Boyle’s Law is the gas law that states that in a closed space, pressure and volume are inversely related. The relationship between gas pressure and volume helps to explain the mechanics of breathing.
There is always a slightly negative pressure within the thoracic cavity, which aids in keeping the airways of the lungs open. During inhalation, volume increases as a result of contraction of the diaphragm, and pressure decreases (according to Boyle’s Law). This decrease of pressure in the thoracic cavity relative to the environment makes the cavity less than the atmosphere (Figure 2a). Because of this drop in pressure, air rushes into the respiratory passages. To increase the volume of the lungs, the chest wall expands. This results from the contraction of the intercostal muscles, the muscles that are connected to the rib cage. Lung volume expands because the diaphragm contracts and the intercostals muscles contract, thus expanding the thoracic cavity. This increase in the volume of the thoracic cavity lowers pressure compared to the atmosphere, so air rushes into the lungs, thus increasing its volume. The resulting increase in volume is largely attributed to an increase in alveolar space, because the bronchioles and bronchi are stiff structures that do not change in size.
The chest wall expands out and away from the lungs. The lungs are elastic; therefore, when air fills the lungs, the elastic recoil within the tissues of the lung exerts pressure back toward the interior of the lungs and pushes air back out of the lungs. These outward and inward forces compete to inflate and deflate the lung with every breath. Upon exhalation, the lungs recoil to force the air out of the lungs, and the intercostal muscles relax, returning the chest wall back to its original position (Figure 2b).
The diaphragm also relaxes and moves higher into the thoracic cavity. This increases the pressure within the thoracic cavity relative to the environment, and air rushes out of the lungs. The movement of air out of the lungs is a passive event; no muscles are contracting to expel the air.
Each lung is surrounded by an invaginated sac. The layer of tissue that covers the lung and dips into spaces is called the visceral pleura. A second layer of parietal pleura lines the interior of the thorax (Figure 3). The space between these layers, the intrapleural space, contains a small amount of fluid that protects the tissue and reduces the friction generated from rubbing the tissue layers together as the lungs contract and relax. Pleurisy results when these layers of tissue become inflamed; it is painful because the inflammation increases the pressure within the thoracic cavity and reduces the volume of the lung.
View how Boyle’s Law is related to breathing and watch this video:
A link to an interactive elements can be found at the bottom of this page.
The alveolar and intrapleural pressures are dependent on certain physical features of the lung. However, the ability to breathe—to have air enter the lungs during inspiration and air leave the lungs during expiration—is dependent on the air pressure of the atmosphere and the air pressure within the lungs.
Inspiration (or inhalation) and expiration (or exhalation) are dependent on the differences in pressure between the atmosphere and the lungs. In a gas, pressure is a force created by the movement of gas molecules that are confined. For example, a certain number of gas molecules in a two-liter container has more room than the same number of gas molecules in a one-liter container (Figure 22.3.1). In this case, the force exerted by the movement of the gas molecules against the walls of the two-liter container is lower than the force exerted by the gas molecules in the one-liter container. Therefore, the pressure is lower in the two-liter container and higher in the one-liter container. At a constant temperature, changing the volume occupied by the gas changes the pressure, as does changing the number of gas molecules. Boyle’s law describes the relationship between volume and pressure in a gas at a constant temperature. Boyle discovered that the pressure of a gas is inversely proportional to its volume: If volume increases, pressure decreases. Likewise, if volume decreases, pressure increases. Pressure and volume are inversely related (P = k/V). Therefore, the pressure in the one-liter container (one-half the volume of the two-liter container) would be twice the pressure in the two-liter container. Boyle’s law is expressed by the following formula:
In this formula, P1 represents the initial pressure and V1 represents the initial volume, whereas the final pressure and volume are represented by P2 and V2, respectively. If the two- and one-liter containers were connected by a tube and the volume of one of the containers were changed, then the gases would move from higher pressure (lower volume) to lower pressure (higher volume).
Figure 22.3.1 – Boyle’s Law: In a gas, pressure increases as volume decreases.
