Anatomy of your Respiratory System

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 many other muscles are also involved in the process of ventilation. The action of breathing is tightly controlled by the respiratory center located inside the brain stem.

The Thoracic Cage

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

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. ll 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.

The Muscles of Respiration

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. 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 inferoanterior 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 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 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 muscles 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 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 labored breathing when grasping a support or staying in the so-called tripod position.

Abdominal Muscles

The 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.

The Airways and Lungs

Subdivided into conducting zones (airways) and respiratory zones. 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: The nose, nasopharynx, larynx, trachea, bronchi, bronchioles, terminal bronchioles.

Your lungs are in your chest and are so big that they take up most of the space in there. You have two lungs, but they aren't the same size the way your eyes or nostrils are. Instead, the lung on the left side of your body is a bit smaller than the lung on the right. This extra space on the left leaves room for your powerful heart.

Your lungs are protected by your rib cage, which is made up of 12 sets of ribs. These ribs are connected to your spine in your back and go around your lungs to keep them safe. Beneath the lungs is the diaphragm.

You can't see your lungs, but it's easy to feel them in action: Put your hands on your chest and breathe in very deeply. You will feel your chest getting slightly bigger. Now breathe out the air, and feel your chest return to its regular size. You've just felt the power of your lungs!

“The lungs produce the most DMT in the body. Conscious Breathing is psychedelic”.

A Look Inside the Lungs

These soft and spongy structures are very elastic and 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. From the outside, lungs are pink and a bit squishy, like a sponge. But the inside contains the real lowdown on the lungs! At the bottom of the trachea, or windpipe, there are two large tubes. These tubes are called the main stem bronchi, and one heads left into the left lung, while the other heads right into the right lung.

Each main stem bronchus — the name for just one of the bronchi — then branches off into tubes, or bronchi, that get smaller and even smaller still, like branches on a big tree. The tiniest tubes are called bronchioles, and there are about 30,000 of them in each lung. Each bronchiole is about the same thickness as a hair.

At the end of each bronchiole is a special area that leads into clumps of teeny tiny air sacs called alveoli. There are about 600 million alveoli in your lungs and if you stretched them out, they would cover a large space.

Each alveolus — what we call just one of the alveoli — has a mesh-like covering of very small blood vessels called capillaries. These capillaries are so tiny that the cells in your blood need to line up a single file just to march through them.

Every time you inhale air, dozens of body parts work together to help get that air in there without you ever thinking about it.


Inspiration involves air entering the lungs from the external environment. Normal and quiet inspiration. As you breathe in, your diaphragm contracts and flattens out. This allows it to move down, so your lungs have more room to grow larger as they fill up with air. And the diaphragm isn't the only part that gives your lungs the room they need. Your rib muscles also lift the ribs up and outward to give the lungs more space.

At the same time, you inhale air through your mouth and nose, and the air heads down your trachea, or windpipe. On the way down the windpipe, tiny hairs called cilia move gently to keep mucus and dirt out of the lungs. The air then goes through the series of branches in your lungs, through the bronchi and the bronchioles.

The Alveoli

The air finally ends up in the 600 million alveoli. As these millions of alveoli fill up with air, the lungs get bigger. It's the alveoli that allow oxygen from the air to pass into your blood.

All the cells in the body need oxygen every minute of the day. Oxygen passes through the walls of each alveolus into the tiny capillaries that surround it. The oxygen enters the blood in the tiny capillaries, hitching a ride on red blood cells and traveling through layers of blood vessels to the heart. The heart then sends the oxygenated (filled with oxygen) blood out to all the cells in the body.


While inspiration is active, expiration is a passive process because it uses the elastic recoil of the muscles and lungs. When it's time to exhale (breathe out), everything happens in reverse.  Your diaphragm relaxes and moves up, pushing air out of the lungs. Your rib muscles become relaxed, and your ribs move in again, creating a smaller space in your chest.

By now your cells have used the oxygen they need, and your blood is carrying carbon dioxide and other wastes that must leave your body. The blood comes back through the capillaries and the wastes enter the alveoli. Then you breathe them out in the reverse order of how they came in — the air goes through the bronchioles, out the bronchi, out the trachea, and finally out through your mouth and nose.

The air that you breathe out not only contains wastes and carbon dioxide, but it's warm too. As air travels through your body, it picks up heat along the way. You can feel this heat by putting your hand in front of your mouth or nose as you breathe out. With all this movement, you might be wondering why things don't get stuck as the lungs fill and empty! Luckily, your lungs are covered by two really slick special layers called pleural membranes. These membranes are separated by a fluid that allows them to slide around easily while you inhale and exhale.

Time for Talk

Your lungs are important for breathing and also for talking. Above the trachea is the larynx, which is sometimes called the voice box. Across the voice box are two tiny ridges called vocal cords, which open and close to make sounds. When you exhale air from the lungs, it comes through the trachea and larynx and reaches the vocal cords. If the vocal cords are closed and the air flows between them, the vocal cords vibrate, and a sound is made.

The amount of air you blow out from your lungs determines how loud a sound will be and how long you can make the sound. The next time you're outside, try shouting and see what happens — shouting requires lots of air, so you'll need to breathe in more frequently than you would if you were only saying the words. Experiment with different sounds and the air it takes to make them.

Regulation of Breathing

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.

The breathing cycle is controlled by the respiratory center located inside the medulla oblongata and the pons of the brain stem. Three major collections of neurons form this center. 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 center 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.

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