The respiratory system (also respiratory apparatus, ventilatory system) is

a biological system consisting of specific organs and structures used for gas
exchange in animals and plants. The anatomy and physiology that make this happen
varies greatly, depending on the size of the organism, the environment in which it lives
and its evolutionary history. In land animals, the respiratory surface is internalized as
linings of the lungs.[1] Gas exchange in the lungs occurs in millions of small air sacs; in
mammals and reptiles, these are called alveoli, and in birds, they are known as atria.
These microscopic air sacs have a very rich blood supply, thus bringing the air into
close contact with the blood.[2] These air sacs communicate with the external
environment via a system of airways, or hollow tubes, of which the largest is
the trachea, which branches in the middle of the chest into the two main bronchi. These
enter the lungs where they branch into progressively narrower secondary and tertiary
bronchi that branch into numerous smaller tubes, the bronchioles. In birds, the
bronchioles are termed parabronchi. It is the bronchioles, or parabronchi that generally
open into the microscopic alveoli in mammals and atria in birds. Air has to be pumped
from the environment into the alveoli or atria by the process of breathing which involves
the muscles of respiration.

In most fish, and a number of other aquatic

animals (both vertebrates and invertebrates), the respiratory system consists of gills,
which are either partially or completely external organs, bathed in the watery
environment. This water flows over the gills by a variety of active or passive means.
Gas exchange takes place in the gills which consist of thin or very
flat filaments and lammellae which expose a very large surface area of
highly vascularized tissue to the water.

Other animals, such as insects, have respiratory systems with very simple anatomical
features, and in amphibians, even the skin plays a vital role in gas
exchange. Plants also have respiratory systems but the directionality of gas exchange
can be opposite to that in animals. The respiratory system in plants includes anatomical
features such as stomata, that are found in various parts of the plant.[3]

Mammals
Anatomy
Main articles: Lung and Respiratory tract
 Fig. 1. Respiratory system

Fig. 2. The lower respiratory tract, or "Respiratory Tree"

1. Trachea
2. Mainstem bronchus
3. Lobar bronchus
4. Segmental bronchus
5. Bronchiole
6. Alveolar duct
7. Alveolus
In humans and other mammals, the anatomy of a typical respiratory system is
the respiratory tract. The tract is divided into an upper and a lower respiratory tract. The
upper tract includes the nose, nasal cavities, sinuses, pharynx and the part of
the larynx above the vocal folds. The lower tract (Fig. 2.) includes the lower part of
the larynx, the trachea, bronchi, bronchioles and the alveoli.

The branching airways of the lower tract are often described as the respiratory
tree or tracheobronchial tree (Fig. 2).[4] The intervals between successive branch points
along the various branches of "tree" are often referred to as branching "generations", of
which there are, in the adult human, about 23. The earlier generations (approximately
generations 0–16), consisting of the trachea and the bronchi, as well as the larger
bronchioles which simply act as air conduits, bringing air to the respiratory bronchioles,
alveolar ducts and alveoli (approximately generations 17–23), where gas
exchange takes place.[5][6] Bronchioles are defined as the small airways lacking
any cartilaginous support.[4]

The first bronchi to branch from the trachea are the right and left main bronchi. Second,
only in diameter to the trachea (1.8 cm), these bronchi (1–1.4 cm in diameter)[5] enter
the lungs at each hilum, where they branch into narrower secondary bronchi known as
lobar bronchi, and these branch into narrower tertiary bronchi known as segmental
bronchi. Further divisions of the segmental bronchi (1 to 6 mm in diameter)[7] are known
as 4th order, 5th order, and 6th order segmental bronchi, or grouped together as
subsegmental bronchi.[8][9]

Compared to the 23 number (on average) of branchings of the respiratory tree in the
adult human, the mouse has only about 13 such branchings.

The alveoli are the dead end terminals of the "tree", meaning that any air that enters
them has to exit via the same route. A system such as this creates dead space, a
volume of air (about 150 ml in the adult human) that fills the airways after exhalation
and is breathed back into the alveoli before environmental air reaches them. [10][11] At the
end of inhalation, the airways are filled with environmental air, which is exhaled without
coming in contact with the gas exchanger.[10]

Ventilatory volumes
 Fig.
3 Output of a 'spirometer'. Upward movement of the graph, read from the left, indicates the
intake of air; downward movements represent exhalation.
Main articles: Breathing and Lung volumes
The lungs expand and contract during the breathing cycle, drawing air in and out of the
lungs. The volume of air moved in or out of the lungs under normal resting
circumstances (the resting tidal volume of about 500 ml), and volumes moved during
maximally forced inhalation and maximally forced exhalation are measured in humans
by spirometry.[12] A typical adult human spirogram with the names given to the various
excursions in volume the lungs can undergo is illustrated below (Fig. 3):