Pulmonary ventilation is dependent on three types of pressure: atmospheric, intra-alveolar, and interpleural. Atmospheric pressure is the amount of force that is exerted by gases in the air surrounding any given surface, such as the body. Atmospheric pressure can be expressed in terms of the unit atmosphere, abbreviated atm, or in millimeters of mercury (mm Hg). One atm is equal to 760 mm Hg, which is the atmospheric pressure at sea level. Typically, for respiration, other pressure values are discussed in relation to atmospheric pressure. Therefore, negative pressure is pressure lower than the atmospheric pressure, whereas positive pressure is pressure that it is greater than the atmospheric pressure. A pressure that is equal to the atmospheric pressure is expressed as zero.
Intra-alveolar pressure is the pressure of the air within the alveoli, which changes during the different phases of breathing (Figure 22.3.2). Because the alveoli are connected to the atmosphere via the tubing of the airways (similar to the two- and one-liter containers in the example above), the interpulmonary pressure of the alveoli always equalizes with the atmospheric pressure.
Figure 22.3.2 – Intrapulmonary and Intrapleural Pressure Relationships: Alveolar pressure changes during the different phases of the cycle. It equalizes at 760 mm Hg but does not remain at 760 mm Hg.
Intrapleural pressure is the pressure of the air within the pleural cavity, between the visceral and parietal pleurae. Similar to intra-alveolar pressure, intrapleural pressure also changes during the different phases of breathing. However, due to certain characteristics of the lungs, the intrapleural pressure is always lower than, or negative to, the intra-alveolar pressure (and therefore also to atmospheric pressure). Although it fluctuates during inspiration and expiration, intrapleural pressure remains approximately –4 mm Hg throughout the breathing cycle.
Competing forces within the thorax cause the formation of the negative intrapleural pressure. One of these forces relates to the elasticity of the lungs themselves—elastic tissue pulls the lungs inward, away from the thoracic wall. Surface tension of alveolar fluid, which is mostly water, also creates an inward pull of the lung tissue. This inward tension from the lungs is countered by opposing forces from the pleural fluid and thoracic wall. Surface tension within the pleural cavity pulls the lungs outward. Too much or too little pleural fluid would hinder the creation of the negative intrapleural pressure therefore, the level must be closely monitored by the mesothelial cells and drained by the lymphatic system. Since the parietal pleura is attached to the thoracic wall, the natural elasticity of the chest wall opposes the inward pull of the lungs. Ultimately, the outward pull is slightly greater than the inward pull, creating the –4 mm Hg intrapleural pressure relative to the intra-alveolar pressure. Transpulmonary pressure is the difference between the intrapleural and intra-alveolar pressures, and it determines the size of the lungs. A higher transpulmonary pressure corresponds to a larger lung.
We all know that breathing is important — there’s a reason why the Bible refers to the “breath of life.” Life doesn’t exist without breath, which is one reason this pandemic and the respiratory distress it can cause is so frightening.
What you might not know is that there’s a right way to breathe, and a wrong way … and unfortunately, most of us are doing it wrong. Which is bad, because improper breathing not only makes us more susceptible to respiratory illness, but also lowers our immune system by inducing a state of chronic stress. N ow more than ever, it’s vital that we make sure we’re breathing right — so here’s how to do it.
1 Breathe through your nose
I know, I know – most of us hate being told to breathe through our noses, especially during exercise. It’s just plain hard to breathe through your nose if you’ve become habituated to the feeling of a great rush of oxygen rapidly fill your chest. But our lungs weren’t designed to work like that. Human beings naturally breathe through the nose as infants, which filters the air and prompts the sinuses to produce nitric oxide. Nitric oxide has powerful vasodilating effects, which means when it reaches the lungs it expands blood vessels, allowing for a much greater exchange of oxygen and carbon dioxide. Basically, breathing through your nose gives you more oxygen and gets rid of more carbon dioxide.
