Anatomy of breathing

Components

The thoracic cage is a component of the thoracic wall and encloses the majority of the structures of the respiratory system. It forms the bony framework for breathing. The dome shaped thoracic cage provides the necessary rigidity for organ protection, weight support for the upper limbs and anchorage for muscles. In spite of its resistance, the cage is dynamic, allowing pulmonary ventilation to take place. The potential for movement is related to the flexibility provided by the ribs and their joints. The thoracic cage is composed of the thoracic skeleton, which includes the sternum, 12 pairs of ribs and 12 thoracic vertebrae, associated with the costal cartilages and intervertebral discs, respectively.

Ribs

The ribs are lightweight and resilient, consisting of three types: true, false and floating ribs. They form most of the thoracic cage, extending from the posterior to the anterior thoracic walls. They are attached at their anterior ends by costal cartilages, which either provide direct attachment to the sternum , or the costal margin. A few ribs, the so-called floating ribs, have no anterior attachment.. The flexible costal cartilages provide the thoracic wall with its necessary elasticity.

The sternum forms the middle portion of the anterior thoracic cage and it consists of three parts: the manubrium, the body and the xiphoid process. Running along its lateral borders, the sternum has costal notches where the costal cartilages attach. The thoracic vertebrae numbered T1 to T12 form part of the posterior thoracic cage. They contain bilateral costal facets on the vertebral bodies where the heads of the ribs attach. The heads also attach partially to the intervertebral discs. With the exception of the last two or three thoracic vertebrae, they also contain costal facets on the transverse processes for articulations with the tubercles of the ribs.

All of the above skeletal components complete the thoracic cage from anterior to posterior, offering both protection and flexibility for ventilation. However, the thoracic cage is opened superiorly and inferiorly at the so-called apertures (openings). The superior aperture permits the passage of the trachea, which facilitates the movement of air during breathing The larger inferior thoracic aperture is completely covered by the diaphragm.

You can find out more details about the thorax below:

Thoracic muscles

While the thoracic cage offers a resistant, yet flexible framework, it would be impossible for you to breathe without the action of the thoracic muscles. Further details will be given below, but ventilation is carried out by expanding and contracting the lungs. One way of doing this is to change the anteroposterior diameter of the chest cavity by elevating or depressing the ribs. The most important muscles raising the ribcage are the external intercostal muscles. These muscles are part of the intercostal muscle group located in the intercostal spaces between the ribs. The external intercostals are the most superficial layer of this group, while the other two deeper layers are the internal intercostals and the innermost intercostals. There are 11 pairs of external intercostals, extending between the tubercles of the ribs and the costochondral joints. They run in an infero-anterior direction between the borders of two adjacent ribs.

The internal intercostal muscles are also important in altering the anteroposterior dimension of the chest cavity. Also consisting of 11 pairs, these muscles run along the bodies and costal cartilages of the ribs between the sternum and the angle of the ribs. They attach between the costal groove and the superior border of two different ribs within the intercostal spaces.

Neck muscles

The external and internal intercostals do not work individually during breathing. They are assisted by the sternocleidomastoid and scalene muscles on the neck.

The sternocleidomastoid muscles originate from the manubrium of the sternum and sternal end of each clavicle; they insert on the mastoid process of the temporal bone and superior nuchal line of the occipital bone. This allows them to elevate the sternum and clavicle, subsequently lifting the ribs during inhalation. Therefore, they are used as accessory muscles in pulmonary ventilation. Therefore, they are used as accessory, or secondary muscles in pulmonary ventilation.

The scalene muscles also play a role in inspiration. They consist of scalenus anterior, scalenus medius and scalenus posterior. All three are involved in breathing. Scalenus anterior muscles extend from the anterior tubercles of transverse processes of C3 to C6 vertebrae to the first rib, contributing to its elevation. Scalenus medius runs from transverse processes of the axis and the transverse process of C3 to C7 until the first rib, also raising it. The scalenus medius is the most significant for breathing in this group. The scalenus posterior passes from the posterior tubercles of the transverse process of C4-6 to the second rib. Therefore, it helps elevate the second rib.

Pectoral girdle muscles

The muscle of this region that is important in breathing is the serratus anterior. It overlies the lateral part of the thorax and forms the lateral wall of the axilla. It arises from the the 1st to 8th pairs of ribs and inserts onto the medial border of the scapula. By fixing the scapula in position, this muscle has an important role in laboured breathing when grasping a support or staying in the so-called tripod position.  

Abdominal muscles

As you can see, the action of breathing that you take for granted and are almost unaware of is quite complex with quite a few muscles at play. It is not surprising then that muscles in the abdominal region also play a role. Specifically, the rectus abdominis pulls the ribs down during active expiration. Its point of origin is the pubic symphysis and pubic crest and it attaches to the xiphoid process and the 5th to 7th costal cartilages. This pair of muscles is separated by the linea alba.

The diaphragm is another crucial structure which makes breathing possible. While all other muscles mostly change the anteroposterior diameter of the chest cavity, the diaphragm lengthens and shortens the cavity by moving up and down. This action also expands and contracts the lungs. The diaphragm is dome shaped and separates the thoracic and abdominal cavities. During breathing, it is the chief muscle of inspiration. It originates from its fixed and circular periphery, which extends around the inferior margin of the thoracic cage and the superior lumbar vertebrae. As such, only the central part is allowed to move during breathing.The diaphragm consists of a right and left dome which rise all the way to the level of the 4th intercostal space.

Inspiration

Inspiration involves air entering the lungs from the external environment. Normal and quiet inspiration is carried out by the diaphragm, which lengthens and shortens the chest cavity. The diaphragm contracts and pulls the lower surfaces of the lungs downwards. Simultaneously, muscles of inspiration elevate the rib cage. These muscles are mainly the external intercostals. By lifting the ribs and pushing the abdominal organs down, the intrathoracic volume increases. Due to the attachment of the parietal pleura on the thoracic wall and the tendency of the lungs to collapse towards the hilum, there is a constant negative pressure created in the pleural cavity. This is similar to a thin layer of water keeping two pieces of plastic attached. Therefore, the lungs are attached to the visceral pleura, which is kept in contact with the parietal pleura through the fluid, which in turn is in contact with the wall. As a result, when the inhalation muscles expand the wall, the lungs have no choice but to expand as well. This action in turn lowers the intrapulmonary pressure compared to the external pressure. As air flows from high pressure to low pressure, air rushes into the lungs. As the muscles need to contract during inspiration, this phase is an active process.

There are moments when you might need to breathe more deeply or forcefully, such as during exercise. In this instance, accessory muscles intervene and increase the size of the thoracic cavity even more by further pulling the sternum and first two ribs. These muscles are the sternocleidomastoid, scalene and serati anterior muscles.

Expiration

While inspiration is active, expiration is a passive process because it uses the elastic recoil of the muscles and lungs. During normal expiration, the external intercostals together with the diaphragm relax. Everything happens in reverse compared to inspiration. The intrathoracic volume decreases, intrapulmonary pressure increases and air is expelled from the lungs. The expulsion of air continues until the pressure inside the lungs equals the external pressure, after which inspiration is re-started.

Similar to inspiration, expiration can become active in certain situations like exercise or playing an instrument. The rectus abdominis and internal intercostal muscles are recruited. The first pair increases intra-abdominal pressure, pushing the diaphragm even more upwards. The second pair pull the ribs downwards and inward, further reducing the size of the thoracic cavity.

Lung anatomy can get quite complicated extremely quickly. Ease into the topic and cement your knowledge using Kenhub's respiratory system quizzes and labeled diagrams.