IAQ UK is an independent organisation with the aim of 'raising the agenda of indoor air quality within the home and workplace'

IAQUK Resources - Respiratory System



The Airway


The airways are passages that carry oxygen-rich air to the lungs and carbon dioxide out of the lungs. The airways include the:

  • Nose and linked air passages called nasal cavities
  • Mouth
  • Larynx, or voice box
  • Trachea, or windpipe
  • Tubes called bronchial tubes or bronchi, and their branches, called bronchioles


Air first enters the body through the nose or mouth, which wets and warms the air. (Cold, dry air can irritate the lungs). The air then travels through the larynx box and down the trachea. The trachea divides into two bronchi that enter the lungs. 

A thin flap of tissue called the epiglottis covers the trachea when a person swallows. This prevents food or drink from entering the air passages that lead to the lungs. 

Except for the mouth and some parts of the nose, all of the airways have special hairs called cilia that are coated with sticky mucus. The cilia trap germs and other foreign particles that enter the airways when a person breathes in air. 

These fine hairs then sweep the particles up to the nose or mouth. There, they are swallowed, coughed, or sneezed out of the body. Nose hairs and mouth saliva also trap particles and germs.


For your lungs to perform their best, these airways need to be open during inhalation and exhalation and free from inflammation or swelling and excess or abnormal amounts of mucus.



Consider the basics of your indoor air environment and the quality of the air you breathe.  If you replace the contents of a 5 gallon bottle that sits aloft a water cooler, substituting water with air, you will breathe an equivalent of 600 bottles every day, taking approximately 20,000 breaths.  It is often an unconscious action and you probably do not consider the quality of air that travels along dusty ventilation ducting or exhaled by adjoining occupants, which passes through our nose and mouth and into our lungs.


The air consists of oxygen (21%) with nitrogen (78%) and the remaining is small amounts of argon, carbon dioxide and other gases and water vapour (1%).  The composition of inhaled air remains relatively constant.  Although earth is a leaky vessel with small quantities of the atmosphere escaping into space every year.  The loss rate is currently tiny, only about three kilograms of hydrogen and 50 grams of helium per second; with the knowledge that oxygen is too heavy to leave the atmosphere, we do not need to worry about wearing oxygen packs yet.    Within the air, dusts, chemical components and biological agents will be present; these variables can affect our health, both physiologically and psychologically, they can affect our perception of our environment, determining whether our surroundings are conducive and comfortable.


Individually and collectively, we take the air we breathe for granted and yet every breath is vital for sustaining our lives, supplying our blood with oxygen and removing metabolic waste in the form of carbon dioxide.  Unconscious breathing is regulated by neural signals in our brain, the medulla (regulates the rhythm of inhalation and exhalation) and pons (controls the speed of inhalation and exhalation). 


Each inhalation takes approximately 1 second, exhalation follows immediately, which takes slightly longer. A pause of a second or two between the end of exhalation and the beginning of the next inhalation occurs as the CO2 content of the blood increases to the point where it triggers the next cycle to start. Normally the rate of respiration at rest is between 12 to 15 breaths per minute. The rate and depth of breathing depend on the body’s needs; it is a rising concentration of carbon dioxide, not a declining concentration of oxygen that stimulates the ventilation of the lungs. 


As most individuals spend approximately 80-90% of their time indoors, they are therefore exposed to the indoor environment to a much greater extent than the outdoor. Information obtained from laboratory and epidemiological studies suggest that indoor air pollutants are an important cause of avoidable morbidity and mortality in the UK’s life expectancy.


Anatomy of the Lung

  • alveolus - tiny, thin-walled air sac at the end of the bronchiole branches where gas exchange occurs (plural - alveoli).
  • bronchioles - numerous small tubes that branch from each bronchus into the lungs. They get smaller and smaller.
  • bronchus - a branch of the trachea that goes from the trachea into the lung (plural - bronchi)
  • diaphragm - muscle at the base of the chest cavity that contracts and relaxes during breathing.
  • epiglottis - a flap of tissue that closes over the trachea when you swallow so that food does not enter your airway.
  • intercostal muscles - muscles along the rib cage that assist in breathing
  • larynx - voice box where the vocal cords are located.
  • nasal cavity - chamber in from the nose where air is moistened and warmed .
  • pleural membranes - thin, membranes that cover the lungs, separate them from other organs and form a fluid-filled chest cavity.
  • pulmonary capillaries - small blood vessels that surround each alveolus.
  • trachea -rigid tube that connects the mouth with the bronchi.

Your Lungs


Your lungs are located within your chest cavity inside the rib cage.  They are made of spongy, elastic tissue that stretches and constricts as you breathe. The airways that bring air into the lungs (the trachea and bronchi) are made ­of smooth muscle and cartilage, allowing the airways to constrict and expand. The lungs and airways bring in fresh, oxygen-enriched air and get rid of waste carbon dioxide made by your cells. They also help in regulating the concentration of hydrogen ion (pH) in your blood.


