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For other uses, see Breathing (disambiguation).

Breathing is the process that moves air in and out of the lungs, or oxygen through other respiratory organs such as gills. For organisms with lungs, breathing is also called ventilation, which includes both inhalation and exhalation. Breathing is one part of physiological respiration required to sustain life.[1] Aerobic organisms of birds, mammals, and reptiles—require oxygen to release energy via cellular respiration, in the form of the metabolism of energy-rich molecules such as glucose. Breathing is only one of the processes that deliver oxygen to where it is needed in the body and remove carbon dioxide. Another important process involves the movement of blood by the circulatory system.[2] Gas exchange occurs in the pulmonary alveoli by passive diffusion of gases between the alveolar gas and the blood in lung capillaries. Once these dissolved gases are in the blood, the heart powers their flow around the body (via the circulatory system). The medical term for normal relaxed breathing is eupnea.

In addition to removing carbon dioxide, breathing results in loss of water from the body. Exhaled air has a relative humidity of 100% because of water diffusing across the moist surface of breathing passages and alveoli. When a person exhales into very cold outdoor air, the moisture-laden atmosphere from the lungs becomes chilled to the point where the water condenses into a fog, making the exhale visible by anyone.


X-ray video of a female American alligator while breathing.

In mammals, breathing in (inhalation) at rest is primarily due to the contraction and flattening of the diaphragm, a domed muscle that separates the thoracic cavity from the abdominal cavity. When the diaphragm contracts it pushes the abdominal organs downward, but since the pelvic floor prevents the lowermost abdominal organs from moving in that direction, the abdomen, in fact, bulges forwards (or outwards). In the process the size of the thoracic cavity has increased in volume (as has the volume of the body as a whole). This increased thoracic volume results in a fall in pressure in the thorax, which causes the expansion of the lungs. During exhalation (breathing out), at rest, the diaphragm relaxes, returning the chest and abdomen to a position which is determined by their anatomical elasticity (i.e. the position in the cadaver, or in an animal that has been given a muscle relaxant under anesthesia). This is the "resting mid-position" of the thorax when the lungs contain the functional residual capacity of air, which in the adult human has a volume of about 2.5 liters.[3] Resting exhalation lasts about twice as long as inhalation because the diaphragm relaxes more gently than it contracts during inhalation. This prevents undue narrowing of the airways, from which the air escapes more easily than from the alveoli.

During heavy breathing (hyperventilation), as, for instance, during exercise, the "accessory muscles of inhalation" (of which the first to be recruited are the intercostal muscles, but include a large number of other muscles - see below) pull the ribs upwards, both in the front and on the sides. This increases the volume of the rib cage, adding to the volume increase caused by the descending diaphragm. During the ensuing exhalation the rib cage is actively pulled downwards (front and sides) by the abdominal muscles, which 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 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.

The entirely unconscious and automatic breathing on which the life of the animal depends can be temporarily over-ridden by conscious or emotion-driven movements of air in and out of the lungs. Speech in humans is generated by a specialized form of exhalation, but other forms of communication (e.g. crying, yelping, yawning, barking, baying, hissing, panting, sighing, shouting, laughing etc.) also rely on a balance between breathing for blood gas homeostasis and the emotional or other messages that need to be conveyed to the animal's conspecifics.

Ten muscles can be used for inhalation:[4]

Diaphragm, Intercostal Muscles, Scalenes, Pectoralis Minor, Serratus Anterior, Sternocleidomastoid, Levator Costarum, Upper / Superior Trapezius, Latissimus Dorsi, and Subclavius.

Eight are used for forced exhalation:[5]

Internal intercostal, Obliquus Internus, Obliquus Externus, Levator Ani, Triangularis Sterni, Transversalis, Pyramidalis, and Rectus Abdominus.

In amphibians, the process used is positive pressure ventilation. Muscles lower the floor of the oral cavity, enlarging it and drawing in air through the nostrils (which uses the same mechanics - pressure, volume, and diffusion - as a mammalian lung). With the nostrils and mouth closed, the floor of the oral cavity is forced up, which forces air down the trachea into the lungs.


