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(a) Shape of the thorax at the end of normal inspiration. (b) Shape of the thorax at the end of maximal inspiration, aided by contraction of the sternocleidomastoid and pectoralis minor muscles.

The ease with which the lungs can expand as a result of pressure changes occurring during breathing is called compliance (distensibility). In a normal lung, compliance decreases as lung volume increases, because an inflated lung is more difficult to expand than a lung at rest. Conditions that obstruct air passages, destroy lung tissue, or impede lung expansion in other ways also decrease compliance.


The forces responsible for normal resting expiration come from elastic recoil of lung tissues and from surface tension. The lungs contain a considerable amount of elastic tissue, which stretches as the lungs expand during inspiration. When the diaphragm lowers, the abdominal organs inferior to it are compressed. As the diaphragm and the external intercostal muscles relax following inspiration, the elastic tissues cause the lungs to recoil, and they return to their original shapes. Similarly, elastic tissues within the abdominal organs cause them to spring back into their previous shapes, pushing the diaphragm upward. At the same time, surface tension that develops between the moist surfaces of the alveolar linings shrinks alveoli. Each of these factors increases the intra-alveolar pressure about 1 mm Hg above atmospheric pressure, so the air inside the lungs is forced out through the respiratory passages. Because normal resting expiration occurs without the contraction of muscles, it is considered a passive process.

The recoil of the elastic fibers within the lung tissues reduces pressure in the pleural cavity. Consequently, the pressure between the pleural membranes (intrapleural pressure) is usually about 4 mm Hg less than atmospheric pressure.

Because of the low intrapleural pressure, the visceral and parietal pleural membranes are held closely together, and no significant space normally separates them in the pleural cavity. However, if the thoracic wall is punctured, atmospheric air may enter the pleural cavity and create a substantial space between the membranes. This condition is called pneumothorax, and when it occurs, the lung on the affected side may collapse because of its elasticity.

Pneumothorax may be treated by covering the chest wound with an impermeable bandage, passing a tube (chest tube) through the thoracic wall into the pleural cavity, and applying suction to the tube. The suction reestablishes negative pressure within the cavity, and the collapsed lung expands.

If a person needs to exhale more air than normal, the posterior internal (expiratory) intercostal muscles can be contracted. These muscles pull the ribs and sternum downward and inward, increasing the pressure in the lungs. Also, the abdominal wall muscles, including the external and internal obliques, the transversus abdominis,


Abdominal organs recoil and press diaphragm upward


Abdominal organs recoil and press diaphragm upward

Posterior internal intercostal muscles pull ribs down and inward


Abdominal organs force diaphragm higher

Abdominal wall muscles contract and compress abdominal organs

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