View source for Breathing

{{Short description|Process of moving air in and out of the lungs}}
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{{Other uses}}
{{Redirect2|Breath|Breathed}} }}
{{use dmy|date=September 2022}}
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[[File:Real-time MRI - Thorax.ogv|thumb|upright=1.4|Real-time [[magnetic resonance imaging]] of the human thorax during breathing]]
[[File:X-ray video of a female American alligator (Alligator mississippiensis) while breathing - pone.0004497.s009.ogv|thumb|upright=1.4|X-ray video of a female [[American alligator]] while breathing]]
'''Breathing''' ('''spiration'''<ref>{{Cite web |title=Definition of SPIRATION |url=https://www.merriam-webster.com/dictionary/spiration |access-date=2023-10-16 |website=www.merriam-webster.com |language=en}}</ref> or '''ventilation''') is the [[neuroscience of rhythm|rhythmical process]] of moving air into ([[inhalation]]) and out of ([[exhalation]]) the [[lung]]s to facilitate [[gas exchange]] with the [[Milieu intérieur|internal environment]], mostly to flush out [[carbon dioxide]] and bring in [[oxygen]].

All [[Aerobic respiration|aerobic]] creatures need oxygen for [[cellular respiration]], which extracts energy from the reaction of oxygen with molecules derived from [[food]] and produces carbon dioxide as a [[Waste|waste product]]. Breathing, or external respiration, brings [[Atmosphere of Earth|air]] into the lungs where gas exchange takes place in the [[Pulmonary alveolus|alveoli]] through [[diffusion]]. The body's [[circulatory system]] transports these gases to and from the cells, where cellular [[Respiration (physiology)|respiration]] takes place.<ref>{{cite book|last1=Hall|first1=John|title=Guyton and Hall Textbook of Medical Physiology|date=2011|publisher=Saunders/Elsevier|location=Philadelphia, Pennsylvania|isbn=978-1-4160-4574-8|page=5|edition=12th}}</ref><ref name="Pocock2">{{cite book|last1=Pocock|first1=Gillian|last2=Richards|first2=Christopher D.|title=Human physiology: the basis of medicine|date=2006|publisher=Oxford University Press|location=Oxford|isbn=978-0-19-856878-0|page=311|edition=3rd}}</ref>

The breathing of all [[vertebrate]]s with lungs consists of repetitive cycles of [[inhalation]] and [[exhalation]] through a highly branched system of tubes or [[Respiratory tract|airways]] which lead from the nose to the alveoli.<ref name="Pocock">{{cite book|last1=Pocock|first1=Gillian|last2=Richards|first2=Christopher D.|title=Human physiology: the basis of medicine|date=2006|publisher=Oxford University Press|location=Oxford|isbn=978-0-19-856878-0|page=320|edition=3rd}}</ref> The number of respiratory cycles per minute is the breathing or [[respiratory rate]], and is one of the four primary [[vital signs]] of life.<ref>{{cite web|title=Vital Signs 101|url=http://www.hopkinsmedicine.org/healthlibrary/conditions/cardiovascular_diseases/vital_signs_body_temperature_pulse_rate_respiration_rate_blood_pressure_85,P00866/|website=Johns Hopkins Medicine|date=14 June 2022 |language=en}}</ref> Under normal conditions the breathing depth and rate is automatically, and unconsciously, controlled by several [[Homeostasis|homeostatic mechanisms]] which keep the [[partial pressure]]s of [[carbon dioxide]] and [[oxygen]] in the arterial blood constant. Keeping the partial pressure of carbon dioxide in the arterial blood unchanged under a wide variety of [[Physiology|physiological]] circumstances contributes significantly to [[acid–base homeostasis|tight control of the pH]] of the [[extracellular fluids]] (ECF). Over-breathing ([[hyperventilation]]) increases the arterial partial pressure of carbon dioxide, causing a rise in the pH of the ECF. Under-breathing ([[hypoventilation]]), on the other hand, decreases the arterial partial pressure of carbon dioxide and lowers the pH of the ECF. Both cause distressing symptoms.

Breathing has other important functions. It provides a mechanism for [[speech]], [[laughter]] and similar expressions of the emotions. It is also used for [[reflex]]es such as [[yawning]], [[cough reflex|coughing]] and [[sneezing]]. Animals that cannot [[Thermoregulation|thermoregulate]] by [[perspiration]], because they lack sufficient [[sweat gland]]s, may lose heat by evaporation through panting.

==Mechanics==
{{Further|Muscles of respiration}}
{{Multiple image
| direction = horizontal
| align = right
| header = The "pump handle" and "bucket handle movements" of the ribs 
| width1 = 200
| image1 = ribcage during inhalation.jpg
| caption1 = The effect of the [[Muscles of respiration|muscles of inhalation]] in expanding the [[rib cage]]. The particular action illustrated here is called the [[pump handle movement]] of the rib cage.
| width2 = 200
| image2 = Costillas (gray).png
| caption2 = 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]].
}}
{{Multiple image
| direction = horizontal
| align = top
| header = Breathing
| width2 = 200
| image2 = Forceful breathing.jpg
| caption2 = 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.
| width1 = 200
| image1 = Quiet breathing.jpg
| caption1 = 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 [[Thoracic diaphragm|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 another 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 these diagrams with the MRI video at the top of the page.
}}

