By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
Breathing is something we do so automatically that we hardly ever pay attention to it or stop to ponder how it works. Through the carefully orchestrated actions of specific muscles, our thoracic cavity moves in synchrony with the lungs to supply us with our next breath…
Take a breath in…
Inspiration, or taking air in, is an active process. The respiratory muscles increase the thoracic volume by contracting.
The main muscle that accomplishes this is the diaphragm. It is a skeletal muscle that projects into the thoracic cavity like a dome. The diaphragm originates along the walls of the thorax and inserts into a centrally placed tendon. It is innervated by the phrenic nerve arising from the cervical segments 3 to 5.
During contraction, the diaphragm pulls down the central tendon to allow an increase in the volume of the thoracic cavity horizontally. 75% of the air volume movement in the lungs is can be attributed just to the action of the diaphragm.
The external intercostal muscles also play an important role by elevating the ribs. These muscles originate in the upper rib and insert in the one below. Their fibres run inferiorly and anteriorly, hence when they contract this pulls the ribs up. So, the remaining 25% of air volume movement can be attributed to the external intercostals.
By OpenStax [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons
and slowly breathe out completely…
Expiration normally is a passive process. The diaphragm and the external intercostals relax and the volume of the thorax is decreased again which compresses the lungs and air is expelled.
Forced expiration, on the other hand, is an active process assisted by the internal intercostals and transversus thoracis. Both of these depress the ribs, which further reduces thorax volume expelling and assists with expelling the remaining air in the lungs.
The internal intercostals originate from the superior border of each rib and insert into the inferior border of the rib above. This enables them to pull the ribs down.
There is a thin muscle called transversus thoracis located on the inner surface of the anterior thoracic wall. It originates the dorsal surface of the xiphoid process and the body of the sternum and inserts through five craniolateral tendons to the cartilages of ribs 2 to 6. Its attachments and the exact number of tendons vary greatly among people. Transversus thoracis supports active expiration by pulling the rib cartilages caudally.
There is a number of other muscles, which all play their part in the process of breathing.
The sternocleidomastoid, scalenes, serratus anterior, pectoralis minor, pectoralis major, and upper trapezius, for example, assist inspiration. The sternum is elevated thanks to sternocleidomastoid, while the scalenes elevate the upper few ribs, which increases the anteroposterior diameter of the thorax. The insertion of serratus anterior, pectoralis major and minor is fixed and their origin moves when the upper arm is fixed into position. In this case, they pulling the ribs toward the arms by reverse muscle action. Trapezius helps with elevating the clavicle. These accessory muscles become more active during forced inspiration, like in people with asthma.
During expiration, the abdominal muscles including the external obliques, internal obliques, transversus abdominis, and the rectus abdominis, force the diaphragm upward by compressing the abdomen.
Last but not least, the abductor muscles in the larynx pull the vocal cords apart early in inspiration. The reflexive contraction of the adductor muscles closes the glottis and prevents food from entering the larynx during swallowing.
We can see that a great number of muscles work in symphony to take care of our most basic need – the need for air. So, why not pay a bit more attention to this miraculous process of breathing from time to time. Perhaps it will allow us to connect with the deep wisdom of our bodies more closely…
Martin Stefanov Petkov