Respiratory System
Breathing: How And Why We Breathe

Health Topics


Respiratory System


What Is The Respiratory System?
How Oxygen Enters The Body
How Breathing Works
How Does The Body Know When To Breathe?
What Does Air Contain?
Diseases Of The Respiratory System
Other Systems It Links To

What Is The Respiratory System?

It is the body's breathing equipment. The respiratory system consists of the nose, lungs, diaphragm and the air passages, such as the trachea, which connect them. Similar to the digestive system, it takes substances from outside the body (gases, particularly oxygen), circulates them through the body to cells and tissues, then dumps (excretes) the excess and waste. Oxygen is the respiratory system's 'food' and carbon dioxide is its 'waste'. Breathing is probably the most important and fundamental action of the human body: if we stop breathing for more than a couple of minutes we die.


The Passage of Air From Nose To Lungs

What Is Breathing?

Breathing is the inhalation and exhalation of air and the gases it contains.

How Do We Breathe?

We breathe through the nose and the system of passageways and organs with which it connects. The nose is the only organ of respiration that we can see. The following section explains the structure and function of the organs of the respiratory system.

What Is The Nose?

The nose is an organ on the face. It acts as the first passageway for air entering the body.
Structure: The nose is made of cartilage and 2 nasal bones. It is covered with skin, both inside and out and lined with a mucous membrane that is ciliated (that means it has microscopic hairs). The 2 nostrils lead into a bony nasal cavity, which has 2 chambers divided by a nasal septum. The septum is made of cartilage. Thus the outside of the nose which we can see, is mostly made of cartilage whereas the inside of the nose is mostly made of bone. The nasal cavity connects to the paranasal sinuses, hollow spaces inside the bones surrounding the nose which are full of air and are also lined with mucous membrane.
Functions: The nose is the first organ that air enters. It has 3 functions:
1. To smell things.
2. To moisten and warm the air entering the nostrils.
3. To filter dust, bacteria, and other unwelcome particles from the air using the mucous membrane and its hairs. The mucus collects any dirt and bacteria and prevents it from passing into the lungs. The cilia push the mucus into the throat. It is then swallowed and travels to the stomach where any bacteria are neutralized by gastric acids.


Once air has been filtered, moistened and warmed in the nose it travels to the pharynx, a tube which leads from the back of the nose and mouth and divides into the esophagus (posteriorly) and larynx (anteriorly). It works as part of both the digestive and respiratory systems.
Structure: The pharynx is about 12.5cm long and made of fibrous and muscular. At the back of the section of the pharynx which connects to the nose are small masses of lymphoid tissue which form the pharyngeal tonsils, or adenoids. Like the palatine tonsils (at the junction of the mouth and throat) the pharyngeal tonsils filter bacteria.
Function: It acts as an air passage and also warms and moistens the air.


From the pharynx, air travels down to the larynx (also known as the voice box).
Structure: The larynx is a tube positioned between the tongue at the back of the mouth and the trachea (the tube leading to the lungs). It is made of rings of cartilage, attached to each other by membranes and ligaments. The thyroid cartilage at the top of the larynx, which is larger in men than in women, forms the Adam's apple which is often visible in the throat.
Function: The larynx is a passageway for air between the pharynx and trachea. It filters bacteria, helps in voice production and warms and moistens the air.


From the larynx, air travels to the trachea.
Structure: The trachea is a continuation of the larynx. It is a tube about 10cm long which runs from the front of the neck to the chest where it divides into two bronchi, tubes which lead to the lungs. The trachea is made of incomplete rings of hyaline cartilage (anteriorly) and involuntary muscle and connective tissue (posteriorly). It is lined with ciliated epithelium which contains mucus-secreting goblet cells.
Function: The trachea is a passageway for air between the larynx and bronchi. The goblet secretory cells in the lining secrete mucus which collects any foreign matter or bacteria and the cilia then push this up to the larynx.

The bronchi are the branches of the respiratory tube which transport air in and out of each lung.
Structure: Bronchi (singular: bronchus) connect the trachea to the lungs. There are 2 of them, one on the left and one on the right which enter the lungs at the hilum, where they subdivide into different branches for different lobes of the lungs. Like the trachea, they are made of hyaline cartilage, involuntary muscle and connective tissue and are lined with ciliated (hairy) epithelium.
Function: To pass air from the trachea into the bronchioles, and thus to the lungs.

The final and finest tubes in the passage of air from the nose to the lungs are the bronchioles.
Structure: Bronchioles are made of muscular, fibrous and elastic tissue. They become progressively smaller as they spread further into the lungs until they are no more than a single layer of flattened epithelial cells (just like blood capillaries). These microscopic tubes are called terminal bronchioles.
Function: Bronchioles take air to the alveoli of the lungs.