The [[lung]]s are not capable of inflating themselves and will expand only when there is an increase in the volume of the [[thoracic cavity]].<ref>{{cite book|last1=Pocock|first1=Gillian|last2=Richards|first2=Christopher D.|title=Human physiology : the basis of medicine|date=2006|publisher=Oxford University Press|location=Oxford|isbn=978-0-19-856878-0|page=316|edition=3rd}}</ref><ref name="Levitzky2013_1">{{cite book|last1=Levitzky|first1=Michael G.|title=Pulmonary physiology|date=2013|publisher=McGraw-Hill Medical|location=New York|isbn=978-0-07-179313-1|page=Chapter 1. Function and Structure of the Respiratory System|edition=Eighth}}</ref> In humans, as in the other [[mammals]], this is achieved primarily through the contraction of the [[Thoracic diaphragm|diaphragm]], but also by the contraction of the [[intercostal muscles]] which pull the [[Rib cage#Function|rib cage]] upwards and outwards as shown in the diagrams on the right.<ref name=tortora1>{{cite book |last1= Tortora |first1= Gerard J. |last2=Anagnostakos|first2=Nicholas P.| title=Principles of anatomy and physiology |url= https://archive.org/details/principlesofanat05tort |url-access= registration |pages=[https://archive.org/details/principlesofanat05tort/page/556 556–582]|edition= Fifth |location= New York |publisher= Harper & Row, Publishers|date= 1987 |isbn= 978-0-06-350729-6 }}</ref> During forceful inhalation (figure on the right) the [[accessory muscles of breathing|accessory muscles of inhalation]], which connect the ribs and [[sternum]] to the [[cervical vertebrae]] and base of the skull, in many cases through an intermediary attachment to the [[clavicles]], exaggerate the [[Pump handle movement|pump handle]] and [[bucket handle movement]]s (see illustrations on the left), bringing about a greater change in the volume of the chest cavity.<ref name=tortora1 /> During exhalation (breathing out) at rest, all the muscles of inhalation relax, returning the chest and abdomen to a position called the "resting position", which is determined by their anatomical elasticity.<ref name=tortora1 /> At this point the lungs contain the [[functional residual capacity]] of air, which, in the adult human, has a volume of about 2.5–3.0 liters.<ref name=tortora1 />

During heavy breathing ([[hyperpnea]]) as, for instance, during exercise, exhalation is brought about by relaxation of all the muscles of inhalation, (in the same way as at rest), but, in addition, the abdominal muscles, instead of being passive, now contract strongly causing the rib cage to be pulled downwards (front and sides).<ref name=tortora1 /> This not only decreases the size of the rib cage but also pushes the abdominal organs upwards against the diaphragm which consequently bulges deeply into the thorax. The end-exhalatory lung volume is now less air than the resting "functional residual capacity".<ref name=tortora1 /> However, in a normal mammal, the lungs cannot be emptied completely. In an adult human, there is always still at least one liter of residual air left in the lungs after maximum exhalation.<ref name=tortora1 />

[[Diaphragmatic breathing]] causes the abdomen to rhythmically bulge out and fall back. It is, therefore, often referred to as "abdominal breathing". These terms are often used interchangeably because they describe the same action.

When the accessory muscles of inhalation are activated, especially during [[labored breathing]], the clavicles are pulled upwards, as explained above. This external manifestation of the use of the accessory muscles of inhalation is sometimes referred to as [[clavicular breathing]], seen especially during [[asthma]] attacks and in people with [[chronic obstructive pulmonary disease]].

==Passage of air==
{{Main|Respiratory tract}}

[[File:Inhalation and Exhalation Diagram.jpg|thumb|This is a diagram showing how inhalation and exhalation is controlled by a variety of muscles, and what that looks like from a general overall view.]]

===Upper airways===
[[File:Nose in cold weather.gif|thumb|Inhaled air is warmed and moistened by the wet, warm nasal mucosa, which consequently cools and dries. When warm, wet air from the lungs is breathed out through the nose, the cold hygroscopic mucus in the cool and dry nose re-captures some of the warmth and moisture from that exhaled air. In very cold weather the re-captured water may cause a "dripping nose".]]

Ideally, air is breathed [[Obligate nasal breathing|first out and secondly in through the nose]].<ref>{{Cite journal |last1=Watso |first1=Joseph C. |last2=Cuba |first2=Jens N. |last3=Boutwell |first3=Savannah L. |last4=Moss |first4=Justine E. |last5=Bowerfind |first5=Allison K. |last6=Fernandez |first6=Isabela M. |last7=Cassette |first7=Jessica M. |last8=May |first8=Allyson M. |last9=Kirk |first9=Katherine F. |date=2023-11-21 |title=Acute nasal breathing lowers diastolic blood pressure and increases parasympathetic contributions to heart rate variability in young adults |journal=American Journal of Physiology-Regulatory, Integrative and Comparative Physiology |volume=325 |issue=6 |pages=R797–R808 |doi=10.1152/ajpregu.00148.2023 |pmid=37867476 |issn=0363-6119|pmc=11178300 }}</ref> The [[nasal cavity|nasal cavities]] (between the [[nostril]]s and the [[pharynx]]) are quite narrow, firstly by being divided in two by the [[nasal septum]], and secondly by [[Lateral (anatomy)|lateral]] walls that have several longitudinal folds, or shelves, called
[[nasal concha]]e,<ref name="grays" /> thus exposing a large area of [[Mucous membrane of nose|nasal mucous membrane]] to the air as it is inhaled (and exhaled). This causes the inhaled air to take up moisture from the wet [[mucus]] and warmth from the underlying blood vessels so that the air is very nearly saturated with [[water vapor]] and is at almost body temperature by the time it reaches the [[larynx]].<ref name=tortora1 /> Part of this moisture and heat is recaptured as the exhaled air moves out over the partially dried-out, cooled mucus in the nasal passages during exhalation. The sticky mucus also traps much of the particulate matter that is breathed in, preventing it from reaching the lungs.<ref name=tortora1 /><ref name=grays>{{cite book |last1=Williams |first1=Peter L |last2=Warwick |first2=Roger |last3=Dyson|first3=Mary |last4=Bannister |first4=Lawrence H. |title=Gray's Anatomy| pages=1172–1173, 1278–1282 |location=Edinburgh|publisher=Churchill Livingstone | edition=Thirty-seventh |date=1989|isbn= 0443-041776 }}</ref>