When you inhale, the diaphragm and intercostal muscles (those are the muscles between your ribs) contract and expand the chest cavity. This expansion lowers the pressure in the chest cavity below the outside air pressure. Air then flows in through the airways (from high pressure to low pressure) and inflates the lungs. When you exhale, the diaphragm and intercostal muscles relax and the chest cavity gets smaller. The decrease in volume of the cavity increases the pressure in the chest cavity above the outside air pressure. Air from the lungs (high pressure) then flows out of the airways to the outside air (low pressure). The cycle then repeats with each breath.

The distinction between respiration and breathing

When the respiratory system is mentioned, people generally think of breathing, but breathing is only one of the activities involved in respiration. The body cells need a continuous supply of oxygen for the metabolic processes that are necessary to maintain life. The respiratory system works with the circulatory system to provide this oxygen and to remove the waste products of metabolism. It also helps to regulate pH of the blood. 

Every 3 to 5 seconds, nerve impulses stimulate the breathing process, orventilation, which, as described above, moves air through a series of passages into and out of the lungs. After this, there is an exchange of gases between the lungs and the blood. This is called external respiration. The blood transports the gases to and from the tissue cells. The exchange of gases between the blood and tissue cells is internal respiration. Finally, the cells utilize the oxygen for their specific activities. This is cellular metabolism, or cellular respiration. Together these activities constitute respiration.

The Respiratory Tract


The respiratory tract is divided into two main parts: the upper respiratory tract, consisting of the nose, nasal cavity and the pharynx; and the lower respiratory tract consisting of the larynx, trachea, bronchi and the lungs.


The trachea, which begins at the edge of the larynx, divides into two bronchi and continues into the lungs.


The trachea allows air to pass from the larynx to the bronchi and then to the lungs. The bronchi divide into smaller bronchioles which branch in the lungs forming passageways for air.


The terminal parts of the bronchi are the alveoli. The alveoli are the functional units of the lungs and they form the site of gaseous exchange.

Breathing in (inhalation) 

When a person breathes in, the diaphragm contracts (tightens) and moves downward. This increases the space in the chest cavity, into which the lungs expand. The intercostal muscles between the ribs also help enlarge the chest cavity. They contract to pull the rib cage both upward and outward when the person inhales.   As the lungs expand, air is sucked in through the nose or mouth. The air travels down the windpipe and into the lungs. After passing through the bronchial tubes, the air finally reaches and enters the alveoli (air sacs).   The bronchial tubes pass through the lungs, they divide into smaller air passages called bronchioles. The bronchioles end in tiny balloon-like air sacs called alveoli. Your body has over 300 million alveoli. The alveoli are surrounded by a mesh of tiny blood vessels called capillaries. Oxygen from the inhaled air passes through the very thin alveoli walls and into the blood.  A red blood cell protein called haemoglobin helps move oxygen from the air sacs to the blood. (Oxygen is especially drawn to haemoglobin.)  At the same time, as the cells use the oxygen, carbon dioxide is produced and absorbed into the blood. Your blood then carries the carbon dioxide back to your lungs through the capillaries, where it is removed from the body when you exhale.  The gas has travelled in the bloodstream from the right side of the heart through the pulmonary artery.  Oxygen-rich blood from the lungs is carried through a network of capillaries, which become the pulmonary vein. This vein delivers the oxygen-rich blood to the left side of the heart. The left side of the heart pumps the blood to the rest of the body. There, the oxygen in the blood moves from blood vessels into surrounding tissues. 

Breathing out (exhalation) 

When a person breathes out, the diaphragm relaxes and moves upward into the chest cavity. The intercostal muscles between the ribs also relax to make the chest cavity size smaller.  As the chest cavity gets smaller, air rich in carbon dioxide is forced out of the lungs and windpipe, and then out of the noseor mouth.  Breathing out requires no effort from the body unless a person has a lung disease or is doing physical activity. When a person is physically active, the abdominal muscles contract and push the diaphragm even more so against the lungs. This pushes the air in the lungs out rapidly.

Clearing the air


The respiratory system has built-in methods to prevent harmful substances in the air from entering the lungs.


Small hairs in your nose, called cilia, help filter out large particles. Cilia are also found along your air passages and move in a sweeping motion to keep the air passages clean. But if harmful substances, such as cigarette smoke, are inhaled, the cilia stop functioning properly, causing health problems like bronchitis.


Mucus produced by cells in the trachea and bronchial tubes keeps air passages moist and helps in stopping dust, bacteria and viruses, allergy-causing substances, and other substances from entering the lungs.


Impurities that do reach the deeper parts of the lungs can be moved up via mucus and coughed out or swallowed.

Breathing and the Autonomic Nervous System

You don'­t have to think about breathing because your body's autonomic nervous system controls it, as it does many other functions in your body. If you try to hold your breath, your body will override your action and force you to let out that breath and start breathing again. The respiratory centers that control your rate of breathing are in the brainstem or medulla. The nerve cells that live within these centers automatically send signals to the diaphragm and intercostal muscles to contract and relax at regular intervals. However, the activity of the respiratory centers can be influenced by these factors:


Oxygen: Specialized nerve cells within the aorta and carotid arteries called peripheral chemoreceptors monitor the oxygen concentration of the blood and feed back on the respiratory centers. If the oxygen concentration in the blood decreases, they tell the respiratory centers to increase the rate and depth of breathing.