Nasal breathing is breathing through the nose. The importance of breathing through the nose rather than the mouth was recognised in the 19th century. Hendrik Zwaardemaker (1857-1930) studied this and invented a device to measure the amount of airflow through each nostril. This rhinomanometer used cold mirrors; more recent devices use acoustic technology.[6]

It is often considered superior to mouth breathing[7][8] for several reasons. Air travels to and from the external environment and the lungs through the nasal passages, as opposed to the mouth. The nasal passages do a better job of filtering the air as it enters the lungs. In addition, the smaller diameter of the nasal passages creates pressure in the lungs during exhalation, allowing the lungs to have more time to extract oxygen from them. When there is proper oxygen-carbon dioxide exchange, the blood will maintain a balanced pH. If carbon dioxide is lost too quickly, as in mouth breathing, oxygen absorption decreases. Nasal breathing is especially important in certain situations such as dehydration, cold weather, laryngitis, and when the throat is sore or dry because it does not dry the throat as much.


Breathing is one of the few bodily functions which, within limits, can be controlled both consciously and unconsciously.


Conscious control of breathing is common in many forms of meditation, specifically forms of yoga for example pranayama.[9]

In swimming, cardio fitness, speech or vocal training, one learns to discipline one's breathing, initially consciously but later sub-consciously, for purposes other than life support.

Human speech is also dependent on conscious breath control.

Also breathing control is used in Buteyko method, an alternative physical therapy that proposes the use of breathing exercises as a treatment for asthma and other conditions.


Unconsciously, breathing is controlled by specialized centers in the brainstem, which automatically regulates the rate and depth of breathing depending on the body’s needs at any time. When carbon dioxide levels increase in the blood, it reacts with the water in blood, producing carbonic acid. Lactic acid produced by fermentation during exercise also lowers pH. The drop in the blood's pH stimulates chemoreceptors in the carotid and aortic bodies as well as those inside the respiratory center in the medulla oblongata. Chemoreceptors send more nerve impulses to the respiration centre in the medulla oblongata and pons in the brain. These, in turn send nerve impulses through the phrenic and thoracic nerves to the diaphragm.

Breathing on stage

For instance, while exercising, the level of carbon dioxide in the blood increases due to increased cellular respiration by the muscles, which activates carotid and aortic bodies and the respiration center, which ultimately cause a higher rate of respiration.

During rest, the level of carbon dioxide is lower, so breathing rate is lower. This ensures an appropriate amount of oxygen is delivered to the muscles and other organs. It is important to reiterate that it is the buildup of carbon dioxide making the blood acidic that elicits the desperation for a breath much more than lack of oxygen.


It is not possible for a healthy person to voluntarily stop breathing indefinitely. If one does not inhale, the level of carbon dioxide builds up in the blood, and one experiences overwhelming air hunger. This irrepressible reflex is not surprising given that without breathing, the body's internal oxygen levels drop dangerously low within minutes, leading to permanent brain damage followed eventually by death. However, there have been instances where people have survived for as long as two hours without air; this is only possible when submerged in cold water, as this triggers the mammalian diving reflex[10] as well as putting the subject into a state of suspended animation.

If a healthy person were to voluntarily stop breathing (i.e. hold his or her breath) for a long enough amount of time, he or she would lose consciousness, and the body would resume breathing on its own. Because of this one cannot commit suicide with this method, unless one's breathing was also restricted by something else (e.g. water, see drowning).

Voluntary hyperventilation can cause arterial carbon dioxide levels to fall to dangerously low levels, leading to paresthesias (a sensation of "pins and needles") round the mouth and in the hands and feet, and a peculiar form of muscular spasms (tetany) of the hands, arms, feet and face. This is usually elicited by anxiety or agitation, and can be very distressing, causing the person to think that they are suffocating when, in fact, they are over-riding their blood gas homeostat, and over-breathing, causing the blood pH to rise to dangerously high levels. The symptoms are brought about by calcium being less soluble under alkaline conditions than under acidic conditions. Thus, during a hyperventilation-induced alkaemia the plasma ionized calcium (Ca2+) level falls, causing many proteins to change their tertiaty (or 3D) configuration. Among the proteins that are most prominently affected are the voltage gated sodium channels of nerve fibres[11] - causing them to randomly generate inappropriate action potentials, which cause the abnormal sensations (paresthesias) and spontaneous muscle contractions (tetany).


