IAQUK Resources - Respiratory System
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
- Larynx, or voice box
- Trachea, or windpipe
- Tubes called bronchial tubes or bronchi, and their branches,
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
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.
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
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
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
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
- 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 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
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
distinction between respiration and breathing
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.
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.
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
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
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
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
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.
diseases and conditions
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.