The two lungs are the centre of the respiratory system. It is in these two spongy organs that gases enter and exit the blood.
Structure: The lungs are positioned on either side of the heart. The left lung is divided into 2 lobes, the superior and inferior lobes, whereas the right lung is divided into 3, the superior, middle and inferior. Lobes are subdivided into lobules. The tissue of the lungs is made of bronchioles, alveoli, blood vessels, nerves, connective tissue and elastic tissue. They are covered with the pleura, a special membrane.
Function: Lungs allow the exchange of gases into and out of the blood.

Structure: the pleura is a serous membrane that surrounds each lung. It has 2 layers, the inner, visceral layer which sticks to the lung tissue and covers the surface and the outer, parietal layer which sticks to the chest wall and the top of the diaphragm. The 2 layers are separated by a space called the pleural cavity which is filled with a fluid.
Function: The pleural cavity prevents friction between the two layers during respiration.

The exchange of gases in the lungs takes place in tiny sacs called alveoli (alveolus, singular) at the end of the terminal bronchioles. There are 300 million alveoli in your lungs.
Structure: Alveoli are made of a thin layer of squamous epithelial cells and are surrounded by a capillary network.
Function: The function of the alveoli is to exchange gases between the circulatory and respiratory systems. Deoxygenated blood is delivered to the capillary network via the pulmonary artery. It is then oxygenated by contact with the air in the alveoli. The oxygenated blood then leaves the lungs via the capillary network and the pulmonary veins and travels to the heart to be pumped around the body.


Internal and External Respiration


Mechanism Of Respiration

Although all of these separate tubes and passageways have individual functions, it is their function as a whole that is important i.e. to allow us, and every cell in our body, to breathe. The entrance and exit of air in and out of the body is a process known as breathing, whereas the entrance and exit of oxygen and carbon dioxide in and out of cells is known as gaseous exchange.

What Is External Respiration?

External respiration is the breathing in and out of air, and the diffusion of oxygen from the alveoli into the blood and carbon dioxide from the blood to the alveoli. In order to understand how the gases pass from one tissue to the next it is important to know the following physical law:

gases diffuse from a higher pressure to a lower pressure until equal pressure is achieved.

Diffusion occurs when a strong concentration of a gas comes into contact with a weak concentration of the same gas. The dissolved gas molecules will move from the strong concentration to the weak concentration until the concentration is equal on both sides. In the case of oxygen and carbon dioxide this occurs through the capillary and alveoli walls. The oxygen in the alveoli is under more pressure than the venous, deoxygenated blood in the capillaries so the oxygen passes from the alveoli (high pressure) into the capillaries (low pressure). Once the pressure in both is the same, the exchange stops. The carbon dioxide in the blood is under more pressure than the carbon dioxide in the alveoli so it diffuses through the capillary walls to the alveoli. The blood is thus oxygenated and its waste removed and it now travels back to the heart ready to be pumped round the body. The lungs then expel the carbon dioxide through the process of exhalation.

What Is Internal Respiration?

Internal respiration is the diffusion of oxygen from the blood to the body cells, and of carbon dioxide from the body cells to the blood. Once blood has been oxygenated in the lungs it travels back to the heart and is then pumped round the body. When blood reaches the various cells of the body, oxygen is again transferred by diffusion: the pressure of the oxygen in the blood is high whereas the pressure of the oxygen in the cells is low, so the oxygen passes into the cells. The amount of oxygen delivered depends on how busy the cell is. For example, more oxygen will be delivered to a muscle cell when it is exercising than when it is resting. The blood delivers its oxygen and collects the carbon dioxide (pressure in the blood is lower than in the cells so the carbon dioxide passes into the blood), carrying it back to the lungs where it will be delivered to the alveoli and then exhaled.

How Does Blood Travel To And From The Lungs?

It travels via the pulmonary circulation system, which is the movement of blood from the heart to the lungs and back again.

How Does Air Get Into The Body In The First Place?

Through the same gaseous pressure principle. Air enters the respiratory system (this is known as inspiration or inhalation) when the pressure is lower inside the lungs and leaves the lungs when the pressure in the atmosphere around the body is lower (known as expiration or exhalation). But it is the action of the muscles involved in respiration that make these changes in pressure, and the movement of air, happen. The main muscle involved in the mechanics of respiration is the diaphragm which is helped by the intercostal muscles (positioned between the ribs).

  What Is The Diaphragm?

