Breathing is a remarkably robust process compared with other activities involving skeletal (voluntary) muscle. It performs some 400 million or so operational cycles in a sedentary lifetime and, except in disease, does so without fatigue, thus being able to increase in pace with the most strenuous exercise that the heart can support. Also, breathing continues in sleep and in coma and, fortuitously for patient and surgeon alike, even under anaesthesia that is sufficiently deep to abolish pain. Breathing occurs spontaneously and without conscious attention, notwithstanding its immediate cessation or augmentation in response to volition. Thus it is both the most highly automated of movements, yet, when powering vocalization during speech and singing, the most voluntary.
The neural mechanism responsible for the generation of the basic respiratory rhythm is now attributed to a network of neurons located in the brain stem. The search for such a rhythm generator, originally conceived as the ‘Respiratory Centre’, began early in the nineteenth century and notably, perhaps with Madame Guillotine in mind, initiated by the French scientist Le Gallois. In rabbits he successively transected the brain stem and spinal cord at different levels in different sequences. He discovered that if the lower brain stem remains connected to the spinal cord breathing continues, a result in sharp distinction to that following the human experiment by ‘Madame,’ of sectioning in the neck between the brain stem and spinal cord!
In the fully automatic mode the rate and depth (tidal volume) of breathing movements is regulated on the basis of information from sensors in the brain (‘central’ chemoreceptors) and a variety of peripheral reflex mechanisms, using sensors detecting the concentrations of oxygen and carbon dioxide in the arterial blood (peripheral chemoreceptors) and mechanical events in the heart, lungs and arteries (heart, lung, and arterial mechanoreceptors), and also receptors elsewhere in the body. Collectively, these maintain homeostasis of the blood gases and pH, thus satisfying tissue metabolic requirements for oxygen uptake and the elimination of carbon dioxide in the face of changing behavioural demands.
The act of breathing can be considered from two points of view: mechanical properties, and the central nervous control mechanism. Insight into the former dictates the problem to be solved by the central nervous system, as through its ‘command’ signals breathing is maintained and regulated according to the metabolic or behavioural demands of the moment.
Mechanical factorsThe lungs are paired, lobed structures which when inflated are in some ways balloon-like: when punctured or surgically removed from the thorax, they collapse under the combined influence of internal elastic forces and surface tension within the terminal air sacs (alveoli). As with a balloon they can be inflated through their neck (trachea) by air under positive pressure (greater than atmospheric pressure), and the relationship between pressure and volume can be determined experimentally. However, in the living body, the situation is different. For example, at the end of a normal expiration, there is a volume of air in the lungs known as the Functional Residual Capacity (FRC); at this point the pressure in the alveoli is atmospheric, there is no airflow, but nevertheless the lungs are held inflated within the chest; this is now by ‘suction’ from without. Suction exists because the outer surfaces of the lungs are exposed to a pressure less than atmospheric (‘negative’ pressure) within the thin film of fluid between the two layers of the pleura that cover the lungs and line the inside of the chest wall. This pleura fluid both separates the lungs from the chest wall and also links them to it, whilst allowing the lobes to move freely as they inflate and deflate. At FRC this surface pressure, of about –5.0 to –8.0 cm H2O, is determined by the balance between the elastic recoil forces of the lung pulling inwards and the net elastic recoil of the chest wall pulling outwards. Evidence of the latter is seen during surgery when opening of the chest is accompanied by the collapse of the lungs and the outwards motion of the ribcage. Entry of air into the pleural ‘space’ due to lung or chest wall puncture constitutes a ‘pneumothorax’; this is not life-threatening when only one lung collapses, but bilateral collapse is fatal unless artificial ventilation is immediately available.
Even when the breath has been fully expelled by voluntary effort to reach the lungs' minimal volume (Residual Volume, RV) the lungs still remain slightly inflated, with pleural pressure still negative at about -2.0 cm H2O; the expiratory muscular effort to reach RV is needed to overcome the greatly increasing elastic recoil outwards of the chest wall as lung volume becomes low. Similarly, if voluntary effort is used to inflate the lungs to their maximal volume (Total Lung Capacity, TLC) the lungs remain intimately applied to the chest wall, held there by a negative pressure of some -25 to -30 cm H2O; this reflects the increased elastic recoil of the lungs at this higher volume; and the recoil of the chest wall itself is now also a force acting inwards, opposing the voluntary muscular effort to reach TLC. The total volume of air that can be breathed in from residual volume to TLC (or vice versa) is called the Vital Capacity (VC), and is in the order of 5.0 litres in the adult. Lung volume, or changes in lung volume, are often expressed as a percentage of vital capacity. The tidal volume during quiet breathing, about 0.5 litres, therefore represents only 10% VC and is achieved by swings in pleural pressure of -2 to -3 cm H2O (see figure).
