Allaerobic creatures need oxygen forcellular respiration, which extracts energy from the reaction of oxygen with molecules derived fromfood and produces carbon dioxide as awaste product. Breathing, or external respiration, bringsair into the lungs where gas exchange takes place in thealveoli throughdiffusion. The body'scirculatory system transports these gases to and from the cells, where cellularrespiration takes place.[2][3]
The breathing of allvertebrates with lungs consists of repetitive cycles ofinhalation andexhalation through a highly branched system of tubes orairways which lead from the nose to the alveoli.[4] The number of respiratory cycles per minute is the breathing orrespiratory rate, and is one of the four primaryvital signs of life.[5] Under normal conditions the breathing depth and rate is automatically, and unconsciously, controlled by severalhomeostatic mechanisms which keep thepartial pressures ofcarbon dioxide andoxygen in the arterial blood constant. Keeping the partial pressure of carbon dioxide in the arterial blood unchanged under a wide variety ofphysiological circumstances, contributes significantly totight control of the pH of theextracellular 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.
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 thebucket handle movement.
Breathing
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 thediaphragm 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 therib 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 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.
Thelungs are not capable of inflating themselves, and will expand only when there is an increase in the volume of thethoracic cavity.[6][7] In humans, as in the othermammals, this is achieved primarily through the contraction of thediaphragm, but also by the contraction of theintercostal muscles which pull therib cage upwards and outwards as shown in the diagrams on the right.[8] During forceful inhalation (Figure on the right) theaccessory muscles of inhalation, which connect the ribs andsternum to thecervical vertebrae and base of the skull, in many cases through an intermediary attachment to theclavicles, exaggerate thepump handle andbucket handle movements (see illustrations on the left), bringing about a greater change in the volume of the chest cavity.[8] 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.[8] At this point the lungs contain thefunctional residual capacity of air, which, in the adult human, has a volume of about 2.5–3.0 liters.[8]
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).[8] 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".[8] 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.[8]
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 duringlabored 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 asclavicular breathing, seen especially duringasthma attacks and in people withchronic obstructive pulmonary disease.
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 breathedfirst out and secondly in through the nose.[9] Thenasal cavities (between thenostrils and thepharynx) are quite narrow, firstly by being divided in two by thenasal septum, and secondly bylateral walls that have several longitudinal folds, or shelves, callednasal conchae,[10] thus exposing a large area ofnasal mucous membrane to the air as it is inhaled (and exhaled). This causes the inhaled air to take up moisture from the wetmucus, and warmth from the underlying blood vessels, so that the air is very nearly saturated withwater vapor and is at almost body temperature by the time it reaches thelarynx.[8] 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.[8][10]
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 arespiratory tree ortracheobronchial 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 themouse 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 forgas exchange.[8][11]
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 thelungs.
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 createsdead 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 ml.
The primary purpose of breathing is to refresh air in the alveoli so thatgas 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 bydiffusion. After exhaling, adult human lungs still contain 2.5–3 L of air, theirfunctional 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 theperipheral andcentral 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 maintainsa constant pH of the blood.[8]
The rate and depth of breathing is automatically controlled by therespiratory centers that receive information from theperipheral andcentral chemoreceptors. Thesechemoreceptors 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 themedulla oblongata of thebrain stem which are particularly sensitive topH as well as the partial pressure of carbon dioxide in the blood andcerebrospinal fluid.[8] 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 theaortic andcarotid bodies.[8] Information from all of these chemoreceptors is conveyed to therespiratory centers in thepons andmedulla 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 kPa (40 mm Hg), the pH to 7.4 and, to a lesser extent, the partial pressure of oxygen to 13 kPa (100 mm Hg).[8] For example,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 thephrenic nerves, which innervate the diaphragm, are probably the most important.[8]
Automatic breathing can be overridden to a limited extent by simple choice, or to facilitateswimming,speech,singing or othervocal 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.[12]
Other automatic breathing control reflexes also exist. Submersion, particularly of the face, in cold water, triggers a response called thediving reflex.[13][14] This has the initial result of shutting down the airways against the influx of water. Themetabolic 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.[13] The diving reflex is an often-used response in animals that routinely need to dive, such as penguins, seals and whales.[15][16] It is also more effective in very young infants and children than in adults.[17]
Following on from the above diagram, if the exhaled air is breathed out through the mouth in cold andhumid conditions, thewater vapor willcondense into a visiblecloud ormist.