Carbon dioxide: Peripheral chemoreceptors also monitor the carbon dioxide concentration in the blood. In addition, a central chemoreceptor in the medulla monitors the carbon dioxide concentration in the cerebrospinal fluid (CSF) that surrounds the brain and spinal cord; carbon dioxide diffuses easily into the CSF from the blood. If the carbon dioxide concentration gets too high, then both types of chemoreceptors signal the respiratory centers to increase the rate and depth of breathing. The increased rate of breathing returns the carbon dioxide concentration to normal and the breathing rate then slows down.


Hydrogen ion (pH): The peripheral and central chemoreceptors are also sensitive to the pH of the blood and CSF. If the hydrogen ion concentration increases (that is, if the fluid becomes more acidic), then the chemoreceptors tell the respiratory centers to speed up. Hydrogen ion concentration is heavily influenced by the carbon dioxide concentration and bicarbonate concentration in the blood and CSF.


Stretch: Stretch receptors in the lungs and chest wall monitorthe amount of stretch in these organs. If the lungs become over-inflated (stretch too much), they signal the respiratory centers to exhale and inhibit inspiration. This mechanism prevents damage to the lungs that would be caused by over-inflation.


Signals from higher brain centers: Nerve cells in the hypothalamus and cortex also influence the activity of the respiratory centers. During pain or strong emotions, the hypothalamus will tell the respiratory centers to speed up. Nerve centers in the cortex can voluntarily tell the respiratory center to speed up, slow down or even stop (holding your breath). Their influence, however, can be overridden by chemical factors (oxygen, carbon dioxide, pH).


Chemical irritants: Nerve cells in the airways sense the presence of unwanted substances in the airways such as pollen, dust, noxious fumes, water, or cigarette smoke. These cells then signal the respiratory centers to contract the respiratory muscles, causing you to sneeze or cough. Coughing and sneezing cause air to be rapidly and violently exhaled from the lungs and airways, removing the offending substance.


Of these factors, the strongest influence is the carbon dioxide concentration in your blood and CSF followed by the oxygen concentration.


Sometimes the respiratory centers go temporarily awry and sends extra impulses to the diaphragm. These impulses cause unwanted contractions (hiccups). The same thing happens in unborn children; many pregnant women often feel their babies hiccup. This happens because the respiratory centers of the developing child's brain are working just like those of an adult even though they are not yet breathing air.

Where the Air Goes

As you breathe air in through your noseor mouth, it goes past the epiglottis and into the trachea. It continues down the trachea through your vocal cords in the laryn­x until it reaches the bronchi. From the bronchi, air passes into each lung.  The air then follows narrower and narrower bronchioles until it reaches the alveoli.

Within each air sac, the oxygen concentration is high, so oxygen passesor diffuses across the alveolar membrane into the pulmonary capillary. At the beginning of the pulmonary capillary, the haemoglobin in the red blood cells has carbon dioxide bound to it and very little oxygen. 


The oxygen binds to haemoglobin and the carbon dioxide is released. Carbon dioxide is also released from sodium bicarbonate dissolved in the blood of the pulmonary capillary. The concentration of carbon dioxide is high in the pulmonary capillary, so carbon dioxide leaves the blood and passes across the alveolar membrane into the air sac.


This exchange of gases occurs rapidly (fractions of a second). The carbon dioxide then leaves the alveolus when you exhale and the oxygen-enriched blood returns to the heart. Thus, the purpose of breathing is to keep the oxygen concentration high and the carbon dioxide concentration low in the alveoli so this gas exchange can occur.


Lung diseases and conditions

Many steps are involved in breathing. If injury, disease, or other factors affect any of the steps, a person may have trouble breathing. For example, the fine hairs (cilia) that line the upper airways may not trap all of the germs that are breathed in. These germs can cause an infection in the bronchi (bronchitis) or deep in the lungs (pneumonia). These infections cause a buildup of mucus and/or fluid that narrows the airways and hinders airflow in and out of the lungs. 

If a person has asthma, breathing in certain substances to which that individual is sensitive can trigger the airways to narrow. This makes it hard forair to flow in and out of the lungs. 

Over a long period, breathing in cigarette smoke or air pollutants can damage the airways and the air sacs. This can lead to a condition called chronic obstructive pulmonary disease (COPD). COPD prevents proper airflow in and out of the lungs and can hinder gas exchange in the air sacs. 

An important step to breathing is the movement of the diaphragm and other muscles in the chest, neck, and abdomen. This movement lets a person inhale and exhale. Nerves that run from the brain to these muscles control their movement. Damage to these nerves in the upper spinal cord can cause breathing to stop, unless a machine is used to help in breathing. (This machine is called a ventilator or respirator). 

A steady flow of blood in the small blood vessels that surround the air sacs is vital for gas exchange. Long periods of inactivity or surgery can cause a blood clot called a pulmonary embolism to block the lung artery. This reduces orstops the flow of blood in the small blood vessels and interferes with gas exchange.