Cross-section through a healthy lung showing the air-filled alveoli. (b) Cross-section of a diseased lung showing deposits ...

High compliance results when the lungs are too pliable and move in response to small changes in pressure. This makes exhalation difficult because the elastic recoil of the lungs (i.e. their ability to ‘snap back’ after inhalation) is decreased. High lung compliance is a characteristic of chronic obstructive pulmonary disease (COPD), a general term for a collection of diseases that are associated with lung damage, such as emphysema and chronic bronchitis, which are often associated with smoking (Figure 9).

Figure 9 (a) Cross-section through a healthy lung (left) and a lung from a smoker (right). (b) and (c) Scanning electron microscope images showing alveoli in the lungs of (b) a healthy individual and (c) a person with COPD.

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(a) Photographs of lung sections from a healthy person showing dense, spongy lung tissue (left) and from a smoker showing a larger lung, extensive cavitation in the lung and black-brown tar deposits (right).

(b) Artificially-coloured scanning electron-micrograph image of alveoli in a healthy lung showing intact, well-defined alveoli as sections of spherical hollows.

(c) Artificially-coloured scanning electron-micrograph image of alveoli from the lung of a person suffering from COPD (chronic obstructive pulmonary disease) showing damaged alveoli with thinner walls and larger air spaces.

Figure 9 (a) Cross-section through a healthy lung (left) and a lung from a smoker (right). (b) and (c) Scanning electron microscope ...

Pulmonary ventilation is also affected by the resistance of the airways to the flow of air. This resistance is caused by the friction that is generated when the air passes along the structures in the conduction and respiratory zones. Because the airways are made up of a series of tubes, resistance is largely affected by the diameter of the trachea, bronchi and bronchioles. Resistance is inversely proportional to radius, so structures with a small diameter have a higher resistance.

During an asthma attack, the airway resistance increases because the bronchial smooth muscle cells contract and reduce the diameter of the bronchi and bronchioles. This results in the characteristic wheezing, coughing and shortness of breath. Fast-acting reliever inhalers release drugs that relax the smooth muscle cells and thereby increase airflow.

In the next section, you will see how lung capacity and function are measured and used as a guide for overall lung health.