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Blood and the respiratory system
Blood and the respiratory system

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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).

Described image
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).

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).

Described image
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.

Activity 5 Comparing tissues

Timing: Allow about 10 minutes

Compare the tissue sections in Figure 9. List the differences you observe between:

  • 1. the healthy lung versus the smoker's lung
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The tissue from the healthy lung is uniform and pale in colour throughout. The lung from the smoker is bigger and contains large holes and has black-brown discolourations (probably due to tar and other particulates contained in cigarette smoke).

  • 2. the healthy alveoli versus the COPD alveoli
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The alveoli in the healthy lung are well defined and interconnected. The alveoli in the lung affected by COPD have incomplete and thinner walls, do not make as many connections with other alveoli and have larger air spaces that reduce the respiratory surface.

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.

Question 6 Airflow resistance






neither, they are the same

The correct answer is a.


The radius of a bronchiole is smaller than that of a bronchus. A smaller radius results in higher resistance. Therefore, the resistance to airflow is higher in the bronchiole compared with the bronchus.

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.