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Real-time magnetic resonance imaging of the human thorax during breathing
X-ray video of a female American alligator while breathing.

Breathing (or respiration, or ventilation) is the process of moving air into and out of the lungs to facilitate gas exchange, mostly by bringing in oxygen and flushing out carbon dioxide.
All animals need oxygen for cellular respiration, which uses the oxygen to break down foods for energy and produces carbon dioxide as a waste product. Breathing, or external respiration, brings air into the body where gas exchange can take place in the alveoli of the lungs through diffusion. The body's internal respiration transports these gasses to and from the cells.[1][2]

The respiratory system of all vertebrates other than fish consists of repetitive respiratory cycles of inhalation and exhalation.[3] The number of respiratory cycles per minute is the breathing or respiratory rate, and is one of the four primary vital signs of life.[4] Under normal conditions the breathing rate is automatically, and unconsciously, controlled by a complex process of homeostasis but this can be overridden by conscious intent or disrupted by physiological or psychological respiratory disorders leading to over-breathing (hyperventilation) and under-breathing (hypoventilation), causing a range of medical problems.

Breathing has other important functions including the homeostatic regulation of the pH of extracellular fluid. It also provides a mechanism for speech and laughter and in reflexes such as yawning, coughing and sneezing. Animals that cannot thermoregulate by perspiration because they lack sufficient sweat glands may lose heat by evaporation through panting.



The "pump handle" and "bucket handle movements" of the ribs
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.
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.]]
The muscles of breathing at rest: inhalation on the left, exhalation on the right. Contracting muscles are shown in red; relaxed muscles in blue. The intercostal muscles pull the ribs upwards (their effect is indicated by arrows) causing the rib cage to expand during inhalation (see diagram on other side of the page); their relaxation during exhalation cause the rib cage and abdomen to elastically return to their resting position. Compare these diagrams with the MRI video at the top of the page.
The muscles of forceful breathing: inhalation and exhalation, and the the color code are the same as on the left. 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.

The lungs are not capable of expanding themselves, and will expand only when there is an increase in the volume of the thoracic cavity.[5] In humans, as in the other mammals, this is achieved primarily through the contraction of the diaphragm, but also by the contraction of the intercostal muscles which pull the rib cage upwards as shown in the diagrams on the left.[6] During forceful inhalation (Figure on the right) the 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 and bucket handle movements (see illustrations on the left), bringing about a greater change in the volume of the chest cavity.[6] 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.[6] 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.[6]

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).[6] 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".[6] 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.[6]

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 refer to the same phenomenon (i.e. the contraction of the diaphragms pushing the abdominal contents down and outwards, and vice versa during exhalation), though, in mammals there is, strictly speaking, no "abdominal breathing" as there is in other vertebrates.

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 airEdit

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".
Following on from the above diagram, if the exhaled air is breathed out through the mouth, on a cold day, as in this photograph the water vapor will condense into a visible cloud or mist, not normally seen when breathing out through the nose.

Usually air is breathed in and out through the nose. The nasal passages consist of narrow slits, exposing a large area of mucous membrane to the air moving in (during inhalation) and out (during exhalation) through the nose during each breath. 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.[6] Part of this moisture and heat is recaptured as the exhaled air moves over the partially dried-out, hygroscopic, cooled mucus in the nose. The sticky mucus also traps much of the particulate matter that is breathed in, preventing it from reaching the lungs.[6][7]

Gas exchangeEdit

The primary purpose of breathing is to bring atmospheric air (in small doses) into the alveoli where gas exchange with the gases in the blood takes place. The equilibration of the partial pressures of the gases in the alveolar blood and the alveolar air occurs by passive diffusion. At the end of each exhalation the adult human lungs still contain 2,500-3,000 mL of air, their functional residual capacity or FRC. With each breath (inhalation) only as little as about 350 mL of warm, moistened atmospheric is added, and well mixed, with the FRC. Consequently, the gas composition of the FRC changes very little during the breathing cycle. Since the pulmonary capillary blood equilibrates with this virtually unchanging mixture of air in the lungs (which has a substantially different composition from that of the ambient air), the partial pressures of the arterial blood gases also do not change with each breath. The tissues are therefore not exposed to swings in oxygen and carbon dioxide tensions in the blood during the breathing cycle, and the blood gas homeostats do not need to "choose" the point in the breathing cycle at which the blood gases need to be measured, and responded to. Thus the homeostatic control of the breathing rate simply depends on the partial pressures of oxygen and carbon dioxide in the arterial blood. This then also maintains the constancy of the pH of the blood.[6]


