Editing Respiratory system (section)

===Mechanics of breathing===
[[File:Real-time MRI - Thorax.ogv|thumb|right|'''Fig. 6''' Real-time [[magnetic resonance imaging]] (MRI) of the chest movements of human thorax during breathing]]

{{Main|Breathing#Mechanics}}
{{Multiple image
| direction = vertical
| align = left
| header = The "pump handle" and "bucket handle movements" of the ribs 
| width1 = 200
| image1 = ribcage during inhalation.jpg
| caption1 = '''Fig. 4''' 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.png
| caption2 = '''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. }}
{{Multiple image
| direction = horizontal
| align = top
| header = Breathing
| width2 = 200
| image2 = Forceful breathing.jpg
| caption2 = '''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.
| width1 = 200
| image1 = Quiet breathing.jpg
| caption1 = '''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 [[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 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.
}}
In [[mammals]], inhalation at rest is primarily due to the contraction of the [[Thoracic diaphragm|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.&nbsp;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.&nbsp;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.&nbsp;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.&nbsp;4. All the ribs slant downwards from the rear to the front (as shown in Fig.&nbsp;4); but the lowermost ribs ''also'' slant downwards from the midline outwards (Fig.&nbsp;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.&nbsp;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, [[Pleural cavity|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 [[Obligate nasal breathing|begin with the nose]].<ref name=cc>{{cite web
|url=https://health.clevelandclinic.org/breathe-mouth-nose/
|title=Should You Breathe Through Your Mouth or Your Nose?
|access-date=2020-06-28
|last=Turowski
|first=Jason
|date=2016-04-29
|publisher=[[Cleveland Clinic]]
}}</ref><ref name="guardian">{{cite web|title=Your Nose, the Guardian of Your Lungs|url=https://www.bmc.org/otolaryngology-head-neck-surgery/resources/your-nose-guardian-your-lungs|access-date=2020-06-29|publisher=[[Boston Medical Center]]}}</ref> (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.<ref name=harmful>{{cite web
|url=https://www.nbcnews.com/healthmain/mouth-breathing-gross-harmful-your-health-1C6437430
|title='Mouth-breathing' gross, harmful to your health
|access-date=2020-06-28
|last=Dahl
|first=Melissa
|date=2011-01-11
|publisher=NBC News
}}</ref><ref name="role">{{cite web
|url=https://www.journal-imab-bg.org/issues-2018/issue1/JofIMAB-2018-24-1p1878-1882.pdf
|title=THE ROLE OF MOUTH BREATHING ON DENTITION DEVELOPMENT AND FORMATION
|access-date=2020-05-31
|last=Valcheva
|first=Zornitsa
|date=January 2018
|publisher=Journal of IMAB
}}</ref><ref name="nesnpr">{{cite web
|url=https://www.npr.org/transcripts/862963172
|title=How The 'Lost Art' Of Breathing Can Impact Sleep And Resilience
|access-date=2020-06-23
|last=Gross
|first=Terry
|date=2020-05-27
|publisher=[[NPR|National Public Radio (NPR)]]/[[Fresh Air]]
}}</ref>) It ends in the microscopic dead-end sacs called [[Pulmonary alveolus|alveoli]], which are always open, though the diameters of the various sections can be changed by the [[Sympathetic nervous system|sympathetic]] and [[parasympathetic nervous system]]s. The alveolar air pressure is therefore always close to atmospheric air pressure (about 100&nbsp;[[Pascal (unit)|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&nbsp;kPa.<ref name="Chrisvan L 1995">{{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=176–178 |doi=10.1152/physiologyonline.1995.10.4.176}}</ref><ref name="Williams & Wilkins">{{cite book |last1=West |first1=J.B. |title=Respiratory physiology: the essentials. |location=Baltimore |publisher=Williams & Wilkins |date=1985| pages= 21–30, 84–84, 98–101 }}</ref>

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.&nbsp;7) when the lungs contain their [[functional residual capacity]] of air (the light blue area in the right hand illustration of Fig.&nbsp;7), which in the adult human has a volume of about 2.5–3.0&nbsp;liters (Fig.&nbsp;3).<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/principlesofan1987tort |url-access= registration |pages=[https://archive.org/details/principlesofan1987tort/page/556 556–586]|edition= Fifth |location= New York |publisher= Harper & Row, Publishers|date= 1987 |isbn= 0-06-350729-3 }}</ref> Resting exhalation lasts about twice as long as inhalation because the diaphragm relaxes passively more gently than it contracts actively during inhalation.

[[File:Alveolar air.png|thumb|right|400 px|'''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&nbsp;ml per breath. At the end of exhalation, the airways contain about 150&nbsp;ml of alveolar air which is the first air that is breathed back into the alveoli during inhalation.<ref name=fowler1948>{{cite journal | author = Fowler W.S. | year = 1948 | title = Lung Function studies. II. The respiratory dead space | journal = Am. J. Physiol. | volume = 154 | issue = 3| pages = 405–416 | doi=10.1152/ajplegacy.1948.154.3.405| pmid = 18101134 }}</ref><ref>{{cite journal|last=Burke |first=TV |author2=Küng, M |author3=Burki, NK |title=Pulmonary gas exchange during histamine-induced bronchoconstriction in asthmatic subjects. |journal=Chest |year=1989 |volume=96 |issue=4 |pages=752–6 |pmid=2791669 |doi=10.1378/chest.96.4.752|s2cid=18569280 }}</ref> This volume air that is breathed out of the alveoli and back in again is known as [[Dead space (physiology)|dead space]] ventilation, which has the consequence that of the 500&nbsp;ml breathed into the alveoli with each breath only 350&nbsp;ml (500&nbsp;ml – 150&nbsp;ml = 350&nbsp;ml) is fresh warm and moistened air.<ref name=tortora1 /> Since this 350&nbsp;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&nbsp;liters), it is clear that the composition of the alveolar air changes very little during the breathing cycle (see Fig. 9). The oxygen [[Partial pressure|tension]] (or partial pressure) remains close to 13–14&nbsp;kPa (about 100&nbsp;mm&nbsp;Hg), and that of carbon dioxide very close to 5.3&nbsp;kPa (or 40&nbsp;mm&nbsp;Hg). This contrasts with composition of the dry outside air at sea level, where the partial pressure of oxygen is 21&nbsp;kPa (or 160&nbsp;mm&nbsp;Hg) and that of carbon dioxide 0.04&nbsp;kPa (or 0.3&nbsp;mmHg).<ref name=tortora1 />

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 respiration|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 [[clavicle]]s.<ref name=tortora1 /> 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#Function|rib cage]] downwards (front and sides) (Fig.&nbsp;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.&nbsp;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.<ref name=tortora1 />

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.<ref name="taylor">{{cite journal |last=Taylor |first=D |title=The Valsalva Manoeuvre: A critical review |journal=South Pacific Underwater Medicine Society Journal |volume=26 |issue=1 |year=1996 |issn=0813-1988 |oclc=16986801 |url=http://archive.rubicon-foundation.org/6264 |access-date=14 March 2016 |archive-date=31 January 2010 |archive-url=https://web.archive.org/web/20100131114931/http://archive.rubicon-foundation.org/6264 |url-status=usurped }}</ref> 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.