The Muscles of Respiration (Abridged)

The muscles of the oral cavity, pharynx and larynx as well as the structure of the glottis and the protection of the airway via the vocal folds will be discussed in future posts. Here, we will review the additional muscle groups involved in inspiration and expiration (Ferrand, 2014; Samsam, 2015; Shier Butler, Lewis, 2015; Talaro & Chess, 2012).

The muscles of the respiratory system aid in respiration in healthy adults, but are not primarily responsible for making it happen. The diaphragm separates the thorax and the abdomen, and is involved in inspiration. It is a thin, dome-shaped sheet of muscle that inserts into the lower ribs, and along with the pectoralis major, pectoralis minor, scalene, platysma, serratus anterior, sternocleidomastoid, internal and external intercostal muscles, the shape of the thoracic cavity and lungs, changes. The muscles of the abdomen, such as the external obliques, internal obliques, transverse abdominus, and rectus abdominus, work together to compress the abdominal cavity during expiration, exerting its effects on the diaphragm and decreases the volume of the thoracic cavity (Ferrand, 2014; Samsam, 2015; Shier Butler, Lewis, 2015; Talaro & Chess, 2012).

The primary and accessory muscles of inspiration facilitate contraction of the diaphragm, which pulls the lungs downward and spread out to increase the vertical dimension of the chest cavity resulting in a change in abdominal pressure. Abdominal pressure increases that drives the abdomen down and out and this pulls the chest cavity in the transverse dimension. Together, the lungs are pulled down and out, which expands the lungs, dropping the intra-lobar pressure and forces air into the alveoli. During passive breathing, the diaphragm relaxes and returns the thorax and lungs to their original shape, increasing intra-lobar pressure that forces air back out of the body. Overall, the diaphragm moves about a centimeter in either direction during passive breathing. During active breathing, expiration involves the contraction of abdominal muscles that further pushes the diaphragm upward nearly 10cm (Ferrand, 2014; Samsam, 2015; Shier Butler, Lewis, 2015; Talaro & Chess, 2012). These muscles are recruited during conscious, active breathing.