2 Release your diaphragm
The diaphragm is the muscle that sits at the bottom of your chest and pulls down like an umbrella to expand your lungs and allow them to fill with air. When your diaphragm releases properly, your belly expands along with your lungs. But many of us have been conditioned to hold our stomachs in, preventing proper diaphragmatic breathing and causing us to take shallow breaths through the chest and shoulders. If your shoulders rise up towards your ears when you breathe and your belly stays flat, your diaphragm isn’t releasing to allow your lungs to expand.
To correct this, get on all fours in a tabletop position and relax your belly. Then relax it more. Now take a deep breath in (through your nose!) — you should feel your belly expand as your diaphragm releases and contract as your diaphragm engages. Your diaphragm might have become tight after years of improper release, so it might take some practice to get the hang of proper diaphragmatic breathing. But don’t give up – releasing the diaphragm is essential to maintaining healthy lung function, so relax that belly and fill those lungs the way God intended.
3 Expand your ribs
Another crucial element of proper breathing mechanics is the expansion and contraction of the rib cage. Once you have the hang of breathing through your nose and letting your belly expand, look in the mirror and watch what happens with your rib cage during inhalation and exhalation. It should expand 360° and then contract as you exhale—but the front of your rib cage shouldn’t rise up toward the ceiling. Rib flare is a common phenomenon that not only prevents full expansion of the lungs, but also contributes to poor posture, lower back pain, and increased pressure on the lumbar spine. Luckily, there’s an easy way to teach yourself to strengthen your intercostals (the muscles between those ribs): tie a stretchy resistance band or even a pair of tights around your rib cage. As you inhale (through your nose!), expand your rib cage 360°. You should be able to feel the pressure from the band all the way around your rib cage (even those ribs in your back) before exhaling and reversing the process by contracting those ribs as much as possible. If you’re doing this properly, your rib cage and your abdomen should expand and contract with each breath — you’ll be able to feel your abdominal muscles engage as you exhale and your ribs contract. Practice a few times a day until you get familiar with the sensation of expanding and contracting your ribs, and then continue this without the band.
Don’t be discouraged if any of these three steps are difficult for you to master — a lifetime of poor breathing mechanics is an incredibly hard habit to unlearn! But commit to regular practice. Block off time to practice breathing, which might sound silly until you realize how different breathing feels when you’re doing it right. Your stress level will drop, your energy will increase, and most importantly, you’ll have strong, healthy lungs to keep you living your best life!
Breathing exercise that leads to peace in the soul
Induction and Regulation of Xenobiotic-Metabolizing Cytochrome P450s in the Human A549 Lung Adenocarcinoma Cell Line
Several cytochrome P450 (CYP) enzymes are expressed in the human lung, where they participate in metabolic inactivation and activation of numerous exogenous and endogenous compounds. In this study, the expression pattern of all known xenobiotic-metabolizing CYP genes was characterized in the human alveolar type II cell–derived A549 adenocarcinoma cell line using qualitative reverse transcriptase/polymerase chain reaction (RT-PCR). In addition, the mechanisms of induction by chemicals of members in the CYP1 and CYP3A subfamilies were assessed by quantitative RT-PCR. The expression of messenger RNAs (mRNAs) of CYPs 1A1, 1B1, 2B6, 2C, 2E1, 3A5, and 3A7 was detected in the A549 cells. The amounts of mRNAs of CYPs 1A2, 2A6, 2A7, 2A13, 2F1, 3A4, and 4B1 were below the limit of detection. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induced CYP1A1 and CYP1B1 mRNAs 56-fold and 2.5-fold, respectively. CYP3A5 was induced 8-fold by dexamethasone and 11-fold by phenobarbital. CYP3A4 was not induced by any of the typical CYP3A4 inducers used. The tyrosine kinase inhibitor genistein and the protein kinase C inhibitor staurosporine blocked TCDD-elicited induction of CYP1A1, but they did not affect CYP1B1 induction. Protein phosphatase inhibitors okadaic acid and calyculin A enhanced TCDD-induction of CYP1B1 slightly, but had negligible effects on CYP1A1 induction. These results suggest that CYP1A1 and CYP1B1 are differentially regulated in human pulmonary epithelial cells and give the first indication of the induction of CYP3A5 by glucocorticoids in human lung cells. These results establish that having retained several characteristics of human lung epithelial cell CYP expression, the A549 lung cell line is a valuable model for mechanistic studies on induction of the pulmonary CYP system.
