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How can diaphragm paralysis reduce a person's breathing capacity?

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Medical illustration of the human chest showing the lungs, diaphragm, ribs, trachea, bronchus, and bronchioles, with one lung cut away to reveal internal structures. — respiratory disease The bronchioles of the lungs are the site where oxygen is exchanged for carbon dioxide during the process of respiration. Inflammation, infection, or obstruction of the bronchioles is often associated with acute or chronic respiratory disease, including bronchiectasis, pneumonia, and lung abscesses.

The diaphragm is the primary muscle of respiration. Under normal conditions, its contraction increases the height of the thoracic cavity, which lowers internal pressure and facilitates inspiration.2

When the diaphragm is paralyzed, this essential mechanism for expanding the chest cavity is lost, leading to a significant reduction in vital capacity. This impairment is particularly severe in cases of bilateral diaphragmatic paralysis, where the reduction in breathing capacity is most pronounced when a person is in a recumbent (lying down) position. While unilateral paralysis (paralysis on one side) is more common and generally better tolerated, it often still results in shortness of breath during physical exertion. By failing to contract effectively, the paralyzed diaphragm cannot properly lower the thoracic pressure required to draw air into the lungs, thereby compromising the body's overall respiratory efficiency.12