Not all the air in the lungs can be expelled during maximally forced exhalation (ERV).
This is the residual volume (volume of air remaining even after a forced exhalation) of
about 1.0–1.5 liters which cannot be measured by spirometry. Volumes that include the
residual volume (i.e. functional residual capacity of about 2.5–3.0 liters, and total lung
capacity of about 6 liters) can therefore also not be measured by spirometry. Their
measurement requires special techniques.[12]

The rates at which air is breathed in or out, either through the mouth or nose or into or
out of the alveoli are tabulated below, together with how they are calculated. The
number of breath cycles per minute is known as the respiratory rate. An average
healthy human breathes 12–16 times a minute.

Measurement Equation Description

Minute tidal volume * the total volume of air entering, or leaving,

ventilation respiratory rate the nose or mouth per minute or normal
 respiration.

(tidal volume – dead

Alveolar the volume of air entering or leaving the
space) * respiratory
ventilation alveoli per minute.
rate

the volume of air that does not reach the

Dead space dead space *
alveoli during inhalation, but instead remains
ventilation respiratory rate
in the airways, per minute.

Mechanics of breathing
Duration: 16 seconds.0:16Subtitles available.CCFig. 6 Real-time magnetic resonance
imaging (MRI) of the chest movements of human thorax during breathing
Main article: Breathing § Mechanics
The "pump handle" and "bucket handle movements" of the ribs

Fig. 4 The effect of the muscles of inhalation in expanding the rib cage. The particular action
illustrated here is called the pump handle movement of the rib cage.
 Fig. 5 In this view of the rib cage the downward slope of the lower ribs from the midline outwards
can be clearly seen. This allows a movement similar to the "pump handle effect", but in this case, it
is called the bucket handle movement. The color of the ribs refers to their classification, and is not
relevant here.

Breathing

Fig. 7 The muscles of breathing at rest: inhalation on the left, exhalation on the right. Contracting
muscles are shown in red; relaxed muscles in blue. Contraction of the diaphragm generally
contributes the most to the expansion of the chest cavity (light blue). However, at the same time, the
intercostal muscles pull the ribs upwards (their effect is indicated by arrows) also causing the rib
cage to expand during inhalation (see diagram on other side of the page). The relaxation of all these
muscles during exhalation causes the rib cage and abdomen (light green) to elastically return to their
resting positions. Compare with Fig. 6, the MRI video of the chest movements during the breathing
cycle.

Fig. 8 The muscles of forceful breathing (inhalation and exhalation). The color code is the same as
on the left. In addition to a more forceful and extensive contraction of the diaphragm, the intercostal
muscles are aided by the accessory muscles of inhalation to exaggerate the movement of the ribs
upwards, causing a greater expansion of the rib cage. During exhalation, apart from the relaxation of
the muscles of inhalation, the abdominal muscles actively contract to pull the lower edges of the rib
cage downwards decreasing the volume of the rib cage, while at the same time pushing the
diaphragm upwards deep into the thorax.

In mammals, inhalation at rest is primarily due to the contraction of the diaphragm. This
is an upwardly domed sheet of muscle that separates the thoracic cavity from the
abdominal cavity. When it contracts, the sheet flattens, (i.e. moves downwards as
shown in Fig. 7) increasing the volume of the thoracic cavity in the antero-posterior axis.
The contracting diaphragm pushes the abdominal organs downwards. But because the
pelvic floor prevents the lowermost abdominal organs from moving in that direction, the
pliable abdominal contents cause the belly to bulge outwards to the front and sides,
because the relaxed abdominal muscles do not resist this movement (Fig. 7). This
entirely passive bulging (and shrinking during exhalation) of the abdomen during normal
breathing is sometimes referred to as "abdominal breathing", although it is, in fact,
"diaphragmatic breathing", which is not visible on the outside of the body. Mammals
only use their abdominal muscles during forceful exhalation (see Fig. 8, and discussion
below). Never during any form of inhalation.

As the diaphragm contracts, the rib cage is simultaneously enlarged by the ribs being
pulled upwards by the intercostal muscles as shown in Fig. 4. All the ribs slant
downwards from the rear to the front (as shown in Fig. 4); but the lowermost
ribs also slant downwards from the midline outwards (Fig. 5). Thus the rib cage's
transverse diameter can be increased in the same way as the antero-posterior diameter
is increased by the so-called pump handle movement shown in Fig. 4.