The control of breathing is normally an automatic and unconscious homeostatic reflex. The rate and depth of breathing is automatically controlled by the body's respiratory centers. Exercise increases the partial pressure of carbon dioxide in the venous blood, and from there the arterial blood, which is sensed by the central blood gas chemoreceptors on the surface of the medulla oblongata. This, in turn, controls the rate and depth of respiratory movements of the diaphragm and other respiratory muscles. This diffuses carbon dioxide out of the body while importing oxygen maintaining the partial pressures of carbon dioxide and oxygen in the arterial blood at almost the same levels as at rest.

Automatic breathing can be overridden to a limited extent by simple choice, or to facilitate swimming, speech, singing or other vocal training. 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.[8] It is impossible to suppress the urge to breath to the point of hypoxia but training can increase the ability to breath-hold, for example, in February 2016, a Spanish, professional freediver broke the world record for holding the breath under water at just over 24 minutes.[9]

Other automatic breathing control reflexes also exist. Submersion, particularly of the face, in cold water, triggers a response called the diving reflex.[10][11] This firstly has the result of shutting down the airways against the influx of water. The metabolic rate slows right down. This is coupled with intense vasoconstriction of the arteries to the limbs and abdominal viscera. This reserves the oxygen that is in blood and lungs at the beginning of the dive almost exclusively for the heart and the brain.[10] The diving reflex is an often-used response in other animals that routinely need to dive, such as penguins, seals and whales.[12][13] It is also more effective in very young infants and children than in adults.[14]


Inhaled air is by volume 78.08% nitrogen, 20.95% oxygen and small amounts include argon, carbon dioxide, neon, helium, and hydrogen.[15]

The gas exhaled is 4% to 5% by volume more of carbon dioxide and 4% to 5% by volume less of oxygen than was inhaled and is typically composed of:[16]

In addition to air, underwater divers practising 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 suits is pure oxygen.[17]

Effects of ambient air pressureEdit

Breathing at altitudeEdit

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 5,500 metres (18,000 ft) rise in altitude.[18] The composition of atmospheric air is, however, almost constant below 80 km, as a result of the continuous mixing effect of the weather.[19] The concentration of oxygen in the air (mmols O2 per liter of air) therefore decreases at the same rate as the atmospheric pressure.[19] At sea level, where the ambient pressure is about 100 kPa, oxygen contributes 21% of the atmosphere and the partial pressure of oxygen (PO2) is 21 kPa (i.e. 21% of 100 kPa). At the summit of Mount Everest, 8,848 metres (29,029 ft), where the total atmospheric pressure is 33.7 kPa, oxygen still contributes 21% of the atmosphere but its partial pressure is only 7.1 kPa (i.e. 21% of 33.7 kPa = 7.1 kPa).[19] Therefore, a greater volume of air must be inhaled at altitude than at sea level in order to breath in the same amount of oxygen in a given period.