Abbreviations: aryl hydrocarbon receptor, AHR benzo(a)pyrene, B(a)P complementary DNA, cDNA cytochrome P450, CYP messenger RNA, mRNA protein kinase C, PKC reverse transcriptase/polymerase chain reaction, RT-PCR 2,3,7,8-tetrachlorodibenzo-p-dioxin, TCDD.
22.3: The Mechanics of Human Breathing - Biology
Optional Breathing: Activating the Diaphragm
The everyday experiences of breathing for most untrained individuals is much more inconsistent than one would assume. Practices in yoga often first teach individuals to observe their own breathing to ultimately familiarize the student with the sensations of respiration. Thus, one meaningful aspect in learning breathing techniques is the awareness in the difference in smooth, even breathing to erratic breathing. Modifications in respiratory patterns come naturally to some individuals after one lesson, however, it may take up to six months to replace bad habits, and ultimately change the way one breathes (Sovik, 2000). The general rule, often noted in studies, and particularly observed by Gallego et al. (2001) was that if a voluntary act is repeated, learning occurs, and the neurophysiological and cognitive processes underpinning its control may change. Gallego et al. continue that while some changes can be made, the need for longer-term studies is warranted to better understand the attention demanding phases involved with these breathing changes.
Although the diaphragm is one of the primary organs responsible for respiration, it is believed by some yogics to be under functioning in many people (Sovik, 2000). Thus, there is often emphasis placed upon diaphragmatic breathing, rather than the use of the overactive chest muscles. Anatomically the diaphragm sits beneath the lungs and is above the organs of the abdomen. It is the separation between cavities of the torso (the upper or thoracic and the lower or abdominal). It is attached at the base of the ribs, the spine, and the sternum. As describe earlier, when the diaphragm contracts the middle fibers, which are formed in a dome shape, descend into the abdomen, causing thoracic volume to increase (and pressure to fall), thus drawing air into the lungs. The practice of proper breathing techniques is aimed at eliminating misused accessory chest muscles, with more emphasis on diaphragmatic breathing.
With diaphragmatic breathing the initial focus of attention is on the expansion of the abdomen, sometimes referred to as abdominal or belly breathing. Have a client place one hand on the abdomen above the navel to feel it being pushed outward during the inhalations. Next, the breathing focus includes the expansion of the rib cage during the inhalation. To help a student learn this, try placing the edge of the hands along side the rib cage (at the level of the sternum) correct diaphragmatic breathing will elicit a noticeable lateral expansion of the rib cage. Diaphragmatic breathing should be practiced in the supine, prone and erect positions, as these are the functional positions of daily life. Finally, the diaphragmatic breathing is integrated with physical movements, asanas, during meditation and during relaxation. Analogous to the seasoned cyclist, who is able to maintain balance effortlessly while cycling, the trained practitioner in diaphragmatic breathing can focus attention on activities of daily life while naturally doing diaphragmatic breathing. To summarize, Sovik suggests the characteristics of optimal breathing (at rest) are that it is diaphragmatic, nasal (inhalation and exhalation), smooth, deep, even, quiet and free of pauses.
Answers to Some Common Questions on Breathing
The following are some answers to common questions about breathing adapted from Repich (2002).
1) How do you take a deep breath?
Although many people feel a deep breath comes solely from expansion of the chest, chest breathing (in of itself) is not the best way to take a deep breath. To get a full deep breath, learn how to breathe from the diaphragm while simultaneously expanding the chest.
2) What happens when you feel breathless?
Breathlessness is often a response of your flight or fight hormone and nervous system triggering the neck and chest muscles to tighten. This makes breathing labored and gives a person that breathless feeling.
