Lung volumes and lung capacities refer to the volume of air in the lungs at different phases of the respiratory cycle. The average total lung capacity of an adult human male is about 6 litres of air. Tidal breathing is normal, resting breathing; the average human respiratory rate is 30-60 breaths per minute at birth, decreasing to 12-20 breaths per minute in adults. Several factors affect lung volumes. Lung volumes vary with different people as follows: A person, born and lives at sea level will develop a smaller lung capacity than a person who spends their life at a high altitude; this is because the partial pressure of oxygen is lower at higher altitude which, as a result means that oxygen less diffuses into the bloodstream. In response to higher altitude, the body's diffusing capacity increases in order to process more air. Due to the lower environmental air pressure at higher altitudes, the air pressure within the breathing system must be lower in order to inhale; when someone living at or near sea level travels to locations at high altitudes that person can develop a condition called altitude sickness because their lungs remove adequate amounts of carbon dioxide but they do not take in enough oxygen.
Lung function development is reduced in children who grow up near motorways although this seems at least in part reversible. Air pollution exposure affects FEV1 in asthmatics, but affects FVC and FEV1 in healthy adults at low concentrations. Specific changes in lung volumes occur during pregnancy. Functional residual capacity drops 18–20% falling from 1.7 to 1.35 litres, due to the compression of the diaphragm by the uterus. The compression causes a decreased total lung capacity by 5% and decreased expiratory reserve volume by 20%. Tidal volume increases by 30–40%, from 0.5 to 0.7 litres, minute ventilation by 30–40% giving an increase in pulmonary ventilation. This is necessary to meet the increased oxygen requirement of the body, which reaches 50 mL/min, 20 mL of which goes to reproductive tissues. Overall, the net change in maximum breathing capacity is zero; the tidal volume, vital capacity, inspiratory capacity and expiratory reserve volume can be measured directly with a spirometer. These are the basic elements of a ventilatory pulmonary function test.
Determination of the residual volume is more difficult as it is impossible to "completely" breathe out. Therefore, measurement of the residual volume has to be done via indirect methods such as radiographic planimetry, body plethysmography, closed circuit dilution and nitrogen washout. In absence of such, estimates of residual volume have been prepared as a proportion of body mass for infants, or as a proportion of vital capacity or in relation to height and age. Standard errors in prediction equations for residual volume have been measured at 579 mL for men and 355 mL for women, while the use of 0.24*FVC gave a standard error of 318 mL. Online calculators are available that can compute predicted lung volumes, other spirometric parameters based on a patient's age, height and ethnic origin for many reference sources; the mass of one breath is a gram. A Litre of air weighs about 1.2 grams. A half Litre ordinary tidal breath weighs just over half a gram; the results can be used to distinguish between restrictive and obstructive pulmonary diseases: Lung capacity can be expanded through flexibility exercises such as yoga, breathing exercises, physical activity.
A greater lung capacity is sought by people such as athletes, freedivers and wind-instrument players. A stronger and larger lung capacity allows more air to be inhaled into the lungs. In using lungs to play a wind instrument for example, exhaling an expanded volume of air will give greater control to the player and allow for a clearer and louder tone. Spirometry Lung function fundamentals RT Corner Volume of human lungs
Internal intercostal muscles
The internal intercostal muscles are a group of skeletal muscles located between the ribs. They are eleven in number on either side, they commence anteriorly at the sternum, in the intercostal spaces between the cartilages of the true ribs, at the anterior extremities of the cartilages of the false ribs, extend backward as far as the angles of the ribs, hence they are continued to the vertebral column by thin aponeuroses, the posterior intercostal membranes. Each muscle arises from the ridge on the inner surface of a rib, as well as from the corresponding costal cartilage, is inserted into the inferior border of the rib above; the internal intercostals are innervated by the intercostal nerve. Their fibers are directed obliquely, but pass in a direction opposite to those of the external intercostal muscles. For the most part, they are muscles of exhalation. In exhalation the interosseous portions of the internal intercostal muscles and retracts the ribs, compressing the thoracic cavity and expelling air.
The internal intercostals, are only used in forceful exhalation such as coughing or during exercise and not in relaxed breathing. The external intercostal muscles, the intercartilaginous part of the internal intercostal muscles, are used in inspiration, by aiding in elevating the ribs and expanding the thoracic cavity; this article incorporates text in the public domain from page 403 of the 20th edition of Gray's Anatomy Anatomy photo:18:03-0101 at the SUNY Downstate Medical Center - "The Internal Intercostal Muscle" Anatomy figure: 18:03-03 at Human Anatomy Online, SUNY Downstate Medical Center - "Transverse section of thorax."
