The Role of Muscles in Breathing

What is the role of different muscles in the process of breathing?

How do muscles such as the external intercostals, diaphragm, internal intercostals, and abdominal muscles contribute to quiet and forced inspiration and expiration?

Answer:

During quiet inspiration, the external intercostals contract. During quiet expiration, the diaphragm relaxes. During forced inspiration, the internal intercostal muscles contract. During forced expiration, the abdominal muscles contract.

Pulmonary ventilation consists of the process of inspiration (or inhalation), where air enters the lungs, and expiration (or exhalation), where air leaves the lungs. During quiet inspiration, the external intercostals contract, causing the rib cage to expand and move outward, and expanding the thoracic cavity and lung volume. This creates a lower pressure within the lung than that of the atmosphere, causing air to be drawn into the lungs. During quiet expiration, the diaphragm relaxes, allowing the lungs to passively deflate.

During forced inspiration, such as during exercise, the internal intercostal muscles contract in addition to the external intercostals, drawing the ribs together to constrict the rib cage and further increase lung volume. For forced expiration, the abdominal muscles contract along with the internal intercostals, increasing intra-abdominal pressure and pushing the diaphragm upward, forcing air out of the lungs.

Additional muscles like the internal intercostals and muscles of the neck such as the sternocleidomastoid and scalenes assist during forced inspiration by further elevating the rib cage. For forced expiration, the internal intercostals contract to depress the ribs, while the abdominal muscles contract to increase intra-abdominal pressure, pushing up against the diaphragm and forcing more air out of the lungs.