3) What is hyperventilation syndrome?
Hyperventilation syndrome is also known as overbreathing. Breathing too frequently causes this phenomenon. Although it feels like a lack of oxygen, this is not the case at all. The overbreathing causes the body to lose considerable carbon dioxide. This loss of carbon dioxide triggers symptoms such as gasping, trembling, choking and the feeling of being smothered. Regrettably, overbreathing often perpetuates more overbreathing, lowering carbon dioxide levels more, and thus become a nasty sequence. Repich (2002) notes that this hyperventilation syndrome is common in 10% of the population. Fortunately, slow, deep breathing readily alleviates it. The deliberate, even deep breaths helps to transition the person to a preferable diaphragmatic breathing pattern.
4) When you feel short of breath, do you need to breathe faster to get more air?
Actually, just the opposite. If you breathe fast, you may start to over breathe and lower your carbon dioxide levels. Once again, slow deep diaphragmatic breathing is recommended.
5) How do you know if you are hyperventilating?
Often times a person does not realize when he/she is hyperventilating. Usually more focus is centered on the anxiety-provoking situation causing the rapid breathing. With hyperventilation there is much more rapid chest breathing, and thus the chest and shoulders will visibly move much more. As well, if you take about 15-17 breaths per minute or more (in a non-exercise situation) then this could be a more quantifiable measure of probable hyperventilating.
The research is very clear that breathing exercises (e.g. pranayama breathing) can enhance parasympathetic (inhibit neural responses) tone, decrease sympathetic (excitatory) nervous activity, improve respiratory and cardiovascular function, decrease the effects of stress, and improve physical and mental health (Pal, Velkumary, and Madanmohan, 2004). Health and fitness professionals can utilize this knowledge and regularly incorporate proper slow breathing exercises with their students and clients in their classes and training sessions.
Side Bar 1. What is Asthma? And Five Common Myths Associated with it?
The word "asthma" is derived from the Greek word meaning "to puff or pant. Typical symptoms of asthma include wheezing, shortness of breath, chest tightness, and a persistent cough. Asthma attacks develop from an involuntary response to a trigger, such as house dust, pollen, tobacco, smoke, furnace air, and animal fur.
Asthma provokes an inflammatory response in the lungs. Airway linings swell up, the smooth muscle surrounding them contracts and excess mucus is produced. Airflow is now limited, making it hard for oxygen to get through to the alveoli and into the bloodstream. The severity of an asthma attack is determined by how restricted the airways become. When an asthmatic's airways become chronically inflamed it takes only a slight trigger to cause a major reaction in the airways. Oxygen levels can become low and even life threatening. Below are some of the common myths about asthma.
Myth 1) Asthma is a mental disease
Because asthma sufferers often have attacks when facing emotional stress, some people have identified it as a psychosomatic condition. Asthma is a real physiological condition. However, emotional stimuli can act as an asthma trigger, worsening an asthma flare up.
Myth 2) Asthma is not a serious health condition
Quite the contrary! Asthma attacks may last several minutes or go on for hours. With extended asthma agitation ones health is increasingly threatened. Indeed, if an airway obstruction becomes severe, the sufferer may experience respiratory failure, leading to fainting and possible death.
Myth 3) Children will grow out of asthma as they mature to adulthood
The majority of asthma sufferers will have it for life, although some people do appear to grow out of it.
Myth 4) Asthmatics shouldnt exercise
Asthmatics can and should exercise. Importantly they should find the types of exercise they feel most comfortable w ith as well as the best place and time to do the exercise.
Myth 5) Not that many people are affected by asthma
According to National Center for Health Statistics (2002), 20 million people suffer from asthma in the U.S. Asthma can be life threatening as it took the lives of approximately 4,261 deaths in 2002. Researchers are unclear if this is due to improper preventative care, chronic overuse of asthma medications, or a combination of both factors.
Collins, C. (1998). Yoga: Intuition, preventive medicine, and treatment. Journal of Obstetric, Gynecologic, and Neonatal Nursing, 27 (5) 563-568.