The respiratory system is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies depending on the size of the organism, the environment in which it lives and its evolutionary history. In land animals the respiratory surface is internalized as linings of the lungs. Gas exchange in the lungs occurs in millions of small air sacs called alveoli in mammals and reptiles, but atria in birds; these microscopic air sacs have a rich blood supply, thus bringing the air into close contact with the blood. These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the trachea, which branches in the middle of the chest into the two main bronchi; these enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the bronchioles. In birds the bronchioles are termed parabronchi.
It is the bronchioles, or parabronchi that open into the microscopic alveoli in mammals and atria in birds. Air has to be pumped from the environment into the alveoli or atria by the process of breathing which involves the muscles of respiration. In most fish, a number of other aquatic animals the respiratory system consists of gills, which are either or external organs, bathed in the watery environment; this water flows over the gills by a variety of passive means. Gas exchange takes place in the gills which consist of thin or flat filaments and lammelae which expose a large surface area of vascularized tissue to the water. Other animals, such as insects, have respiratory systems with simple anatomical features, in amphibians the skin plays a vital role in gas exchange. Plants have respiratory systems but the directionality of gas exchange can be opposite to that in animals; the respiratory system in plants includes anatomical features such as stomata, that are found in various parts of the plant.
In humans and other mammals, the anatomy of a typical respiratory system is the respiratory tract. The tract is divided into a lower respiratory tract; the upper tract includes the nose, nasal cavities, sinuses and the part of the larynx above the vocal folds. The lower tract includes the lower part of the larynx, the trachea, bronchi and the alveoli; the branching airways of the lower tract are described as the respiratory tree or tracheobronchial tree. The intervals between successive branch points along the various branches of "tree" are referred to as branching "generations", of which there are, in the adult human about 23; the earlier generations, consisting of the trachea and the bronchi, as well as the larger bronchioles which act as air conduits, bringing air to the respiratory bronchioles, alveolar ducts and alveoli, where gas exchange takes place. Bronchioles are defined as the small airways lacking any cartilagenous support; the first bronchi to branch from the trachea are the right and left main bronchi.
Second only in diameter to the trachea, these bronchi enter the lungs at each hilum, where they branch into narrower secondary bronchi known as lobar bronchi, these branch into narrower tertiary bronchi known as segmental bronchi. Further divisions of the segmental bronchi are known as 4th order, 5th order, 6th order segmental bronchi, or grouped together as subsegmental bronchi. Compared to the, on average, 23 number of branchings of the respiratory tree in the adult human, the mouse has only about 13 such branchings; the alveoli are the dead end terminals of the "tree", meaning that any air that enters them has to exit via the same route. A system such as this creates dead space, a volume of air that fills the airways after exhalation and is breathed back into the alveoli before environmental air reaches them. At the end of inhalation the airways are filled with environmental air, exhaled without coming in contact with the gas exchanger; the lungs contract during the breathing cycle, drawing air in and out of the lungs.
The volume of air moved in or out of the lungs under normal resting circumstances, volumes moved during maximally forced inhalation and maximally forced exhalation are measured in humans by spirometry. A typical adult human spirogram with the names given to the various excursions in volume the lungs can undergo is illustrated below: Not all the air in the lungs can be expelled during maximally forced exhalation; this is the residual volume of about 1.0-1.5 liters. Volumes that include the residual volume can therefore not be measured by spirometry, their measurement requires special techniques. The rates at which air is breathed in or out, either through the mouth or nose, or into or out of the alveoli are tabulated below, together with how they are calculated; the number of breath cycles per minute is known as the respiratory rate. In mammals, inhalation at rest is due to the contraction of the diaphragm; this is an upwardly domed sheet of muscle that separates the thoracic cavity from the abdominal cavity.
When it contracts the sheet flattens. The contracting diaphragm pushes, but because the pelvic floo
Breathing is the process of moving air into and out of the lungs to facilitate gas exchange with the internal environment by bringing in oxygen and flushing out carbon dioxide. All aerobic creatures need oxygen for cellular respiration, which uses the oxygen to break down foods for energy and produces carbon dioxide as a waste product. Breathing, or "external respiration", brings air into the lungs where gas exchange takes place in the alveoli through diffusion; the body's circulatory system transports these gases to and from the cells, where "cellular respiration" takes place. The breathing of all vertebrates with lungs consists of repetitive cycles of inhalation and exhalation through a branched system of tubes or airways which lead from the nose to the alveoli; the number of respiratory cycles per minute is the breathing or respiratory rate, is one of the four primary vital signs of life. Under normal conditions the breathing depth and rate is automatically, unconsciously, controlled by several homeostatic mechanisms which keep the partial pressures of carbon dioxide and oxygen in the arterial blood constant.