During inhalation, air is warmed and saturated with 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 °C it is 6.3 kPa (47.0 mmHg), regardless of any other influences, including altitude.[20] Consequently, at sea level, the tracheal air (immediately before the inhaled air enters the alveoli) consists of: water vapor (PH2O = 6.3 kPa), nitrogen (PN2 = 74.0 kPa), oxygen (PO2 = 19.7 kPa) and trace amounts of carbon dioxide and other gases, a total of 100 kPa. In dry air, the PO2 at sea level is 21.0 kPa, compared to a PO2 of 19.7 kPa in the tracheal air (21% of [100 – 6.3] = 19.7 kPa). At the summit of Mount Everest tracheal air has a total pressure of 33.7 kPa, of which 6.3 kPa is water vapor, reducing the PO2 in the tracheal air to 5.8 kPa (21% of [33.7 – 6.3] = 5.8 kPa), beyond what is accounted for by a reduction of atmospheric pressure alone (7.1 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. Halving the sea level air pressure (100 kPa) results in a pressure gradient of 50 kPa but doing the same at 5500 m, where the atmospheric pressure is 50 kPa, a doubling of the volume of the lungs results in a pressure gradient of only 25 kPa. In practice, because we breathe in a gentle, cyclical manner that generates pressure gradients of only 2–3 kPa, this has little effect on the actual rate of inflow into the lungs and is easily compensated for by breathing slightly deeper.[21][22] 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 blood gas homeostat, which regulates the arterial PO2 and PCO2. This homeostat prioritizes the regulation of the arterial PCO2 over that of oxygen at sea level. That is to say, at sea level the arterial PCO2 is maintained at very close to 5.3 kPa (or 40 mmHg) under a wide range of circumstances, at the expense of the arterial PO2, 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 PO2) 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 2,500 metres (8,200 ft). If this switch occurs relatively abruptly, the hyperventilation at high altitude will cause a severe fall in the arterial PCO2 with a consequent rise in the pH of the arterial plasma leading to respiratory alkalosis. This is one contributor to high altitude sickness. On the other hand, if the switch to oxygen homeostasis is incomplete, then hypoxia may complicate the clinical picture with potentially fatal results.

Breathing at depthEdit

Pressure increases with the depth of water at the rate of about 1 atmosphere (100 kPa) for every 10 metres. Air breathed underwater by 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 disorders which include 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 special gas mixtures.

Respiratory disordersEdit

Disruption to the pattern of breathing 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. Hypopnea is the technical term used to describe physiological (i.e. normal) slow and shallow breathing; hyperpnea is the technical term used to describe fast and deep breathing brought on by physiological activities like 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.[23]

A range of breath tests can be used to diagnose diseases such as dietary intolerances. A rhinomanometer uses acoustic technology to examine the air flow through the nasal passages.[24]

Society and cultureEdit

The word "spirit" comes from the Latin spiritus, meaning "breath". 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, the Hebrew ruach and the psyche in psychology are related to the concept of breath.[25]
T'ai chi and aerobic exercise use breathing exercises to strengthen the diaphragm muscles, improve posture and make better use of the body's Qi, (energy).
In music, wind instruments and singing rely on breath control.
The observation that breathing patterns are associated with moods is used to invoke desirable moods. For example, deeper breathing, using the diaphragm and abdomen, may encourage a more relaxed and confident mood. However, views differ as to how this works and the effects; Buddhists consider that it helps to precipitate a sense of inner-peace while holistic healers that it encourages an overall state of health and psychologists that it provides relief from stress.[26]

Common idioms include: "to catch my breath", "took my breath away", "inspiration", "to expire", "get my breath back".