The diaphragm is a large muscle. It is positioned between the chest and abdomen and separates them from each other.
Structure: The diaphragm is a sheet of muscle below your chest. When relaxed (exhalation) it is a dome shape; when contracted (inhalation) it flattens out.

Inspiration/inhalation: when the diaphragm contracts, it flattens out and since it forms the bottom of the chest cavity, this cavity then increases in size and volume. This lowers the pressure inside the chest. Air is thus sucked in because the pressure is lower inside the body than outside,
Expiration/exhalation: when the diaphragm relaxes it becomes a dome shape and pushes up the chest cavity, thus reducing the cavity's size and volume and increasing the pressure. Air rushes out because the pressure is lower outside.
• The diaphragm also helps with expulsive body actions:
- Urine excretion (micturition)
- Feces expulsion (defecation)
- Giving birth (parturition).

What Are The Intercostal Muscles?

They are the muscles between the ribs. These muscles aid the diaphragm in respiration. During inspiration the external intercostal muscles contract at the same time as the diaphragm, lifting the rib cage up and outwards. The flattened and lowered diaphragm and the raised ribs cause an increase in the size of the chest cavity. During expiration, the external intercostals relax allowing the ribs to fall down and inwards, helping to decrease the size of the chest cavity. Nerve impulses delivered by the intercostal nerves tell the muscles when to contract and relax.

How Does The Body Know When To Breathe?

Nerve cells called chemoreceptors, found in the aorta and carotid arteries (the arteries close to the heart) send impulses to the respiratory centre in the medulla oblongata of the brain with messages about the low levels of oxygen and high levels of carbon dioxide. When the level of carbon dioxide is too high and the level of oxygen too low a nerve impulse is sent to the diaphragm telling it to contract, thus causing inhalation. This is especially important during exercise and illness.

The Brain's Role In Breathing

Two centers of the brain are involved -the respiratory centre in the medulla oblongata and the pons Varolii:
• The respiratory centre stimulates inspiration and controls the depth of breathing and its regularity.
• The pons Varolii stops inspiration thus provoking expiration. When the respiratory centre tells the diaphragm to contract, air is sucked into the lungs, stimulating nerve cells called stretch receptors found in the lung tissue. These receptors send impulses to the pons Varolii which then sends impulses to the diaphragm telling it to relax, thus provoking expiration.

Note: It is important to remember that breathing is not an intermittent process! The body does not stop breathing when the correct levels of oxygen and carbon dioxide are established, although breathing slows down and speeds up depending on our level of activity and health. Cells and tissues need to breathe all the time because every bodily function and movement requires oxygen and produces carbon dioxide. Breathing is a necessary and (in a healthy person) automatic function which continues throughout life.

What Does Air Contain?

Air that comes into the body contains approximately 21 percent oxygen and 0.04 percent carbon dioxide whereas air that leaves the body contains approximately 15 percent oxygen and 4 percent carbon dioxide. Thus, the air we exhale contains 100 times more carbon dioxide and 6 percent less oxygen than the air we inhale.

Diseases And Disorders Of The Respiratory System

Bronchitis is a medical term to indicate inflammation of the bronchial tubes. It causes coughing, shortness of breath and fatigue. Smoking and infections (common cold or influenza) are the most common causes of bronchitis.

Emphysema is where the air sacs (alveoli) stretch and lose their elasticity. As they become gradually destroyed in this way, it prevents effective breathing, causing cough, wheezing and shortness of breath. Smoking and air pollution are the most common causes of emphysema.

An inflammation of the lining (pleural) of the lungs, due to infections such as tuberculosis and pneumonia. It causes pain in the chest when you take a deep breath or cough. It can also cause coughing, rapid breathing, shortness of breath and for your skin to turn a bluish color (cyanosis).

Inflammation of lung tissue caused by infection (viral or bacterial). It can cause coughs, fever, shaking chills, shortness of breath, stabbing chest pain and headaches. In rare cases it can be fatal.

Tuberculosis (TB)
A contagious bacterial infection which can be inhaled or eaten (in infected meat or milk). Symptoms include coughing, night sweats and fever. TB is more common in poorer countries where over-crowding and adequate nutrition is an issue.

A condition that causes the airways of the lungs to narrow, leading to coughing and wheezing. Often caused by allergies. See, what is asthma?

The mucous lining of the nose is irritated and inflamed. Causes stuffy, congested nose and sinuses. Rhinitis is caused by colds, flus, hay fever and sinus infections.

Hay Fever
Also known as allergic rhinitis. An allergy to certain pollens causes rhinitis. Symptoms include sneezing, runny nose and eyes and sometimes swelling/itching. What is hayfever?