===Lower airways===
[[File:illu quiz lung05.jpg|thumb|left|240px|'''The lower airways''':{{ordered list |[[Vertebrate trachea|Trachea]] |[[Main bronchus|Mainstem bronchus]] |[[Secondary bronchus|Lobar bronchus]] |[[Tertiary bronchus|Segmental bronchus]] |[[Bronchiole]] |[[Alveolar duct]] |[[Pulmonary alveolus|Alveolus]]}}]]
The anatomy of a typical mammalian respiratory system, below the structures normally listed among the "upper airways" (the nasal cavities, the pharynx, and larynx), is often described as a '''respiratory tree''' or '''tracheobronchial tree''' (figure on the left). Larger airways give rise to branches that are slightly narrower, but more numerous than the "trunk" airway that gives rise to the branches. The human respiratory tree may consist of, on average, 23 such branchings into progressively smaller airways, while the respiratory tree of the [[mouse]] has up to 13 such branchings. Proximal divisions (those closest to the top of the tree, such as the trachea and bronchi) function mainly to transmit air to the lower airways. Later divisions such as the respiratory bronchioles, alveolar ducts and alveoli are specialized for [[gas exchange]].<ref name=tortora1 /><ref name=gilroy>{{cite book|last1=Gilroy|first1=Anne M.|last2=MacPherson|first2= Brian R.|last3= Ross|first3=Lawrence M.|title= Atlas of Anatomy|publisher=Thieme|location=Stuttgart|date=2008|pages=108–111|isbn=978-1-60406-062-1}}</ref>

The trachea and the first portions of the main bronchi are outside the lungs. The rest of the "tree" branches within the lungs, and ultimately extends to every part of the [[lung]]s.

The alveoli are the blind-ended terminals of the "tree", meaning that any air that enters them has to exit the same way it came. A system such as this creates [[Dead space (physiology)|dead space]], a term for the volume of air that fills the airways at the end of inhalation, and is breathed out, unchanged, during the next exhalation, never having reached the alveoli. Similarly, the dead space is filled with alveolar air at the end of exhalation, which is the first air to be breathed back into the alveoli during inhalation, before any fresh air which follows after it. The dead space volume of a typical adult human is about 150&nbsp;ml.

==Gas exchange==
{{Main|Gas exchange}}
The primary purpose of breathing is to refresh air in the alveoli so that [[gas exchange]] can take place in the blood. The equilibration of the partial pressures of the gases in the alveolar blood and the alveolar air occurs by [[Diffusion#Diffusion vs. bulk flow|diffusion]]. After exhaling, adult human lungs still contain 2.5–3 L of air, their [[functional residual capacity]] or FRC. On inhalation, only about 350 mL of new, warm, moistened atmospheric air is brought in and is well mixed with the FRC. Consequently, the gas composition of the FRC changes very little during the breathing cycle. This means that the pulmonary capillary blood always equilibrates with a relatively constant air composition in the lungs and the diffusion rate with arterial blood gases remains equally constant with each breath. Body tissues are therefore not exposed to large swings in oxygen and carbon dioxide tensions in the blood caused by the breathing cycle, and the [[peripheral chemoreceptor|peripheral]] and [[central chemoreceptors]] measure only gradual changes in dissolved gases. Thus the homeostatic control of the breathing rate depends only on the partial pressures of oxygen and carbon dioxide in the arterial blood, which then also maintains [[Acid–base homeostasis|a constant pH]] of the blood.<ref name=tortora1 />

==Control==

{{Main|Control of ventilation}}
The rate and depth of breathing is automatically controlled by the [[respiratory center]]s that receive information from the [[peripheral chemoreceptors|peripheral]] and [[central chemoreceptors]]. These [[chemoreceptor]]s continuously monitor the partial pressures of carbon dioxide and oxygen in the arterial blood. The first of these sensors are the central chemoreceptors on the surface of the [[medulla oblongata]] of the [[brain stem]] which are particularly sensitive to [[pH]] as well as the partial pressure of carbon dioxide in the blood and [[cerebrospinal fluid]].<ref name=tortora1 /> The second group of sensors measure the partial pressure of oxygen in the arterial blood. Together the latter are known as the peripheral chemoreceptors, and are situated in the [[Aortic body|aortic]] and [[carotid body|carotid bodies]].<ref name=tortora1 /> Information from all of these chemoreceptors is conveyed to the [[respiratory center]]s in the [[pons]] and [[medulla oblongata]], which responds to fluctuations in the partial pressures of carbon dioxide and oxygen in the arterial blood by adjusting the rate and depth of breathing, in such a way as to restore the partial pressure of carbon dioxide to 5.3&nbsp;kPa (40&nbsp;mm&nbsp;Hg), the pH to 7.4 and, to a lesser extent, the partial pressure of oxygen to 13&nbsp;kPa (100&nbsp;mm&nbsp;Hg).<ref name=tortora1 /> For example, [[Physical exercise|exercise]] increases the production of carbon dioxide by the active muscles. This carbon dioxide diffuses into the venous blood and ultimately raises the partial pressure of carbon dioxide in the arterial blood. This is immediately sensed by the carbon dioxide chemoreceptors on the brain stem. The respiratory centers respond to this information by causing the rate and depth of breathing to increase to such an extent that the partial pressures of carbon dioxide and oxygen in the arterial blood return almost immediately to the same levels as at rest. The respiratory centers communicate with the muscles of breathing via motor nerves, of which the [[phrenic nerve]]s, which innervate the diaphragm, are probably the most important.<ref name=tortora1 />