Gallego, J., Nsegbe, E. and Durand, E. (2001). Learning in respiratory control. Behavior Modification, 25 (4) 495-512.
Guz, A. (1997). Brain, breathing and breathlessness. Respiration Physiology. 109, 197-204.
Jerath, R., Edry J.W, Barnes, V.A., and Jerath, V. (2006). Physiology of long pranayamic breathing: Neural respiratory elements may provide a mechanism that explains how slow deep breathing shifts the autonomic nervous system. Medical Hypothesis, 67, 566-571.
National Center for Health Statistics. (2002). U.S. Department of Health and Human Services. Centers for Disease Control and Prevention.
Pal, G.K. Velkumary, S. and Madanmohan. (2004). Effect of short-term practice of breathing exercises on autonomic functions in normal human volunteers. Indian Journal of Medical Research, 120, 115-121.
Repich, D. (2002). Overcoming concerns about breathing. National Institute of Anxiety and Stress, Inc.
Ritz, T. and Roth, W.T. (2003). Behavioral intervention in asthma. Behavior Modification. 27 (5), 710-730.
Sovik, R. (2000). The science of breathing The yogic view. Progress in Brain Research, 122 (Chapter 34), 491-505.
Human Breathing SystemStructure of the human breathing system
Nasal and buccal cavities:
- Mouth and internal areas of the nose
- Function in warming and moistening air entering lungs
- Mucus and small hairs filter the air and then transport the dirt-loaded mucus to the pharynx where it is swallowed
- Area between oesophagus and windpipe (trachea)
- Pharynx has a sphincter (epiglottis) that closes over the opening to the trachea (glottis) that prevents food travelling into the trachea
- Sphincter that closes over the glottis to prevent food getting into the trachea during swallowing
- Swallowing causes the vocal cords to pull on the glottis and the larynx to be pulled upwards thereby closing the epiglottis over the glottis
Larynx (voice box):
- Made of cartilage and sits on top of the trachea
- Three functions:
- Produces sound
- Controls air flowing into and out of the trachea
- Directs food into the oesophagus
- Directs inhaled air into the lungs
- Contains c-shaped rings of cartilage that keeps the trachea open
- Cilia of trachea carry dirt-laden mucus up the pharynx
- Two divisions of the trachea
- Directs air into each lung
- Supported by cartilage
- Tiny divisions of the bronchi
- Air passages that are less then 1 mm in diameter
- Not supported by cartilage
- Composed of spongy, elastic tissue that expands easily during inhalation and recoils rapidly as exhalation occurs
- Thin pair of membranes covering and separating the lungs from other organs, such as the heart
- The lungs are stuck to the rib cage and diaphragm by the pleural fluid (think of a layer of water between a table and a piece of glass and how difficult it is to lift it off the table)
- Composed of 12 thoracic vertebrae, 12 ribs, and the sternum
- First 7 pairs are called ‘true’ ribs (because they attach directly to the sternum)
- Next 3 pairs are called ‘false’ ribs (because they are only attached to the sternum by cartilage)
- Final 2 pairs are called ‘floating’ ribs (because they do not attach to the sternum at all)
- Tiny air sacs at the end of the bronchioles where gas exchange occurs
- Walls of alveoli are only 1 cell thick to maximise diffusion
- Each alveolus has rich blood capillary network surrounding it
- There are
Essential Features of Alveoli and Capillaries
- Alveoli are numerous
- Alveoli have rich blood capillary network nearby
- Alveoli have walls only one-cell thick
- Alveoli surface is moist
- Alveoli walls are elastic
- Capillaries that surround each alveolus have walls that are only one-cell thick
This course provides an introduction to human anatomy and body systems. The laws of physics are used to explain several bodily functions including the mechanics of muscles and body movements, fluid mechanics of blood and air flow, hearing and acoustic properties of the ears, vision optics, heat and energy, acoustics, and electrical signaling. The effects of various environmental phenomena on the body are explored and include discussions on the behavior of the body in low gravity environment (e.g. in space).