Keeping the partial pressure of carbon dioxide in the arterial blood unchanged under a wide variety of physiological circumstances, contributes to tight control of the pH of the extracellular fluids. Over-breathing and under-breathing, which decrease and increase the arterial partial pressure of carbon dioxide cause a rise in the pH of ECF in the first case, a lowering of the pH in the second. Both cause distressing symptoms. Breathing has other important functions, it provides a mechanism for speech and similar expressions of the emotions. It is used for reflexes such as yawning and sneezing. Animals that cannot thermoregulate by perspiration, because they lack sufficient sweat glands, may lose heat by evaporation through panting; the lungs are not capable of inflating themselves, 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 through the contraction of the diaphragm, but by the contraction of the intercostal muscles which pull the rib cage upwards and outwards as shown in the diagrams on the left.
During forceful inhalation 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, bringing about a greater change in the volume of the chest cavity. During exhalation, at rest, all the muscles of inhalation relax, returning the chest and abdomen to a position called the “resting position”, 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 as, for instance, during exercise, exhalation is brought about by relaxation of all the muscles of inhalation, but, in addition, the abdominal muscles, instead of being passive, now contract causing the rib cage to be pulled downwards; this not only decreases the size of the rib cage but pushes the abdominal organs upwards against the diaphragm which bulges 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 fall back, it is, therefore referred to as "abdominal breathing". These terms are used interchangeably because they describe the same action; when the accessory muscles of inhalation are activated 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 during asthma attacks and in people with chronic obstructive pulmonary disease. Air is breathed in and out through the nose; the nasal cavities are quite narrow, firstly by being divided in two by the nasal septum, 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.
This causes the inhaled air to take up moisture from the wet mucus, warmth from the underlying blood vessels, so that the air is nearly saturated with water vapor and is at 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 dried-out, cooled mucus in the nasal passages, during breathing out; the sticky mucus traps much of the particulate matter, breathed in, preventing it from reaching the lungs. The anatomy of a typical mammalian respiratory system, below the structures listed among the "upper airways", is described as a respiratory tree or tracheobronchial tree. Larger airways give rise to branches that are 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 div
Atmosphere of Earth
The atmosphere of Earth is the layer of gases known as air, that surrounds the planet Earth and is retained by Earth's gravity. The atmosphere of Earth protects life on Earth by creating pressure allowing for liquid water to exist on the Earth's surface, absorbing ultraviolet solar radiation, warming the surface through heat retention, reducing temperature extremes between day and night. By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, small amounts of other gases. Air contains a variable amount of water vapor, on average around 1% at sea level, 0.4% over the entire atmosphere. Air content and atmospheric pressure vary at different layers, air suitable for use in photosynthesis by terrestrial plants and breathing of terrestrial animals is found only in Earth's troposphere and in artificial atmospheres; the atmosphere has a mass of about 5.15×1018 kg, three quarters of, within about 11 km of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space.
The Kármán line, at 100 km, or 1.57% of Earth's radius, is used as the border between the atmosphere and outer space. Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 120 km. Several layers can be distinguished in the atmosphere, based on characteristics such as temperature and composition; the study of Earth's atmosphere and its processes is called atmospheric science. Early pioneers in the field include Richard Assmann; the three major constituents of Earth's atmosphere are nitrogen and argon. Water vapor accounts for 0.25% of the atmosphere by mass. The concentration of water vapor varies from around 10 ppm by volume in the coldest portions of the atmosphere to as much as 5% by volume in hot, humid air masses, concentrations of other atmospheric gases are quoted in terms of dry air; the remaining gases are referred to as trace gases, among which are the greenhouse gases, principally carbon dioxide, nitrous oxide, ozone. Filtered air includes trace amounts of many other chemical compounds.
Many substances of natural origin may be present in locally and seasonally variable small amounts as aerosols in an unfiltered air sample, including dust of mineral and organic composition and spores, sea spray, volcanic ash. Various industrial pollutants may be present as gases or aerosols, such as chlorine, fluorine compounds and elemental mercury vapor. Sulfur compounds such as hydrogen sulfide and sulfur dioxide may be derived from natural sources or from industrial air pollution; the relative concentration of gases remains constant until about 10,000 m. In general, air pressure and density decrease with altitude in the atmosphere. However, temperature has a more complicated profile with altitude, may remain constant or increase with altitude in some regions; because the general pattern of the temperature/altitude profile is constant and measurable by means of instrumented balloon soundings, the temperature behavior provides a useful metric to distinguish atmospheric layers. In this way, Earth's atmosphere can be divided into five main layers.