Automatic breathing can be overridden to a limited extent by simple choice, or to facilitate [[Human swimming|swimming]], [[speech]], [[singing]] or other [[vocal]] training. It is impossible to suppress the urge to breathe to the point of hypoxia but training can increase the ability to hold one's breath. [[Conscious breathing]] practices have been shown to promote relaxation and stress relief but have not been proven to have any other health benefits.<ref name=acs>{{cite book |publisher=[[American Cancer Society]] |title=American Cancer Society Complete Guide to Complementary and Alternative Cancer Therapies |edition=2nd |year=2009 |isbn=9780944235713 |editor=Ades TB |pages=[https://archive.org/details/americancancerso0000unse/page/72 72–74] |chapter=Breathwork |chapter-url=https://archive.org/details/americancancerso0000unse/page/72 }}</ref>

Other automatic breathing control reflexes also exist. Submersion, particularly of the face, in cold water, triggers a response called the [[diving reflex]].<ref name="pann">{{cite journal|pmc=3768097|year=2013|last1=Michael Panneton|first1=W|title=The Mammalian Diving Response: An Enigmatic Reflex to Preserve Life?|journal=Physiology|volume=28|issue=5|pages=284–297|doi=10.1152/physiol.00020.2013|pmid=23997188}}</ref><ref name="Physiology and Pathophysiology">{{cite journal|title=The physiology and pathophysiology of human breath-hold diving|url=http://jap.physiology.org/content/106/1/284 |journal=Journal of Applied Physiology|volume=106 |issue=1 |date=1 January 2009 |pages=284–292 |doi=10.1152/japplphysiol.90991.2008 |pmid=18974367 |first1=Peter |last1=Lindholm |first2=Claes EG |last2=Lundgren |access-date=4 April 2015|url-access=subscription }}</ref> This has the initial result of shutting down the airways against the influx of water. The [[metabolic rate]] slows down. This is coupled with intense vasoconstriction of the arteries to the limbs and abdominal viscera, reserving the oxygen that is in blood and lungs at the beginning of the dive almost exclusively for the heart and the brain.<ref name="pann" /> The diving reflex is an often-used response in animals that routinely need to dive, such as penguins, seals and whales.<ref name=pmid15233163>{{cite journal |vauthors=Thornton SJ, Hochachka PW |title=Oxygen and the diving seal |journal=Undersea Hyperb Med |volume=31 |issue=1 |pages=81–95 |year=2004 |pmid=15233163 |url=http://archive.rubicon-foundation.org/6520 |access-date=2008-06-14 |archive-date=2008-12-11 |archive-url=https://web.archive.org/web/20081211020631/http://archive.rubicon-foundation.org/6520 |url-status=usurped }}</ref><ref name=pmid2800051>{{cite journal |vauthors=Zapol WM, Hill RD, Qvist J, Falke K, Schneider RC, Liggins GC, Hochachka PW |title=Arterial gas tensions and hemoglobin concentrations of the freely diving Weddell seal |journal=Undersea Biomed Res |volume=16 |issue=5 |pages=363–73 |date=September 1989 |pmid=2800051 |url=http://archive.rubicon-foundation.org/2531 |access-date=2008-06-14 |archive-date=2008-12-11 |archive-url=https://web.archive.org/web/20081211020626/http://archive.rubicon-foundation.org/2531 |url-status=usurped }}</ref> It is also more effective in very young infants and children than in adults.<ref>{{cite journal|pmid=21881008|year=2012|last1=Pedroso|first1=F. S.|title=The diving reflex in healthy infants in the first year of life|journal=Journal of Child Neurology|volume=27|issue=2|pages=168–71|last2=Riesgo|first2=R. S.|last3=Gatiboni|first3=T|last4=Rotta|first4=N. T.|doi=10.1177/0883073811415269|s2cid=29653062}}</ref>
{{-}}

==Composition==
{{Further|Atmospheric chemistry}}
[[File:Bakó Árpád - Milagro (Santana Tribute) együttes, Miskolc, 2015.02.07 (8).JPG|thumb|Following on from the above diagram, if the exhaled air is breathed out through the mouth in cold and [[humid]] conditions, the [[water vapor]] will [[condense]] into a visible [[cloud]] or [[mist]].]]