In respiratory mechanics, the term ‘chest wall’ also includes the abdomen, because a major factor contributing to the balance of elastic forces is that of posture. This is due to the influence of gravity on the large mass of the abdominal organs, which hydraulically generates a negative pressure transmitted across a relaxed diaphragm to the pleural space at the base of the lungs, so increasing their volume. Thus changing from the supine to the upright posture can increase FRC by some 15–20% of VC (approximately 0.75–1.0 litres). In fact, this major influence of posture can be utilized to achieve artificial ventilation by means of a tilting bed, but the tidal volume achieved is restricted.
The above considerations all refer to the principal ‘passive’ mechanical properties of the system. In quiet, effortless breathing (eupnoea) the resistance to airflow in the lungs and airways is negligible, as also is the viscous resistance to movement of the lungs and surrounding structures, except in disease states. Changes in pleural pressure require appropriate displacements of the structures immediately around the lungs — namely the ribcage and diaphragm — and, through hydraulic coupling, the abdominal wall. This would represent a formidable design challenge in engineering, as it would be the equivalent of asking for the hull of a ship or aircraft to change its volume and shape, although the latter is actually achieved in Concorde by articulation of its nose! The articulated ribcage does exactly this, and aided by cartilaginous extensions at the front which flexibly couple each rib pair to the breastbone or sternum, allows the inspiratory rise (inflation) and expiratory fall (deflation) of the chest in ‘thoracic’ breathing. Similarly, the diaphragm descends relatively freely within the upper abdomen, accommodated by an outward displacement of the abdominal wall — so-called ‘abdominal’ breathing — until this motion is limited by tension in the abdominal wall. Both expansion of the ribcage and descent of the diaphragm normally cause a decrease in pleural pressure, so either acting alone (as can occur following spinal injury or diaphragmatic paralysis) will cause a paradoxical movement of the other, reducing the change in lung volume. Such paradoxing of the ribcage is seen in the newborn, most markedly in the respiratory distress syndrome, when the absence of surfactant which reduces surface tension within the lungs necessitates the development of much greater changes in pleural pressure to achieve an adequate tidal volume. During such inspiratory effort the reduction in pleural pressure due to the diaphragm is greater than the resisting force of the underdeveloped ribcage muscles, so that the chest collapses inwards instead of expanding. Under most circumstances, however, the 2 sets of muscles work in concert and, even when the ribcage may not appear to rise or expand in inspiration, it is stiffened sufficiently to withstand the fall in pleural pressure, enabling the diaphragm to work more efficiently. Similarly, when thoracic breathing is exaggerated the abdomen may move inwards even though the diaphragm is contracting.
The actual elevation and depression of the ribs in quiet breathing is due to the external and internal intercostal muscles, respectively, which bridge each adjacent pair of ribs, aided by other muscles at the front and back, while three layers of abdominal muscle generate active forces for expiratory flow when greater than resting tidal volumes are required. In quiet breathing neither the diaphragm nor all of the chest wall is fully actively engaged in generating the pressures reviewed above. When exercise requires breathing to be increased, the volume and velocity of airflow in and out is greatly accelerated. This is achieved by additional force production involving progressively more of the chest wall and diaphragm; in the athlete this may no longer be fully automatic, but patterned through training to achieve the best performance.
In addition to their role in breathing, the voluntary activation of respiratory muscles is also utilized for generating much higher forces and hence pressures within the system. For example, abdominal pressures in excess of 200 mm Hg help to stabilize the vertebral spine during weight lifting — some 100 times greater than the pressures required for tidal air movement during quiet, effortless breathing. (See respiratory system).