Excluding the exosphere, the atmosphere has four primary layers, which are the troposphere, stratosphere and thermosphere. From highest to lowest, the five main layers are: Exosphere: 700 to 10,000 km Thermosphere: 80 to 700 km Mesosphere: 50 to 80 km Stratosphere: 12 to 50 km Troposphere: 0 to 12 km The exosphere is the outermost layer of Earth's atmosphere, it extends from the exobase, located at the top of the thermosphere at an altitude of about 700 km above sea level, to about 10,000 km where it merges into the solar wind. This layer is composed of low densities of hydrogen and several heavier molecules including nitrogen and carbon dioxide closer to the exobase; the atoms and molecules are so far apart that they can travel hundreds of kilometers without colliding with one another. Thus, the exosphere no longer behaves like a gas, the particles escape into space; these free-moving particles follow ballistic trajectories and may migrate in and out of the magnetosphere or the solar wind. The exosphere is located too far above Earth for any meteorological phenomena to be possible.
However, the aurora borealis and aurora australis sometimes occur in the lower part of the exosphere, where they overlap into the thermosphere. The exosphere contains most of the satellites orbiting Earth; the thermosphere is the second-highest layer of Earth's atmosphere. It extends from the mesopause at an altitude of about 80 km up to the thermopause at an altitude range of 500–1000 km; the height of the thermopause varies due to changes in solar activity. Because the thermopause lies at the lower boundary of the exosphere, it is referred to as the exobase; the lower part of the thermosphere, from 80 to 550 kilometres above Earth's surface, contains the ionosphere. The temperature of the thermosphere increases with height. Unlike the stratosphere beneath it, wherein a temperature inversion is due to the absorption of radiation by ozone, the inversion in the t
Olfaction is a chemoreception that forms the sense of smell. Olfaction has many purposes, such as the detection of hazards and food, it integrates with other senses to form the sense of flavor. Olfaction occurs when odorants bind to specific sites on olfactory receptors located in the nasal cavity. Glomeruli aggregate signals from these receptors and transmit them to the olfactory bulb, where the sensory input will start to interact with parts of the brain responsible for smell identification and emotion. Land organisms will have separate olfaction systems for smell and taste, but water-dwelling organisms have only one system. In vertebrates, smells are sensed by olfactory sensory neurons in the olfactory epithelium; the olfactory epithelium is made up of at least six morphologically and biochemically different cell types. The proportion of olfactory epithelium compared to respiratory epithelium gives an indication of the animal's olfactory sensitivity. Humans have about 10 cm2 of olfactory epithelium, whereas some dogs have 170 cm2.
A dog's olfactory epithelium is considerably more densely innervated, with a hundred times more receptors per square centimeter. Molecules of odorants passing through the superior nasal concha of the nasal passages dissolve in the mucus that lines the superior portion of the cavity and are detected by olfactory receptors on the dendrites of the olfactory sensory neurons; this may occur by the binding of the odorant to odorant-binding proteins. The mucus overlying the epithelium contains mucopolysaccharides, salts and antibodies; this mucus acts as a solvent for odor molecules, flows and is replaced every ten minutes. In insects, smells are sensed by olfactory sensory neurons in the chemosensory sensilla, which are present in insect antenna and tarsa, but on other parts of the insect body. Odorants penetrate into the cuticle pores of chemosensory sensilla and get in contact with insect odorant-binding proteins or Chemosensory proteins, before activating the sensory neurons; the binding of the ligand to the receptor leads to an action potential in the receptor neuron, via a second messenger pathway, depending on the organism.
In mammals, the odorants stimulate adenylate cyclase to synthesize cAMP via a G protein called Golf. CAMP, the second messenger here, opens a cyclic nucleotide-gated ion channel, producing an influx of cations into the cell depolarising it; the Ca2+ in turn opens a Ca2+-activated chloride channel, leading to efflux of Cl−, further depolarizing the cell and triggering an action potential. Ca2+ is extruded through a sodium-calcium exchanger. A calcium-calmodulin complex acts to inhibit the binding of cAMP to the cAMP-dependent channel, thus contributing to olfactory adaptation; the main olfactory system of some mammals contains small subpopulations of olfactory sensory neurons that detect and transduce odors somewhat differently. Olfactory sensory neurons that use trace amine-associated receptors to detect odors use the same second messenger signaling cascade as do the canonical olfactory sensory neurons. Other subpopulations, such as those that express the receptor guanylyl cyclase GC-D or the soluble guanylyl cyclase Gucy1b2, use a cGMP cascade to transduce their odorant ligands.