Inhaled air is by volume 78% [[nitrogen]], 20.95% oxygen and small amounts of other gases including [[argon]], carbon dioxide, [[neon]], [[helium]], and [[hydrogen]].<ref name="NASA">{{cite web|title=Earth Fact Sheet|url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html|website=NASA Space Science Data Coordinated Archive |publisher=NASA}}</ref>

The gas exhaled is 4% to 5% by volume of carbon dioxide, about a hundredfold increase over the inhaled amount. The volume of oxygen is reduced by about a quarter, 4% to 5%, of total air volume. The typical composition is:<ref name=":0">{{Cite book|title = A Textbook of Biology |first1=P. S. |last1=Dhami |first2=G. |last2=Chopra |first3=H. N. |last3=Shrivastava|publisher = Pradeep Publications|year = 2015|location = Jalandhar, Punjab|pages = V/101}}</ref>

*5.0–6.3% water vapor 
*79% nitrogen <ref>{{Cite web|url=https://www.bbc.co.uk/bitesize/guides/z6h4jxs/revision/3|title=Gas exchange in the lungs - Respiratory system - GCSE Biology (Single Science) Revision|website=BBC Bitesize}}</ref>
*13.6–16.0% oxygen
*4.0–5.3% carbon dioxide
*1% argon 
* [[Parts-per notation|parts per million]] (ppm) of [[hydrogen]], from the metabolic activity of microorganisms in the large intestine.<ref>{{cite journal |doi=10.1088/1752-7155/2/4/046002|title=Implementation and interpretation of hydrogen breath tests|year=2008|last1=Eisenmann|first1=Alexander|last2=Amann|first2=Anton|last3=Said|first3=Michael|last4=Datta|first4=Bettina|last5=Ledochowski|first5=Maximilian|journal=Journal of Breath Research|volume=2|issue=4|page=046002|pmid=21386189|bibcode=2008JBR.....2d6002E|s2cid=31706721 }}</ref>{{clarify|train=Looks like there should be a number here, but there isn't. If it's just "a few ppm", say that explicitly|date=September 2023}}
*ppm of [[carbon monoxide]] from degradation of [[heme]] proteins.{{clarify|train=Looks like there should be a number here, but there isn't. If it's just "a few ppm", say that explicitly|date=September 2023}}
* 4.5 ppm of [[methanol]]<ref>{{Cite journal|last1=Turner C|title=A longitudinal study of methanol in the exhaled breath of 30 healthy volunteers using selected ion flow tube mass spectrometry, SIFT-MS|journal=Physiological Measurement|volume=27|issue=7|pages=637–48|pmid=16705261|year=2006|doi=10.1088/0967-3334/27/7/007|bibcode=2006PhyM...27..637T|s2cid=22365066}}</ref>
*1 ppm of [[ammonia]]. 
* Trace many hundreds of [[volatile organic compounds]], especially [[isoprene]] and [[acetone]]. The presence of certain organic compounds indicates disease.<ref>{{cite journal|doi=10.1016/S0378-4347(99)00127-9|pmid=10410929|title=Variation in volatile organic compounds in the breath of normal humans|journal=Journal of Chromatography B: Biomedical Sciences and Applications|volume=729|issue=1–2|pages=75–88|year=1999|last1=Phillips|first1=Michael|last2=Herrera|first2=Jolanta|last3=Krishnan|first3=Sunithi|last4=Zain|first4=Mooena|last5=Greenberg|first5=Joel|last6=Cataneo|first6=Renee N.}}</ref><ref>{{cite journal |doi=10.1088/1752-7155/8/1/014001|title=A review of the volatiles from the healthy human body|year=2014|last1=De Lacy Costello|first1=B.|last2=Amann|first2=A.|last3=Al-Kateb|first3=H.|last4=Flynn|first4=C.|last5=Filipiak|first5=W.|last6=Khalid|first6=T.|last7=Osborne|first7=D.|last8=Ratcliffe|first8=N. M.|journal=Journal of Breath Research|volume=8|issue=1|page=014001|pmid=24421258|bibcode=2014JBR.....8a4001D|s2cid=1998578 }}</ref>

In addition to air, [[Underwater diving|underwater divers]] practicing [[technical diving]] may breathe oxygen-rich, oxygen-depleted or helium-rich [[breathing gas]] mixtures. Oxygen and [[analgesic]] gases are sometimes given to patients under medical care. The atmosphere in [[space suit]]s is pure oxygen. However, this is kept at around 20% of Earthbound atmospheric pressure to regulate the rate of inspiration.{{Citation needed|date=January 2021}}

==Effects of ambient air pressure==
===Breathing at altitude===
{{See also|Effects of high altitude on humans}}
[[File:Altitude and air pressure & Everest.jpg|thumb|Fig. 4 Atmospheric pressure]]
[[Atmospheric pressure]] decreases with the height above sea level (altitude) and since the alveoli are open to the outside air through the open airways, the pressure in the lungs also decreases at the same rate with altitude. At altitude, a pressure differential is still required to drive air into and out of the lungs as it is at sea level. The mechanism for breathing at altitude is essentially identical to breathing at sea level but with the following differences:

The atmospheric pressure decreases exponentially with altitude, roughly halving with every {{convert|5500|m|ft}} rise in altitude.<ref name=altitude>{{cite web|url=http://www.altitude.org/calculators/air_pressure.php|title=Online high altitude oxygen calculator|publisher=altitude.org|access-date=15 August 2007|url-status=dead|archive-url=https://archive.today/20120729214053/http://www.altitude.org/calculators/air_pressure.php|archive-date=29 July 2012}}</ref> The composition of atmospheric air is, however, almost constant below 80&nbsp;km, as a result of the continuous mixing effect of the weather.<ref name = tyson /> The concentration of oxygen in the air (mmols O<sub>2</sub> per liter of air) therefore decreases at the same rate as the atmospheric pressure.<ref name = tyson>{{cite book |last1=Tyson |first1=P.D.|last2=Preston-White|first2=R.A. |title=The weather and climate of Southern Africa. |location=Cape Town |publisher=Oxford University Press |date=2013| pages= 3–10, 14–16, 360|isbn=9780195718065 }}</ref> At sea level, where the [[ambient pressure]] is about 100&nbsp;[[Pascal (unit)|kPa]], oxygen constitutes 21% of the atmosphere and the partial pressure of oxygen ({{math|''P''<sub>O<sub>2</sub></sub>}}) is 21&nbsp;kPa (i.e. 21% of 100&nbsp;kPa). At the summit of [[Mount Everest]], {{convert|8848|m|ft}}, where the total atmospheric pressure is 33.7&nbsp;kPa, oxygen still constitutes 21% of the atmosphere but its partial pressure is only 7.1&nbsp;kPa (i.e. 21% of 33.7&nbsp;kPa = 7.1&nbsp;kPa).<ref name = tyson /> Therefore, a greater volume of air must be inhaled at altitude than at sea level in order to breathe in the same amount of oxygen in a given period.

