Anatomy of breathing
More than likely you were agitated, nervous, excited or energized more than once in your life and you tried to relax. Perhaps you exercised or you were in the most important interview of your life or just before an anatomy exam. No matter what, the words that usually come out of those around you are “breathe, just breathe slowly and relax.” But, what is breathing?
In the medical world, breathing is defined as pulmonary ventilation, described as the movement of air between the atmosphere and the lung alveoli . It involves two events: inspiration, when the air moves into the lungs and expiration, when the air leaves the lungs. Breathing is one of the four components of respiration, the other three being gas diffusion, gas transport and regulation. The pathway towards the lungs is provided by airways and together, these components form the respiratory system, which is located inside the thoracic or chest cavity. The thoracic cage and walls enclose this cavity and its structures, and play an essential role in pulmonary ventilation. The diaphragm and a variety of other muscles are also involved in the process of ventilation. The action of breathing is tightly controlled by the respiratory centre located inside the brain stem. This article will discuss the anatomical basis of breathing and will describe the anatomical components that move every 5 seconds to keep you alive.
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.
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.
Muscles of breathing
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.
The external and internal intercostals do not work individually during breathing. They are assisted by the sternocleidomastoid and scalene muscles on the neck.
The two sternocleidomastoid muscles originate from the mastoid process of the temporal bone and the superior nuchal line of the occipital bone. By attaching to the manubrium, hence sternum, via their sternal heads and the clavicle via their clavicular heads, these muscle can elevate the bones and subsequently lift the anterior ribs. Therefore, they are used as accessory 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.
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.
Airways and lungs
So far, you have seen how the thoracic cage is a frame that encloses the respiratory system and allows breathing to take place. Several muscles that span several regions of the body, such as the thoracic wall itself, neck, shoulder girdle and abdomen, act upon this structure. By altering the shape of the thoracic cage, air moves between the external environment and the lungs through a series of airways, the details of which will be discussed in this section.
The airways are subdivided into conducting zone (airways) and respiratory zone. The conduction airways carry air in and out of the lungs, while the respiratory zone formed by alveoli, is the site of gas exchange. The conducting airways consist of the following:
In addition to carrying the air, they also filter, humidify and warm it. These functions are performed by cilia and mucus secreting cells that line the walls of the airways. In the larynx, the airways are reinforced by C-shaped hyaline cartilage rings. Everything below the larynx is anatomically referred to as the tracheobronchial tree. The trachea, which is found within the superior mediastinum, serves as the trunk of the tree. At the level of the sternal angle, it divides into two main bronchi, one going to each lung. These enter the lungs at the hilum. Inside the lungs, the bronchi divide into smaller bronchi, forming the branches of the tracheobronchial tree. Primary bronchi divide into lobar bronchi supplying different lobes of the lungs. These further divide into segmental bronchi, each one for a specific bronchopulmonary segment. They keep subdividing and branching, ending in terminal and lastly in respiratory bronchioles which bring the air into alveoli.
The air carried by the airways during breathing eventually reaches the lungs. These vital organs of respiration inside the thorax are the site responsible for the exchange of oxygen and carbon dioxide. These soft and spongy structures are very elastic, are located either side of the heart, separated from each other by the mediastinum. Each lung has a superior end called an apex, which extends up to the level corresponding to the neck of the first rib, about 2.5 cm above the level of the clavicle. The base is the concave inferior surface that rests directly on the diaphragm. The right lung has three lobes, while the left one has two. The primary bronchi carrying air enters the lung at its hilum situated on its mediastinal surface.
Critical to the breathing mechanism are the pleural sacs enclosing the lungs. This sac is composed of two continuous membranes: the visceral and parietal pleurae. The visceral pleura comes in contact with the lungs, while the parietal pleura lines the internal surface of the thoracic wall. Between the two layers is the pleural cavity, a potential space containing a very small quantity of fluid, important for lubrication and cohesion of the pleural layers. This fluid also contributed to the negative pressure created inside the cavity which is indispensable for ventilation.
Now you have all the required pieces and understanding to assemble the puzzle illustrating breathing. You know how the components of the respiratory system are located inside a bony and flexible thoracic cage. A number of muscles then act on this cage to change its diameter and allow air to either leave or enter through conducting airways all the way to and from the lungs.
During a breathing cycle, the lungs can be expanded and contracted in two ways. Firstly by lengthening and shortening the chest cavity and secondly by increasing and decreasing its anteroposterior diameter. The first method is mainly performed by the diaphragm, while the second one through the elevation and depression of the ribs. The two phases of breathing are inspiration and expiration.
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.
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.
Regulation of breathing
The breathing cycle is controlled by the respiratory centre located inside the medulla oblongata and the pons of the brain stem. Three major collections of neurons form this centre. The dorsal respiratory group within the dorsal portion of the medulla is responsible for the largest part of the breathing cycle. The ventral respiratory group in the ventrolateral part of the medulla plays a role in forced expiration. The pneumotaxic centre located dorsally in the superior portion of the pons controls the rate and depth of breathing.
To initiate breathing, the dorsal respiratory group sends impulses through the phrenic nerve towards the diaphragm and through the intercostal nerves towards the external intercostal muscles. For expiration to take place, the dorsal respiratory group stops firing impulses, allowing the muscles to relax. When forced expiration is needed, impulses from the respiratory group reaches the ventral group, activating it. In turn, this group initiates impulses, which reach the rectus abdominis through the thoracoabdominal nerves and the internal intercostals through the intercostal nerves.