See alsoEdit


  1. ^ Hall, John (2011). Guyton and Hall textbook of medical physiology (12th ed. ed.). Philadelphia, Pa.: Saunders/Elsevier. p. 5. ISBN 978-1-4160-4574-8. 
  2. ^ Pocock, Gillian; Richards, Christopher D. (2006). Human physiology : the basis of medicine (3rd ed. ed.). Oxford: Oxford University Press. p. 311. ISBN 978-0-19-856878-0. 
  3. ^ Pocock, Gillian; Richards, Christopher D. (2006). Human physiology : the basis of medicine (3rd ed. ed.). Oxford: Oxford University Press. p. 320. ISBN 978-0-19-856878-0. 
  4. ^ "Vital Signs 101". 
  5. ^ Levitzky, Michael G. (2013). Pulmonary physiology (Eighth ed.). New York: McGraw-Hill Medical. p. Chapter 1. Function and Structure of the Respiratory System. ISBN 978-0-07-179313-1. 
  6. ^ a b c d e f g h i j Tortora, Gerard J.; Anagnostakos, Nicholas P. (1987). Principles of anatomy and physiology (Fifth ed.). New York: Harper & Row, Publishers. pp. 556–582. ISBN 0-06-350729-3. 
  7. ^ Williams, Peter L; Warwick, Roger; Dyson, Mary; Bannister, Lawrence H. (1989). Gray’s Anatomy (Thirty-seventh ed.). Edinburgh: Churchill Livingstone. pp. 1172–1173, 1278–1282. ISBN 0443 041776. 
  8. ^ Swami Saradananda, The Power of Breath, Castle House: Duncan Baird Publishers, 2009
  9. ^ "Longest time breath held voluntarily (male)". Guinness World Records. Retrieved 2016-11-29. 
  10. ^ a b Michael Panneton, W (2013). "The Mammalian Diving Response: An Enigmatic Reflex to Preserve Life?". Physiology. 28 (5): 284–297. PMC 3768097 . PMID 23997188. doi:10.1152/physiol.00020.2013. 
  11. ^ Lindholm, Peter; Lundgren, Claes EG (1 January 2009). "The physiology and pathophysiology of human breath-hold diving". Journal of Applied Physiology. 106 (1): 284–292. doi:10.1152/japplphysiol.90991.2008. Retrieved 4 April 2015. 
  12. ^ Thornton SJ, Hochachka PW (2004). "Oxygen and the diving seal". Undersea Hyperb Med. 31 (1): 81–95. PMID 15233163. Retrieved 2008-06-14. 
  13. ^ Zapol WM, Hill RD, Qvist J, Falke K, Schneider RC, Liggins GC, Hochachka PW (September 1989). "Arterial gas tensions and hemoglobin concentrations of the freely diving Weddell seal". Undersea Biomed Res. 16 (5): 363–73. PMID 2800051. Retrieved 2008-06-14. 
  14. ^ Pedroso, F. S.; Riesgo, R. S.; Gatiboni, T; Rotta, N. T. (2012). "The diving reflex in healthy infants in the first year of life". Journal of Child Neurology. 27 (2): 168–71. PMID 21881008. doi:10.1177/0883073811415269. 
  15. ^ "Earth Fact Sheet". 
  16. ^ P.S.Dhami; G.Chopra; H.N. Shrivastava (2015). A Textbook of Biology. Jalandhar, Punjab: Pradeep Publications. pp. V/101. 
  17. ^ Biology. NCERT. 2015. ISBN 81-7450-496-6. 
  18. ^ "Online high altitude oxygen calculator". Retrieved 15 August 2007. 
  19. ^ a b c Tyson, P.D.; Preston-White, R.A. (2013). The weather and climate of Southern Africa. Cape Town: Oxford University Press. pp. 3–10, 14–16, 360. ISBN 9780195718065. 
  20. ^ Diem, K.; Lenter, C. (1970). Scientific Tables (Seventh ed.). Basle, Switzerland: Ciba-Geigy. pp. 257–258. 
  21. ^ Koen, Chrisvan L.; Koeslag, Johan H. (1995). "On the stability of subatmospheric intrapleural and intracranial pressures". News in Physiological Sciences. 10: 176–178. 
  22. ^ West, J.B. (1985). Respiratory physiology: the essentials. Baltimore: Williams & Wilkins. pp. 21–30, 84–84, 98–101. 
  23. ^ Andreoli, Thomas E.; et. al., Dorland's Illustrated Medical Dictionary (30th ed.), Philadelphia, PA: Saunders, pp. 887, 891, 897, 900 
  24. ^ E. H. Huizing; J. A. M. de Groot (2003), Functional Reconstructive Nasal Surgery, p. 101, ISBN 978-1-58890-081-4 
  25. ^ psych-, psycho-, -psyche, -psychic, -psychical, -psychically + (Greek: mind, spirit, consciousness; mental processes; the human soul; breath of life)
  26. ^ Hobert, Ingfried, 'Healthy Breathing – The Right Breathing' in Guide to Holistic Healing in the New Millenium, Munchen: Verlag Peter Erd, 1999, pp. 48–49

Further readingEdit