During inhalation, air is warmed and saturated with [[Vapour pressure of water|water vapor]] as it passes through the nose and [[pharynx]] before it enters the alveoli. The ''saturated'' vapor pressure of water is dependent only on temperature; at a body core temperature of 37&nbsp;°C it is 6.3&nbsp;kPa (47.0&nbsp;mmHg), regardless of any other influences, including altitude.<ref>{{cite book |last1=Diem |first1=K. |last2=Lenter| first2= C.|title=Scientific Tables| location=Basle, Switzerland|publisher=Ciba-Geigy |date=1970|edition=7th| pages= 257–8 }}</ref> Consequently, at sea level, the ''tracheal'' air (immediately before the inhaled air enters the alveoli) consists of: water vapor ({{math|''P''<sub>H<sub>2</sub>O</sub>}} = 6.3&nbsp;kPa), nitrogen ({{math|''P''<sub>N<sub>2</sub></sub>}} = 74.0&nbsp;kPa), oxygen ({{math|''P''<sub>O<sub>2</sub></sub>}} = 19.7&nbsp;kPa) and trace amounts of carbon dioxide and other gases, a total of 100&nbsp;kPa. In dry air, the {{math|''P''<sub>O<sub>2</sub></sub>}} at sea level is 21.0&nbsp;kPa, compared to a {{math|''P''<sub>O<sub>2</sub></sub>}} of 19.7&nbsp;kPa in the tracheal air (21% of [100 – 6.3] = 19.7&nbsp;kPa). At the summit of Mount Everest tracheal air has a total pressure of 33.7&nbsp;kPa, of which 6.3&nbsp;kPa is water vapor, reducing the {{math|''P''<sub>O<sub>2</sub></sub>}} in the tracheal air to 5.8&nbsp;kPa (21% of [33.7 – 6.3] = 5.8&nbsp;kPa), beyond what is accounted for by a reduction of atmospheric pressure alone (7.1&nbsp;kPa).

The [[pressure gradient]] forcing air into the lungs during inhalation is also reduced by altitude. Doubling the volume of the lungs halves the pressure in the lungs at any altitude. Having the sea level air pressure (100&nbsp;kPa) results in a pressure gradient of 50&nbsp;kPa but doing the same at 5500&nbsp;m, where the atmospheric pressure is 50&nbsp;kPa, a doubling of the volume of the lungs results in a pressure gradient of the only 25&nbsp;kPa. In practice, because we breathe in a gentle, cyclical manner that generates pressure gradients of only 2–3&nbsp;kPa, this has little effect on the actual rate of inflow into the lungs and is easily compensated for by breathing slightly deeper.<ref>{{cite journal |last1=Koen |first1=Chrisvan L. |last2=Koeslag |first2=Johan H. | title=On the stability of subatmospheric intrapleural and intracranial pressures |journal= News in Physiological Sciences | date=1995 |volume=10 |issue=4 |pages=176–8 |doi=10.1152/physiologyonline.1995.10.4.176 }}</ref><ref>{{cite book |last1=West |first1=J.B. |title=Respiratory physiology: the essentials |url=https://books.google.com/books?id=FsRqAAAAMAAJ |isbn=978-0-683-08940-0 |publisher=Williams & Wilkins |date=1985| pages= 21–30, 84–84, 98–101 }}</ref> The lower [[viscosity]] of air at altitude allows air to flow more easily and this also helps compensate for any loss of pressure gradient.

All of the above effects of low atmospheric pressure on breathing are normally accommodated by increasing the respiratory minute volume (the volume of air breathed in — ''or'' out — per minute), and the mechanism for doing this is automatic. The exact increase required is determined by the [[Homeostasis#Levels of blood gases|respiratory gases homeostatic mechanism]], which regulates the arterial {{math|''P''<sub>O<sub>2</sub></sub>}} and {{math|''P''<sub>CO<sub>2</sub></sub>}}. This [[Homeostasis|homeostatic mechanism]] prioritizes the regulation of the arterial {{math|''P''<sub>CO<sub>2</sub></sub>}} over that of oxygen at sea level. That is to say, at sea level the arterial {{math|''P''<sub>CO<sub>2</sub></sub>}} is maintained at very close to 5.3&nbsp;kPa (or 40&nbsp;mmHg) under a wide range of circumstances, at the expense of the arterial {{math|''P''<sub>O<sub>2</sub></sub>}}, which is allowed to vary within a very wide range of values, before eliciting a corrective ventilatory response. However, when the atmospheric pressure (and therefore the atmospheric {{math|''P''<sub>O<sub>2</sub></sub>}}) falls to below 75% of its value at sea level, oxygen [[homeostasis]] is given priority over carbon dioxide homeostasis. This switch-over occurs at an elevation of about {{convert|2500|m|ft}}. If this switch occurs relatively abruptly, the hyperventilation at high altitude will cause a severe fall in the arterial {{math|''P''<sub>CO<sub>2</sub></sub>}} with a consequent rise in the [[Homeostasis#Blood pH|pH of the arterial plasma]] leading to [[respiratory alkalosis]]. This is one contributor to [[Altitude sickness|high altitude sickness]]. On the other hand, if the switch to oxygen homeostasis is incomplete, then [[Hypoxia (medical)|hypoxia]] may complicate the clinical picture with potentially fatal results.