Neural aspectsIn order to achieve lung ventilation the central nervous control mechanism has to generate muscular forces, displacements, and hence pressure changes, to oppose the augmenting elastic recoil as lung volume increases. It does this through motor commands issued from the brain stem for a waxing and waning profile of pleural pressure change, with a time course matching the current demand for pulmonary ventilation; in resting breathing this would be approximately 1.0 sec for inspiration, and 2–3 sec for expiration depending on the overall respiratory rate (12–15/min). Inspiration is generated by a progressively increasing co-activation of the diaphragm and external intercostal muscles. This action can either be observed mechanically or more directly visualized by recording the electrical activity of the motor units in the relevant muscles, by electromyography (EMG). The EMG shows a strongly augmenting activity pattern paralleling that of the pressure change. At the end of inspiration such activity ceases fairly rapidly, allowing the lungs to deflate due to elastic recoil back to their starting lung volume at FRC. The cycle then repeats. It is generally held that during breathing at rest expiration is a mainly ‘passive’ process, although some activity in expiratory muscles may maintain the outflow of air.
The inspiratory augmentation in the EMG is due to the progressive recruitment in the number and discharge frequencies (e.g. ranging from 5–25 Hz) of the active motor units in the respective muscles. These discharge patterns correspond to respiratory phased ‘trains’ of nerve impulses in the motor nerve fibres to the muscles; in turn, their origin lies in the pattern of the electrical activity in the cell bodies of the motor neurons within the spinal cord. Recordings through minute electrodes inserted through the cell membrane of these motor neurons (intracellular recording) reveals the presence of rhythmic waxing and waning changes of electrical potential, which are given the name ‘central respiratory drive potentials’. Their time course and amplitude closely mirror a mechanical record of the changes in pleural pressure, and it is during the waxing phase that they generate the augmenting patterns of impulse activity in the motor axons that result in the motor unit activity recorded in the muscles.
The ‘central respiratory drive potentials’ of respiratory motor neurons, with their slow time course of 2–3 sec duration, reflect the summed synaptic action of impulses in respiratory motor pathways descending from the brain stem, wherein lies the neural network that generates the breathing rhythm. Again, intracellular recording from those ‘respiratory’ neurones whose axons descend into the spinal cord reveals central respiratory drive potentials, but, in contrast to the motoneurons, their discharge rates are much higher, ranging typically from 50–300/sec within each cycle, depending on the prevailing ‘demand’ for ventilation. This particular system of neurones corresponds to the ‘upper motor neuron system’, the corticospinal tract which conveys ‘volitional’ as well as automated commands for limb and digit movement from the ‘motor cortex’. It is these vital pathways which are interrupted following trauma to the highest part of the spinal cord in the neck (cervical spinal cord), leading to total paralysis of the whole body below the neck including both voluntary and automatic breathing movements. Death ensues within minutes unless artificial ventilation is immediately available. A spinal transection at the base of the neck (low cervical region) abolishes thoracic and expiratory-phased abdominal movements, whereas diaphragmatic movements are left intact because their motor neurons are located above the transection.
Much research, of necessity based on studies of anaesthetized animals, is devoted to unravelling the neural connectivity that sustains the respiratory rhythm, and how this is regulated by a variety of reflexes. Such insights should help, for example, to solve the mystery of the sudden infant death syndrome (SIDS or ‘cot death’), which is usually attributed to an abrupt cessation of breathing — its absence being the first indication of the tragedy to the unsuspecting parents. By recording the electrical activity of a wide range of neurons in the brain stem of anaesthetized animals, several distinctive patterns of neurone activity have been identified, each having different firing patterns within the respiratory cycle and different connectivities one to the other; it is from such information that deductions are made as to how this system could generate the respiratory rhythm. The brain stem emerges as a truly vital structure in the immediate maintenance of ‘life’ as now understood. Thus following major destruction or inactivation of the cerebral cortex through trauma or deprivation of oxygen, a still viable brain stem can maintain breathing when the former structures are so damaged as not to sustain the conscious state (vegetative state); conversely, if the viability of the brain stem is compromised, breathing ceases; however, although artificial ventilation and other life preserving measures may sustain the metabolic activities of the heart, lungs, and other tissues, the otherwise intact cerebral cortex now lacks the afferent inputs from, and also channelled through, the brain stem that are critical for consciousness itself, a state dependent on a fully-functioning cerebral cortex.