Inhaled air is by volume 78%nitrogen, 20.95% oxygen and small amounts of other gases includingargon, carbon dioxide,neon,helium, andhydrogen.[18]
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:[19]
In addition to air,underwater divers practicingtechnical diving may breathe oxygen-rich, oxygen-depleted or helium-richbreathing gas mixtures. Oxygen andanalgesic gases are sometimes given to patients under medical care. The atmosphere inspace suits is pure oxygen. However, this is kept at around 20% of Earthbound atmospheric pressure to regulate the rate of inspiration.[citation needed]
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.[25] The composition of atmospheric air is, however, almost constant below 80 km, as a result of the continuous mixing effect of the weather.[26] The concentration of oxygen in the air (mmols O2 per liter of air) therefore decreases at the same rate as the atmospheric pressure.[26] At sea level, where theambient pressure is about 100 kPa, oxygen constitutes 21% of the atmosphere and the partial pressure of oxygen (PO2) is 21 kPa (i.e. 21% of 100 kPa). At the summit ofMount Everest, 8,848 metres (29,029 ft), where the total atmospheric pressure is 33.7 kPa, oxygen still constitutes 21% of the atmosphere but its partial pressure is only 7.1 kPa (i.e. 21% of 33.7 kPa = 7.1 kPa).[26] 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 withwater vapor as it passes through the nose andpharynx before it enters the alveoli. Thesaturated 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.[27] Consequently, at sea level, thetracheal 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, thePO2 at sea level is 21.0 kPa, compared to aPO2 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 thePO2 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).
Thepressure 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 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 the 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.[28][29] The lowerviscosity 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 therespiratory gases homeostatic mechanism, which regulates the arterialPO2 andPCO2. Thishomeostatic mechanism prioritizes the regulation of the arterialPCO2 over that of oxygen at sea level. That is to say, at sea level the arterialPCO2 is maintained at very close to 5.3 kPa (or 40 mmHg) under a wide range of circumstances, at the expense of the arterialPO2, 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 atmosphericPO2) falls to below 75% of its value at sea level, oxygenhomeostasis 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 arterialPCO2 with a consequent rise in thepH of the arterial plasma leading torespiratory alkalosis. This is one contributor tohigh altitude sickness. On the other hand, if the switch to oxygen homeostasis is incomplete, thenhypoxia may complicate the clinical picture with potentially fatal results.
Typical breathing effort when breathing through a diving regulator
Pressure increases with the depth of water at the rate of about oneatmosphere – slightly more than 100 kPa, or onebar, for every 10 meters. Air breathed underwater bydivers 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 severaldiving disorders which includepulmonary barotrauma,decompression sickness,nitrogen narcosis, andoxygen toxicity. The effects of breathing gasses under pressure are further complicated by the use of one or morespecial gas mixtures.
Air is provided by adiving regulator, which reduces the high pressure in adiving cylinder to the ambient pressure. Thebreathing performance of regulators is a factor when choosing a suitable regulator for thetype 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 theVenturi 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.
Other breathing disorders includeshortness of breath (dyspnea),stridor,apnea,sleep apnea (most commonlyobstructive sleep apnea),mouth breathing, andsnoring. Many conditions are associated with obstructed airways. Chronic mouth breathing may be associated with illness.[30][31]Hypopnea refers to overlyshallow breathing;hyperpnea refers to fast and deep breathing brought on by a demand for more oxygen, as for example by exercise. The termshypoventilation andhyperventilation 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.[32]
A range ofbreath tests can be used to diagnose diseases such as dietary intolerances.Arhinomanometer uses acoustic technology to examine the air flow through the nasal passages.[33]
The word "spirit" comes from theLatinspiritus, meaning breath. Historically, breath has often been considered in terms of the concept of life force. TheHebrew 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 Polynesianmana, the Hebrewruach and thepsyche in psychology are related to the concept of breath.[34]
Intai chi,aerobic exercise is combined with breathing exercises to strengthen thediaphragm muscles, improve posture and make better use of the body'sqi. Different forms ofmeditation, andyoga advocate various breathing methods. A form ofBuddhist meditation calledanapanasati meaning mindfulness of breath was first introduced byBuddha. Breathing disciplines are incorporated into meditation, certain forms of yoga such aspranayama, and theButeyko method as a treatment for asthma and other conditions.[35]
Common cultural expressions related to breathing include: "to catch my breath", "took my breath away", "inspiration", "to expire", "get my breath back".
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.[12][36] 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[37] and business advisers that it provides relief from work-based stress.
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.[38] 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.
Nasal cycle – Subconscious alternation of the nasal cavities
Nitrogen washout – Test for measuring anatomic dead space in the lung during a respiratory cycle
Obligate nasal breathing – physiological necessity to breathe through the nose rather than the mouthPages displaying wikidata descriptions as a fallback
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