The enlargement of the thoracic cavity's vertical dimension by the contraction of the
diaphragm, and its two horizontal dimensions by the lifting of the front and sides of the
ribs, causes the intrathoracic pressure to fall. The lungs' interiors are open to the
outside air and being elastic, therefore expand to fill the increased space, pleura
fluid between double-layered pleura covering of lungs helps in reducing friction while
lungs expand and contract. The inflow of air into the lungs occurs via the respiratory
airways (Fig. 2). In a healthy person, these airways begin with the nose.[13][14] (It is
possible to begin with the mouth, which is the backup breathing system. However,
chronic mouth breathing leads to, or is a sign of, illness.[15][16][17]) It ends in the
microscopic dead-end sacs called alveoli, which are always open, though the diameters
of the various sections can be changed by the sympathetic and parasympathetic
nervous systems. The alveolar air pressure is therefore always close to atmospheric air
pressure (about 100 kPa at sea level) at rest, with the pressure gradients because of
lungs contraction and expansion cause air to move in and out of the lungs during
breathing rarely exceeding 2–3 kPa.[18][19]

During exhalation, the diaphragm and intercostal muscles relax. This returns the chest
and abdomen to a position determined by their anatomical elasticity. This is the "resting
mid-position" of the thorax and abdomen (Fig. 7) when the lungs contain their functional
residual capacity of air (the light blue area in the right hand illustration of Fig. 7), which
in the adult human has a volume of about 2.5–3.0 liters (Fig. 3).[6] Resting exhalation
lasts about twice as long as inhalation because the diaphragm relaxes passively more
gently than it contracts actively during inhalation.
 Fig. 9 The changes in the
composition of the alveolar air during a normal breathing cycle at rest. The scale on the left, and
the blue line, indicate the partial pressures of carbon dioxide in kPa, while that on the right and
the red line, indicate the partial pressures of oxygen, also in kPa (to convert kPa into mm Hg,
multiply by 7.5).
The volume of air that moves in or out (at the nose or mouth) during a single breathing
cycle is called the tidal volume. In a resting adult human, it is about 500 ml per breath.
At the end of exhalation, the airways contain about 150 ml of alveolar air which is the
first air that is breathed back into the alveoli during inhalation.[10][20] This volume air that is
breathed out of the alveoli and back in again is known as dead space ventilation, which
has the consequence that of the 500 ml breathed into the alveoli with each breath only
350 ml (500 ml – 150 ml = 350 ml) is fresh warm and moistened air.[6] Since this 350 ml
of fresh air is thoroughly mixed and diluted by the air that remains in the alveoli after a
normal exhalation (i.e. the functional residual capacity of about 2.5–3.0 liters), it is clear
that the composition of the alveolar air changes very little during the breathing cycle
(see Fig. 9). The oxygen tension (or partial pressure) remains close to 13–14 kPa
(about 100 mm Hg), and that of carbon dioxide very close to 5.3 kPa (or 40 mm Hg).
This contrasts with composition of the dry outside air at sea level, where the partial
pressure of oxygen is 21 kPa (or 160 mm Hg) and that of carbon dioxide 0.04 kPa (or
0.3 mmHg).[6]

During heavy breathing (hyperpnea), as, for instance, during exercise, inhalation is
brought about by a more powerful and greater excursion of the contracting diaphragm
than at rest (Fig. 8). In addition, the "accessory muscles of inhalation" exaggerate the
actions of the intercostal muscles (Fig. 8). These accessory muscles of inhalation are
muscles that extend from the cervical vertebrae and base of the skull to the upper ribs
and sternum, sometimes through an intermediary attachment to the clavicles.[6] When
they contract, the rib cage's internal volume is increased to a far greater extent than can
be achieved by contraction of the intercostal muscles alone. Seen from outside the
body, the lifting of the clavicles during strenuous or labored inhalation is sometimes
called clavicular breathing, seen especially during asthma attacks and in people
with chronic obstructive pulmonary disease.

During heavy breathing, exhalation is caused by relaxation of all the muscles of

inhalation. But now, the abdominal muscles, instead of remaining relaxed (as they do at
rest), contract forcibly pulling the lower edges of the rib cage downwards (front and
sides) (Fig. 8). This not only drastically decreases the size of the rib cage, but also
pushes the abdominal organs upwards against the diaphragm which consequently
bulges deeply into the thorax (Fig. 8). The end-exhalatory lung volume is now well
below the resting mid-position and contains far less air than the resting "functional
residual capacity". However, in a normal mammal, the lungs cannot be emptied
completely. In an adult human, there is always still at least 1 liter of residual air left in
the lungs after maximum exhalation.[6]

The automatic rhythmical breathing in and out, can be interrupted by coughing,

sneezing (forms of very forceful exhalation), by the expression of a wide range of
emotions (laughing, sighing, crying out in pain, exasperated intakes of breath) and by
such voluntary acts as speech, singing, whistling and the playing of wind instruments.
All of these actions rely on the muscles described above, and their effects on the
movement of air in and out of the lungs.