- Basic anatomy of the human body
- Terminology, modeling, and measurement
- Energy, heat, work, and power of the body
- Muscle and forces
- Physics of the skeleton
- Pressure in the body
- Physics of the lungs and breathing
- Physics of the cardiovascular system
- Electrical signals from the body
- Sound and speech
- Physics of the ear and hearing
- Physics of the eyes and vision.
- Human body in space and microgravity
The course assessment will be conducted as follows:
One 2-hour final written examination 60%
One 1-hour in-course test 20%
Four graded assignments (equally weighted) 20%
Students will be expected to satisfy the examiners in both components.
At the end of the course the students should be able to:
- describe the musculoskeletal and cardiovascular systems of the human body
- apply the principles of physics to explain the biomechanics of the body
- use physical quantities to explain the functioning of cardiovascular and pulmonary systems
- analyse the electrical conduction system of the nerves, the brain and the heart
- explain how physics influences the functions of the visual and auditory system
- solve basic conceptual and numerical problems of human body related to energy, work, acceleration, forces, electricity, magnetism, sound, optics and modern physics
- describe the effects of space flight and microgravity on the human body
Herman, I.P. (2007), Physics of the Human Body, Springer. ISBN: 978-3540296034
Cameron, J. R., Skofronick, J. G. and Grant, R. M. (1999), Physics of the Body, Medical Physics Publishing, 2nd Ed., ISBN: 978-0944838914
Davidovits, P., (2008), Physics in Biology and Medicine, 3rd Edition, Elsevier/Academic Press, ISBN: 978-0123694119
Patton, K., and Thibodeau, G., (2009), Anthony's Textbook of Anatomy & Physiology, 19 th Edition , Mosby. ISBN: 978-0323055390
The lungs are not capable of inflating themselves, and will expand only when there is an increase in the volume of the thoracic cavity.   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 and outwards as shown in the diagrams on the right.  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.  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.  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. 
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).  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".  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. 
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 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.
Ideally, air is breathed first out and secondly in through the nose. The nasal cavities (between the nostrils and the pharynx) are quite narrow, firstly by being divided in two by the nasal septum, and secondly by lateral walls that have several longitudinal folds, or shelves, called nasal conchae,  thus exposing a large area of nasal mucous membrane to the air as it is inhaled (and exhaled). 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.  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.  
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 a respiratory tree or tracheobronchial 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 the mouse 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 for gas exchange.  
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 the lungs.
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 creates dead 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 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 that gas 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 by diffusion. After exhaling, adult human lungs still contain 2.5–3 L of air, their functional 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 the peripheral and central 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 maintains a constant pH of the blood. 
The rate and depth of breathing is automatically controlled by the respiratory centers that receive information from the peripheral and central chemoreceptors. These chemoreceptors 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 the medulla oblongata of the brain stem which are particularly sensitive to pH as well as the partial pressure of carbon dioxide in the blood and cerebrospinal fluid.  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 the aortic and carotid bodies.  Information from all of these chemoreceptors is conveyed to the respiratory centers in the pons and medulla 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).  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 the phrenic nerves, which innervate the diaphragm, are probably the most important. 
Automatic breathing can be overridden to a limited extent by simple choice, or to facilitate swimming, speech, singing or other vocal 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. 
Other automatic breathing control reflexes also exist. Submersion, particularly of the face, in cold water, triggers a response called the diving reflex.   This has the initial 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, reserving the oxygen that is in blood and lungs at the beginning of the dive almost exclusively for the heart and the brain.  The diving reflex is an often-used response in animals that routinely need to dive, such as penguins, seals and whales.   It is also more effective in very young infants and children than in adults. 
Inhaled air is by volume 78% nitrogen, 20.95% oxygen and small amounts of other gases including argon, carbon dioxide, neon, helium, and hydrogen. 