See respiratory system.See also life support; lungs; respiration.
Traditional yoga practice is associated with mystical and psychic powers developed through special breathing techniques known as pranayama. According to Hindu teaching, there is a subtle vitality known as prana in the air that we breath, and management of prana has a special effect on the human organism in energizing the chakras, or subtle centers in the body associated with spiritual development and psychic side effects. Pranayama involves special techniques of breathing alternately with right and left nostrils, with a special period of retention. Other exercises involve rapid breathing.
In some instances, levitation is said to be achieved by breathing exercises. Baron Schrenck-Notzing recorded the case of a young man who thus levitated his own body 27 times. In his book Mysterious Psychic Forces (1907), Camille Flammarion stated: "The breathing seems to have a very great influence. In the way things take place it seems as if the sitters released by breathing an amount of motor energy comparable to that which they release when rapidly moving their limbs. There is something in this [that is] very curious and difficult to explain."
Hereward Carrington and other psychic researchers have often drawn attention to the so-called "lifting game," in which four persons lift a fifth by their fingertips, each of the four rapidly inhaling and exhaling in unison, then lifting the subject by a fingertip under each arm and leg. Sometimes the subject is seated on a chair to facilitate the positioning of fingertips.
Carrington tried the experiment on the platform of a large self-registering scale, and in The Story of Psychic Science (1930) commented:
"On the first lift the recorder stated that the needle on the dial had fallen to 660 lbs. (the combined weight was found previously to be 712 lbs.), a loss of 52 lbs. On the second lift there was an apparent loss of 52 lbs.; on the third lift of 60 lbs.; on the fourth lift of 60 lbs.; and on the fifth lift of 60 lbs. No gain of weight was at any time recorded (owing to the muscular exertion), invariably a loss, which, however, slowly returned to normal as the subject was held for some considerable time in the air. I have no theory to offer as to these observations, which I cannot fully explain."
However, modern commentators suggest that the apparent ease with which the subject is lifted by fingertips is related to the distribution of weight on several points, rather like the principle involved when a fakir lies on a bed of sharp nails without injuring himself. Carrington's method of measuring weight loss was too simple a procedure, ignoring such factors as sudden thrust exerted by the lifters.
The association of pranayama with levitation has been revived more recently by the special "siddhi" program of Maharishi Mahesh Yogi, whose system derives from such standard Hindu texts as the Yoga-Sutras of Patanjali. Bhagwan Shree Rajneesh, another modern Indian religious leader, also prescribed special breathing techniques for his followers, although these did not appear to have similar spectacular psychic results. They were more reminiscent of the techniques of the Western mystic Gurdjieff, whose followers often demonstrated remarkable physical control through special breathing.
Breathing techniques have effects on the human organism and contribute to some yogic feats, and also by the related systems of Japanese martial arts, especially kung-fu, where it is claimed that a subtle energy named ch'i (analagous to the Hindu prana) is accumulated, amplified, and directed by will-power to specific parts of the body, developing astonishing strength and resilience. This process is normally preceded by a sudden exhalation of breath, sometimes accompanied by a shout or yell. The intake of breath that follows appears to result in hyperventilation of the system, generating vitality that can be directed to hands, feet, or other parts of the body. Practitioners demonstrate the ability to break bricks, tiles, or heavy planks of wood with a bare hand.
It is interesting to note that special techniques of breathing associated with mystical and psychic development are common to many Asian countries, from India, to Tibet, China, and Japan. These all postulate that a subtle principle exists in the air, and a system of management of that principle occurs in the subtle centers of the body. Of course, traditional Indian yoga teachings present psychic feats as merely side effects of spiritual development, to be discarded for the higher goals of mysticism, and that advanced pranayama techniques, as well as the special physical positions of hatha yoga, should be preceded by strict preliminary practices of yama and niyama (moral observances and ethical restraints). Pranayama is said to begin spontaneously with the perfection of hatha yoga positions, and proper breathing facilitates the advanced techniques. Also with the development of pranayama, a mystical force known as kundalini is aroused and led from the lowest chakras to the highest, culminating in a mystical center in the head, conferring higher consciousness.