Although not a form of breathing, the Valsalva maneuver involves the respiratory
muscles. It is, in fact, a very forceful exhalatory effort against a tightly closed glottis, so
that no air can escape from the lungs.[21] Instead, abdominal contents are evacuated in
the opposite direction, through orifices in the pelvic floor. The abdominal muscles
contract very powerfully, causing the pressure inside the abdomen and thorax to rise to
extremely high levels. The Valsalva maneuver can be carried out voluntarily but is more
generally a reflex elicited when attempting to empty the abdomen during, for instance,
difficult defecation, or during childbirth. Breathing ceases during this maneuver.

Gas exchange
Main article: Gas exchange
Mechanism of gas exchange
Fig. 11 A highly diagrammatic illustration of the process of gas exchange in the mammalian lungs,
emphasizing the differences between the gas compositions of the ambient air, the alveolar air (light
blue) with which the pulmonary capillary blood equilibrates, and the blood gas tensions in the
pulmonary arterial (blue blood entering the lung on the left) and venous blood (red blood leaving the
lung on the right). All the gas tensions are in kPa. To convert to mm Hg, multiply by 7.5.

Fig. 12 A diagrammatic histological cross-section through a portion of lung tissue showing a

normally inflated alveolus (at the end of a normal exhalation), and its walls containing the pulmonary
capillaries (shown in cross-section). This illustrates how the pulmonary capillary blood is completely
surrounded by alveolar air. In a normal human lung, all the alveoli together contain about 3 liters of
alveolar air. All the pulmonary capillaries contain about 100 ml of blood.
 Fig. 10 A histological cross-section through an
alveolar wall showing the layers through which the gases have to move between the blood
plasma and the alveolar air. The dark blue objects are the nuclei of the capillary endothelial and
alveolar type I epithelial cells (or type 1 pneumocytes). The two red objects labeled "RBC"
are red blood cells in the pulmonary capillary blood.
The primary purpose of the respiratory system is the equalizing of the partial pressures
of the respiratory gases in the alveolar air with those in the pulmonary capillary blood
(Fig. 11). This process occurs by simple diffusion,[22] across a very thin membrane
(known as the blood–air barrier), which forms the walls of the pulmonary
alveoli (Fig. 10). It consists of the alveolar epithelial cells, their basement
membranes and the endothelial cells of the alveolar capillaries (Fig. 10).[23] This blood
gas barrier is extremely thin (in humans, on average, 2.2 μm thick). It is folded into
about 300 million small air sacs called alveoli[23] (each between 75 and 300 μm in
diameter) branching off from the respiratory bronchioles in the lungs, thus providing an
extremely large surface area (approximately 145 m2) for gas exchange to occur.[23]

The air contained within the alveoli has a semi-permanent volume of about 2.5–3.0 liters
which completely surrounds the alveolar capillary blood (Fig. 12). This ensures that
equilibration of the partial pressures of the gases in the two compartments is very
efficient and occurs very quickly. The blood leaving the alveolar capillaries and is
eventually distributed throughout the body therefore has a partial pressure of oxygen of
13–14 kPa (100 mmHg), and a partial pressure of carbon dioxide of 5.3 kPa (40 mmHg)
(i.e. the same as the oxygen and carbon dioxide gas tensions as in the alveoli). [6] As
mentioned in the section above, the corresponding partial pressures of oxygen and
carbon dioxide in the ambient (dry) air at sea level are 21 kPa (160 mmHg) and
0.04 kPa (0.3 mmHg) respectively.[6]

This marked difference between the composition of the alveolar air and that of the
ambient air can be maintained because the functional residual capacity is contained in
dead-end sacs connected to the outside air by fairly narrow and relatively long tubes
(the airways: nose, pharynx, larynx, trachea, bronchi and their branches down to
the bronchioles), through which the air has to be breathed both in and out (i.e. there is
no unidirectional through-flow as there is in the bird lung). This typical mammalian
anatomy combined with the fact that the lungs are not emptied and re-inflated with each
breath (leaving a substantial volume of air, of about 2.5–3.0 liters, in the alveoli after
exhalation), ensures that the composition of the alveolar air is only minimally disturbed
when the 350 ml of fresh air is mixed into it with each inhalation. Thus the animal is
provided with a very special "portable atmosphere", whose composition differs
significantly from the present-day ambient air.[24] It is this portable atmosphere
(the functional residual capacity) to which the blood and therefore the body tissues are
exposed – not to the outside air.