The air we inhale is roughly composed of (by volume):[12]

In addition to air, underwater divers often breathe oxygen-rich or helium-rich gas mixtures. Oxygen and analgesic gases are sometimes given to patients under medical care. The atmosphere in space suits is pure oxygen. Also our reliance on this relatively small amount of oxygen can cause overactivity or euphoria in pure or oxygen-rich environments.[13]

The permanent gases in gas we exhale are 4% to 5% by volume more carbon dioxide and 4% to 5% by volume less oxygen than was inhaled. This expired air typically composed of:[12]

  • 78.04% nitrogen
  • 13.6% - 16% oxygen
  • 4% - 5.3% carbon dioxide
  • 1% argon and other gases

Additionally vapors and trace gases are present: 5% water vapor, several parts per million (ppm) of hydrogen and carbon monoxide, 1 part per million (ppm) of ammonia and less than 1 ppm of acetone, methanol, ethanol (unless ethanol has been ingested, in which case much higher concentrations would occur in the breath, cf. Breathalyzer) and other volatile organic compounds. Oxygen is used by the body for cellular respiration and other uses, and carbon dioxide is a product of these processes. The exact amount of exhaled oxygen and carbon dioxide when breathing and the amount of gases exhaled may vary based on diet, exercise and fitness.

Air pressure[edit]

Atmospheric air at altitude is at a lower pressure than at sea level due to the lesser weight of the air above. This lower pressure can lead to altitude sickness, or hypoxia.

Gases breathed underwater are at higher pressure than at sea level due to the added weight of water. This can lead to nitrogen narcosis, oxygen toxicity, or decompression sickness.

Cultural significance[edit]

In t'ai chi ch'uan, aerobic training is combined with breathing to exercise the diaphragm muscles and to train effective posture; making better use of the body's energy. In music, breath is used to play wind instruments and many aerophones. Laughter, physically, is simply repeated sharp breaths. Hiccups, yawns, and sneezes are other breath-related phenomena.

Ancients commonly linked the breath to a 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," "qi," "prana" and "psyche"[14] are related to the concept of breath. Also cognate are Polynesian Mana and Hebrew ruach.

In his book Your Atomic Self: The Invisible Elements That Connect You to Everything Else in the Universe, excerpted in Wired Magazine, Curt Stager explores the atomic and molecular basis of links through which breathing connects humans and other Aerobic organisms of birds, mammals, and reptiles to the entire planet.[15]

Common phrases in English relate to breathing e.g. "catch my breath", "took my breath away", "inspiration", "to expire".

See also[edit]


  1. ^ Peter Raven; George Johnson; Kenneth Mason; Jonathan Losos; Susan Singer (2007). "The capture of oxygen: Respiration". Biology (8 ed.). McGraw-Hill Science/Engineering/Math;. ISBN 0-07-322739-0. 
  2. ^ Kevin T. Patton; Gary A. Thibodeau (2009). Anatomy & Physiology (7 ed.). Mosby. ISBN 0-323-05532-X. 
  3. ^ Tortora, Gerard J.; Anagnostakos, Nicholas P. (1987). Principles of anatomy and physiology (Fifth ed.). New York: Harper & Row, Publishers. pp. 570–572. ISBN 0-06-350729-3. 
  4. ^ All You Need to Know About Inspiratory Muscles Part I | Swimming Science
  5. ^ All You Need to Know About Inspiratory Muscles Part II
  6. ^ E. H. Huizing, J. A. M. de Groot (2003), Functional Reconstructive Nasal Surgery, p. 101, ISBN 978-1-58890-081-4 
  7. ^ The Importance of Nasal Breathing « Distinct’s Ultimate Health & Conditioning
  8. ^ Nose Breathing Has Many Benefits Over Mouth Respiration
  9. ^ Swami Saradananda, The Power of Breath, Castle House: Duncan Baird Publishers, 2009
  10. ^ Ramey CA, Ramey DN, Hayward JS. Dive response of children in relation to cold-water near drowning. J Appl Physiol 2001;62(2):665-8.Source: Diana Hacker (Boston: Bedford/St. Martin’s, 2002).Adapted from Victoria E. McMillan (Boston: Bedford/St. Martin’s, 2001). See it cited here [1]
  11. ^ Armstrong CM, Cota G (Mar 1999). "Calcium block of Na+ channels and its effect on closing rate". Proceedings of the National Academy of Sciences of the United States of America 96 (7): 4154–7. Bibcode:1999PNAS...96.4154A. doi:10.1073/pnas.96.7.4154. PMC 22436. PMID 10097179. 
  12. ^ a b P.S.Dhami, G.Chopra, H.N. Shrivastava (2015). A Textbook of Biology. Jalandhar, Punjab: Pradeep Publications. pp. V/101. 
  13. ^ Biology. NCERT. 2015. ISBN 81-7450-496-6. 
  14. ^ psych-, psycho-, -psyche, -psychic, -psychical, -psychically + (Greek: mind, spirit, consciousness; mental processes; the human soul; breath of life)
  15. ^ The Surprising Ways Your Breath Connects You to the Entire Planet (2014-12-17), Curt Stager, Wired Magazine

Further reading[edit]