===Breathing at depth===
[[File:Breathing Resistance.svg|thumb|Typical breathing effort when breathing through a diving regulator]]

Pressure increases with the depth of water at the rate of about one [[Atmosphere (unit)|atmosphere]] – slightly more than 100 kPa, or one [[Bar (unit)|bar]], for every 10 meters. Air breathed underwater by [[underwater diving|divers]] is at the ambient pressure of the surrounding water and this has a complex range of physiological and biochemical implications. If not properly managed, breathing compressed gasses underwater may lead to several [[diving disorder]]s which include [[Barotrauma#Pulmonary barotrauma|pulmonary barotrauma]], [[decompression sickness]], [[nitrogen narcosis]], and [[oxygen toxicity]]. The effects of breathing gasses under pressure are further complicated by the use of one or more [[Breathing gas|special gas mixtures]].

Air is provided by a [[diving regulator]], which reduces the high pressure in a [[diving cylinder]] to the ambient pressure. The [[breathing performance of regulators]] is a factor when choosing a suitable regulator for the [[Scuba diving#Types of diving|type of diving]] to be undertaken. It is desirable that breathing from a regulator requires low effort even when supplying large amounts of air. It is also recommended that it supplies air smoothly without any sudden changes in resistance while inhaling or exhaling. In the graph, right, note the initial spike in pressure on exhaling to open the exhaust valve and that the initial drop in pressure on inhaling is soon overcome as the [[Venturi effect]] designed into the regulator to allow an easy draw of air. Many regulators have an adjustment to change the ease of inhaling so that breathing is effortless.

==Respiratory disorders==
{{Infobox medical condition (new)
|  name        = Breathing patterns
|  image       = File:Breathing abnormalities.svg
|  caption     = Graph showing normal as well as different kinds of pathological breathing patterns

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}}
Abnormal breathing patterns include [[Kussmaul breathing]], [[Biot's respiration]] and [[Cheyne–Stokes respiration]].

Other breathing disorders include [[shortness of breath]] (dyspnea), [[stridor]], [[apnea]], [[sleep apnea]] (most commonly [[obstructive sleep apnea]]), [[mouth breathing]], and [[snoring]]. Many conditions are associated with obstructed airways. Chronic mouth breathing may be associated with illness.<ref>{{Cite news|last=Wollan|first=Malia|date=2019-04-23|title=How to Be a Nose Breather|language=en-US|work=The New York Times|url=https://www.nytimes.com/2019/04/23/magazine/how-to-be-a-nose-breather.html|access-date=2021-09-06|issn=0362-4331}}</ref><ref>{{Cite journal|last1=Pacheco|first1=Maria Christina Thomé|last2=Casagrande|first2=Camila Ferreira|last3=Teixeira|first3=Lícia Pacheco|last4=Finck|first4=Nathalia Silveira|last5=de Araújo|first5=Maria Teresa Martins|date=2015|title=Guidelines proposal for clinical recognition of mouth breathing children|journal=Dental Press Journal of Orthodontics|volume=20|issue=4|pages=39–44|doi=10.1590/2176-9451.20.4.039-044.oar|issn=2176-9451|pmc=4593528|pmid=26352843}}</ref> [[Hypopnea]] refers to overly [[shallow breathing]]; [[hyperpnea]] refers to fast and deep breathing brought on by a demand for more oxygen, as for example by exercise. The terms [[hypoventilation]] and [[hyperventilation]] also refer to shallow breathing and fast and deep breathing respectively, but under inappropriate circumstances or disease. However, this distinction (between, for instance, hyperpnea and hyperventilation) is not always adhered to, so that these terms are frequently used interchangeably.<ref name="Dorlands">{{Citation |last1=Andreoli |first1=Thomas E. |display-authors=etal |publisher=Saunders |title=Dorland's Illustrated Medical Dictionary |edition=30th |pages=887, 891, 897, 900 |location=Philadelphia, PA |url=http://dorlands.com/ |access-date=17 June 2017 |archive-date=11 January 2014 |archive-url=https://web.archive.org/web/20140111192614/http://dorlands.com/ |url-status=dead }}</ref>

A range of [[breath test]]s can be used to diagnose diseases such as dietary intolerances.
A [[rhinomanometry|rhinomanometer]] uses acoustic technology to examine the air flow through the nasal passages.<ref>{{citation |page=101 |title=Functional Reconstructive Nasal Surgery |author1=E. H. Huizing |author2=J. A. M. de Groot |year=2003 |publisher=Thieme |isbn=978-1-58890-081-4}}</ref>