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breath / bre[unvoicedth]/ • n. an inhalation or exhalation of air from the lungs: she drew in a quick breath. ∎ an exhalation of air by a person or animal that can be seen, smelled, or heard: he sighed, his breath hanging like a cloud in the icy air. ∎ the physiological process of taking air into the lungs and expelling it again, esp. the ability to breathe easily: she paused for breath. ∎ the air taken into or expelled from the lungs: I was gasping for breath. ∎ a brief moment; the time required for one act of respiration: in Las Vegas, they marry you in a breath. ∎ [in sing.] a slight movement of air: the weather was balmy, not a breath of wind. ∎ [in sing.] a sign, hint, or suggestion: he avoided the slightest breath of scandal.PHRASES: a breath of fresh air a refreshing change: the company's no-nonsense attitude is a breath of fresh air.the breath of life a thing that someone needs or depends on: politics has been the breath of life to her for 50 years.catch one's breath1. cease breathing momentarily in surprise or fear.2. rest after exercise to restore normal breathing: she stood for a few moments, catching her breath.don't hold your breath inf. used hyperbolically to indicate that something is likely to take a long time: don't hold your breath waiting for Congress to clean up political action committees.hold one's breath cease breathing temporarily. ∎ fig. be in a state of suspense or anticipation: France held its breath while the Senate chose its new president.in the same (or next) breath at the same time: he congratulated Simon on his victory but in the same breath dismissed it.out of breath gasping for air, typically after exercise: he arrived on the top floor out of breath.save one's breath stop wasting time in futile talk: save your breath; I know all about it.take someone's breath away astonish or inspire someone with awed respect or delight.under (or below) one's breath in a very quiet voice; almost inaudibly: he swore violently under his breath. waste one's breath talk or give advice without effect: I have better things to do than waste my breath arguing.
Breathlessness is an uncomfortable awareness that arduous respiration is taking place, brought about by the need for increased consumption of oxygen during extended muscular activity. The complete cessation of spontaneous breathing, on the other hand, was for many centuries the prime indicator of death. The common tests for extinction of life were the absence of condensation on a mirror held in front of the mouth (the exit point of the soul), or the failure of a feather to flutter in front of the nostrils. But mechanical ventilators have changed all that; and now some relatives will not even accept brain death — which entails the inability to breathe — as the end of life.
For the Greek philosopher Anaximenes (fl. c.550 bc) the breath or pneuma was the primeval life force that bound the universe together; inhaling it invigorated the body. Similarly, in Indian yogic philosophy, prana is the cosmic energy that fills and maintains the body, manifesting in living beings as the breath. The fourth step of Patanjali's system of Raja Yoga is pranayama, or breath control, practised because the breath is held to influence markedly a person's thoughts and emotions. In one sense modern medicine concurs in this association, by directly relating hyperventilation to a disturbed psychological state.
Early advocates of artificial respiration, like the Society for the Recovery of Drowned Persons, formed in Amsterdam in 1767, advocated not only mouth-to-mouth respiration to resuscitate the inanimate but also the application a clyster of tobacco smoke blown into the intestines with a tobacco fumigator. Tobacco smoking is man's masterstroke in breath-tainting, which is today sweetened with medicated mouthwash rather than oil-based troches.
Inspired by Genesis, literalist advocates of good breathing techniques have pleaded divine sanction for nasal rather than mouth breathing. George Catlin, a nineteenth-century ethnographer of the American Indians (a race little subject, he believed, to fatal diseases of the lungs), insisted, since life was originally breathed into a man's nostrils, ‘why should he not continue to live by breathing it in the same manner?’ The unnatural and addictive habit of breathing through the mouth during sleep, he said, was confined to civilized societies. It allowed impurities into the lungs; affected the voice; caused crooked and protruding teeth; led to nervous agitation; made children into idiots and lunatics, and produced confirmed snorers. In a nutshell, he postulated: ‘if I were to endeavour to bequeath to posterity the most important Motto which human language can convey, it should be in three words — Shut your mouth.’
Miller, J. (1978). The breath of life. In The body in question. Jonathan Cape, London.
See also artificial ventilation; breathing; lungs; respiration.
Hence breathe XIII.
breath·ing / ˈbrē[voicedth]ing/ • n. the process of taking air into and expelling it from the lungs: his breathing was shallow.