Inflammation of sinuses caused by a bacterial, fungal or viral infection. It often follows a respiratory infection. It causes headaches and facial pain. Read, how to treat sinus infections.

Stress causes breathing rate to increase. Read about the dangers of stress.

The common cold is a mild viral infection that involves the nose and respiratory passages (but not the lungs). See, how to treat colds.

A cough is the sudden (and noisy) expulsion of air from the lungs which clears your air passages. See how to treat coughs.

Influenza (Flu)
Commonly known as the flu, influenza is a contagious illness of the upper airways and lungs. It is caused by a virus, which rapidly spreads from person to person. See, how to treat flu.

An inflammation of the mucous lining of the larynx (voice box). It causes hoarseness or loss of voice (due to irritation of the vocal cords) and coughing.

A sore throat caused by inflammation of the pharynx.

Pulmonary Embolism
A blockage of the main artery (pulmonary artery) that supplies blood to the lungs. The blockage can be foreign matter or a blood clot which has traveled from another part of the body. Highly dangerous it leads to death in 26 percent of victims. In most cases anticoagulants (blood thinning medications) are the mainstay treatment.

An infection of the tonsils that often causes a sore throat and fever.

Cor Pulmonale
Enlargement and failure of the right side of the heart. It is due to long-term high blood pressure in the pulmonary arteries (the arteries that supply blood to the lungs).

Chronic Obstructive Airways Disease (COPD)
When a patient has both chronic bronchitis and emphysema (which commonly coexist together), they are said to have COPD.

Cystic Fibrosis
Cystic fibrosis is a genetic disease with no known cure. The child's lungs, pancreas and intestines become clogged with thick mucus.

Breathing faster than normal, or over-breathing. You may not even know you are hyperventilating. Symptoms include dizziness and tingling of the fingers and toes and chest pain if continued.

Lung Cancer
One of the biggest causes of death in women (and men), lung cancer is the uncontrolled growth of cells in the lungs. Symptoms include chest pain, coughing up blood, unexplained weight loss, a cough that won't go away, wheezing and shortness of breath.

Thyroid Cancer
Thyroid cancer is a type of cancer that starts in the thyroid gland (located at the front of your neck). It is a relatively rare disease but is fortunately highly curable. Symptoms include a tickley cough, difficulties swallowing and hoarseness.

Whooping Cough
Medically termed pertussis: whooping cough is a highly contagious bacterial disease of the respiratory mucous membrane. It causes violent uncontrolled coughing and the patient makes a whooping sound when they breathe in (more of a childhood illness).

Collapsed Lung
Technically called pneumothorax, a collapsed lung is where there is an abnormal amount of air in the pleural cavity that results in the lung collapsing. It can be spontaneous (due to injury to the chest) or induced (as a treatment for tuberculosis).

Pulmonary Fibrosis
Chronic inflammation of the lung with progressive scarring of the alveolar walls that can lead to death.

A disease that causes lumps (nodules) to form in the lungs, liver, lymph glands and salivary glands. The cause is unknown.

Severe Acute Respiratory Syndrome (SARS)
A severe form of pneumonia that appears to have originated in China in 2003. It is characterized by fever and coughing or breathing difficulties or hypoxia (body becomes deprived of oxygen). It is highly contagious viral infection that can be fatal.

The sound made when your breathing is someway obstructed while sleeping. It may be caused by your mouth anatomy, drinking alcohol before bedtime or sleep apnea.

Other Systems It Links To In The Body

The respiratory system links to:

Circulatory system: The circulation transports oxygen from the respiratory system to every cell of the body and transports carbon dioxide to the respiratory system to be exhaled.

Nervous system: Respiration is closely controlled by the nervous system, which indicates when inhalation or exhalation should happen. Chemoreceptors in the main arteries stimulate the nervous response of the respiratory system to begin the process of inhaling oxygen when required.

Muscular system: The intercostal muscles and diaphragm are fundamental to the process of respiration.

Diagrams and Other Systems

Female Body: Diagrams of the different organs.
Human Body: How it works.
Lymphatic System: Drainage of fluid from cells in the body.
Skin Structure and Function: Diagrams and explanation of what the skin does.
Endocrine System: Hormones, how they are made and what they do.
Urinary System: Urine production and how we excrete it.


The respiratory system:
• Exchanges gases from outside to inside the body and vice versa.
• Is controlled by the nervous system.

Other Useful Guides

Recommended health screenings for women: Tests for all ages.
Latest female health statistics: How long are you likely to live?
Chest problems: Symptom checker.
Bones of the body: How they work and what they do.
Back problems: Symptom checker for diseases and conditions.
Hospital departments explained: Find your way around a hospital.

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