==Society and culture==
The word "spirit" comes from the [[Latin]] ''spiritus'', meaning breath. Historically, breath has often been considered in terms of the concept of life force. The [[Hebrew Bible]] refers to God breathing the breath of life into clay to make Adam a living soul ([[nephesh]]). It also refers to the breath as returning to God when a mortal dies. The terms spirit, [[prana]], the Polynesian [[Mana (Oceanian mythology)|mana]], the Hebrew [[ruach]] and the [[psyche (psychology)|psyche]] in psychology are related to the concept of breath.<ref>{{Cite web|url=https://wordinfo.info/unit/2730?spage=14&letter=P|title=psych-, psycho-, -psyche, -psychic, -psychical, -psychically - Word Information|website=wordinfo.info}}</ref>

In [[tai chi]], [[aerobic exercise]] is combined with breathing exercises to strengthen the [[Thoracic diaphragm|diaphragm muscles]], improve posture and make better use of the body's ''[[qi]]''. Different forms of [[meditation]], and [[yoga]] advocate various breathing methods. A form of [[Buddhist meditation]] called [[anapanasati]] meaning mindfulness of breath was first introduced by [[Gautama Buddha|Buddha]]. Breathing disciplines are incorporated into meditation, certain forms of yoga such as [[pranayama]], and the [[Buteyko method]] as a treatment for asthma and other conditions.<ref>{{cite book |author=Swami Saradananda |title=The Power of Breath: The Art of Breathing Well for Harmony, Happiness, and Health |url=https://books.google.com/books?id=0UQpOAAACAAJ |year=2009 |publisher=Watkins Media |isbn=978-1-84483-798-4}}</ref>

In music, some [[wind instrument]] players use a technique called [[circular breathing]]. [[Singing|Singers]] also rely on [[Singing#Breath control|breath control]].

Common cultural expressions related to breathing include: "to catch my breath", "took my breath away", "inspiration", "to expire", "get my breath back".

===Breathing and mood===
[[File:Austrian Future Cup 2018-11-24 Group 3 Rotation 3 Parallel bars (Martin Rulsch) 056.jpg|thumb|upright=0.7|A young gymnast breathes deeply before performing his exercise.]]
Certain breathing patterns have a tendency to occur with certain moods. Due to this relationship, practitioners of various disciplines consider that they can encourage the occurrence of a particular mood by adopting the breathing pattern that it most commonly occurs in conjunction with. For instance, and perhaps the most common recommendation is that deeper breathing which utilizes the diaphragm and abdomen more can encourage relaxation.<ref name=acs /><ref>{{Cite journal|last1=Zaccaro|first1=Andrea|last2=Piarulli|first2=Andrea|last3=Laurino|first3=Marco|last4=Garbella|first4=Erika|last5=Menicucci|first5=Danilo|last6=Neri|first6=Bruno|last7=Gemignani|first7=Angelo|date=2018|title=How Breath-Control Can Change Your Life: A Systematic Review on Psycho-Physiological Correlates of Slow Breathing|journal=Frontiers in Human Neuroscience|volume=12|pages=353|doi=10.3389/fnhum.2018.00353|pmid=30245619|pmc=6137615|issn=1662-5161|doi-access=free}}</ref> Practitioners of different disciplines often interpret the importance of breathing regulation and its perceived influence on mood in different ways. Buddhists may consider that it helps precipitate a sense of inner-peace, holistic healers that it encourages an overall state of health<ref>{{cite book |first=Ingfried |last=Hobert |chapter=Healthy Breathing — The Right Breathing |title=Guide to Holistic Healing in the New Millennium |chapter-url=https://books.google.com/books?id=OD4hAWbleV4C&pg=PA48 |year=1999 |publisher=Harald Tietze |isbn=978-1-876173-14-2 |pages=48–49}}</ref> and business advisers that it provides relief from work-based stress.

===Breathing and physical exercise===
During physical exercise, a deeper breathing pattern is adapted to facilitate greater oxygen absorption. An additional reason for the adoption of a deeper breathing pattern is to strengthen the body's core. During the process of deep breathing, the thoracic diaphragm adopts a lower position in the core and this helps to generate intra-abdominal pressure which strengthens the lumbar spine.<ref>{{Cite web | url=http://hanslindgren.com/articles/diaphragm-function-for-core-stability/ | title=Diaphragm function for core stability |first=Hans |last=Lindgren}}</ref> Typically, this allows for more powerful physical movements to be performed. As such, it is frequently recommended when lifting heavy weights to take a deep breath or adopt a deeper breathing pattern.

==See also==
{{Div col|colwidth=35em}}
* {{annotated link|Agonal respiration}}
* {{annotated link|Ataxic respiration}}
* {{annotated link|Bad breath}}
* {{annotated link|Breath gas analysis}}
* {{annotated link|Breathing gas}}
* {{annotated link|Carbon cycle}}
* {{annotated link|Central sleep apnea}}
* {{annotated link|Eupnea}}
* {{annotated link|Liquid breathing}}
* {{annotated link|Mouth breathing}}
* {{annotated link|Nasal cycle}}
* {{annotated link|Nitrogen washout}}
* {{annotated link|Obligate nasal breathing}}
* {{annotated link|Respiratory adaptation}}
{{div col end}}

==References==
{{Reflist|30em}}

==Further reading==
*{{Cite book |title=[[Breath: The New Science of a Lost Art]] |last=Nestor |first=James |publisher=Riverhead Books |year=2020 |isbn= 978-0735213616 |ref=none}}
*{{cite journal |last=Parkes |first=M |title=Breath-holding and its breakpoint |journal=Exp Physiol |volume=91 |issue=1 |pages=1–15 |year=2006 |pmid=16272264 |doi=10.1113/expphysiol.2005.031625 |doi-access=free |ref=none}}

==External links==
*{{commons category-inline}}
*{{wikiquote-inline}}

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