The gas exhaled is 4% to 5% by volume of carbon dioxide, about a 100 fold increase over the inhaled amount. The volume of oxygen is reduced by a small amount, 4% to 5%, compared to the oxygen inhaled. The typical composition is: 
- 5.0–6.3% water vapor
- 79% nitrogen 
- 13.6–16.0% oxygen
- 4.0–5.3% carbon dioxide
- 1% argon (ppm) of hydrogen, from the metabolic activity of microorganisms in the large intestine. 
- ppm of carbon monoxide from degradation of heme proteins.
- 1 ppm of ammonia.
- Trace many hundreds of volatile organic compounds especially isoprene and acetone. The presence of certain organic compounds indicate disease. 
In addition to air, underwater divers practicing 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. However, this is kept at around 20% of Earthbound atmospheric pressure to regulate the rate of inspiration. [ citation needed ]
Breathing at altitude
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.  The composition of atmospheric air is, however, almost constant below 80 km, as a result of the continuous mixing effect of the weather.  The concentration of oxygen in the air (mmols O2 per liter of air) therefore decreases at the same rate as the atmospheric pressure.  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).  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 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.  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. 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.   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 respiratory gases homeostatic mechanism, which regulates the arterial PO2 and PCO2 . This homeostatic mechanism 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 depth
Pressure increases with the depth of water at the rate of about one atmosphere — slightly more than 100 kPa, or one bar, for every 10 meters. 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.
Air is provided by a diving regulator, which reduces the high pressure in a diving cylinder to the ambient pressure. The breathing performance of regulators is a factor when choosing a suitable regulator for the type 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 the Venturi 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.
Breathing Patterns Graph showing normal as well as different kinds of pathological breathing patterns.
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 refers to overly shallow breathing hyperpnea refers to fast and deep breathing brought on by a demand for more oxygen, as for example by 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. 
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. 
The word "spirit" comes from the Latin spiritus, meaning breath. Historically, breath has often been considered in terms of the concept of 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, prana, the Polynesian mana, the Hebrew ruach and the psyche in psychology are related to the concept of breath. 
In T'ai chi, aerobic exercise is combined with breathing exercises to strengthen the diaphragm muscles, improve posture and make better use of the body's qi. Different forms of meditation, and yoga advocate various breathing methods. A form of Buddhist meditation called anapanasati meaning mindfulness of breath was first introduced by Buddha. 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. 
In music, some wind instrument players use a technique called circular breathing. Singers also rely on breath control.
Common cultural expressions related to breathing include: "to catch my breath", "took my breath away", "inspiration", "to expire", "get my breath back".
Breathing and mood
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.  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  and business advisers that it provides relief from work-based stress.
Breathing and physical exercise
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.  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.
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2 Answers 2
Breathing is controlled by both the Autonomic nervous system and the voluntary nervous system. You see this in instances where our breath rate increases in flight or fight situations glide to the secretion of Adrenaline and also when we intentionally increase the breathing rate when undergoing high levels of activity. This is due to the fact that the involuntary aspect of breathing is controlled by the medulla oblongata and the voluntary aspect s controlled by the cerebral cortex. The fact that it is controlled by skeletal muscles has nothing to do with how it is innervated. For example, Cardiac muscles are innervated by both the hearts own conducting system and by the Autonomic nervous system. If you're wondering why the skeletal muscles dont get fatigued, it's because there is a small but significant rest period between each Breathing cycle (inhalation and exhalation). Therefore the skeletal muscles have a rest period. However if a high rate of breathing does occur for a sustained period, they will fatigue and that's why you get cramps after a marathon or a sprint.
I would argue that the problem here is more semantic than biological. We artificially classify processes into "voluntary" and "involuntary", but the reality is much more complicated. For example, is walking voluntary or involuntary? Well, if I decide to go walking, it may initially be voluntarily, but when I am walking, I am doing very little in the way of thinking about walking. Breathing is much the same way. There is BOTH a degree of conscious control from the cortex, and a basal regulatory system in the brain stem that keeps things going below conscious perception. Another way to look at this is that the conscious control from the cortex modulates the medulla based breathing system.
Watch the video: Mechanism of Breathing (January 2022).