These distinct subpopulations appear specialized for the detection of small groups of chemical stimuli. This mechanism of transduction is somewhat unusual, in that cAMP works by directly binding to the ion channel rather than through activation of protein kinase A, it is similar to the transduction mechanism for photoreceptors, in which the second messenger cGMP works by directly binding to ion channels, suggesting that maybe one of these receptors was evolutionarily adapted into the other. There are considerable similarities in the immediate processing of stimuli by lateral inhibition. Averaged activity of the receptor neurons can be measured in several ways. In vertebrates, responses to an odor can be measured by an electro-olfactogram or through calcium imaging of receptor neuron terminals in the olfactory bulb. In insects, one can perform calcium imaging within the olfactory bulb. Olfactory sensory neurons project axons to the brain within the olfactory nerve; these nerve fibers, lacking myelin sheaths, pass to the olfactory bulb of the brain through perforations in the cribriform plate, which in turn projects olfactory information to the olfactory cortex and other areas.
The axons from the olfactory receptors converge in the outer layer of the olfactory bulb within small structures called glomeruli. Mitral cells, located in the inner layer of the olfactory bulb, form synapses with the axons of the sensory neurons within glomeruli and send the information about the odor to other parts of the olfactory system, where multiple signals may be processed to form a synthesized olfactory perception. A large degree of convergence occurs, with 25,000 axons synapsing on 25 or so mitral cells, with each of these mitral cells projecting to multiple glomeruli. Mitral cells project to periglomerular cells and granular cells that inhibit the mitral cells surrounding it. Granular cells mediate inhibition and excitation of mitral cells through pathways from centrifugal fibers and the anterior olfactory nuclei. Neuromodulators like acetylcholine and norepinephrine all send axons to the
In humans, the respiratory tract is the part of the anatomy of the respiratory system involved with the process of respiration. Air is breathed in through the mouth. In the nasal cavity, a layer of mucous membrane acts as a filter and traps pollutants and other harmful substances found in the air. Next, air moves into the pharynx, a passage that contains the intersection between the esophagus and the larynx; the opening of the larynx has a special flap of cartilage, the epiglottis, that opens to allow air to pass through but closes to prevent food from moving into the airway. From the larynx, air moves into the trachea and down to the intersection that branches to form the right and left primary bronchi; each of these bronchi branch into secondary bronchi that branch into tertiary bronchi that branch into smaller airways called bronchioles that connect with tiny specialized structures called alveoli that function in gas exchange. The lungs which are located in the thoracic cavity, are protected from physical damage by the rib cage.
At the base of the lungs is a sheet of skeletal muscle called the diaphragm. The diaphragm separates the lungs from intestines; the diaphragm is the main muscle of respiration involved in breathing, is controlled by the sympathetic nervous system. The lungs are encased in a serous membrane that folds in on itself to form the pleurae – a two-layered protective barrier; the inner visceral pleura covers the surface of the lungs, the outer parietal pleura is attached to the inner surface of the thoracic cavity. The pleurae enclose; this fluid is used to decrease the amount of friction. The respiratory tract is divided into lower airways; the upper airways or upper respiratory tract includes the nose and nasal passages, paranasal sinuses, the pharynx, the portion of the larynx above the vocal folds. The lower airways or lower respiratory tract includes the portion of the larynx below the vocal folds, trachea and bronchioles; the lungs can be included in the lower respiratory tract or as separate entity and include the respiratory bronchioles, alveolar ducts, alveolar sacs, alveoli.
The respiratory tract can be divided into a conducting zone and a respiratory zone, based on the distinction of transporting gases or exchanging them. The conducting zone includes structures outside of the lungs – the nose, pharynx and trachea, structures inside the lungs – the bronchi and terminal bronchioles; the conduction zone conducts air breathed in, filtered and moistened, into the lungs. It represents the 1st through the 16th division of the respiratory tract; the conducting zone is most of the respiratory tract that conducts gases into and out of the lungs, but excludes the respiratory zone that exchanges gases. The conducting zone functions to offer a low resistance pathway for airflow, it provides a major defense role in its filtering abilities. The respiratory zone includes the respiratory bronchioles, alveolar ducts and alveoli, is the site of oxygen and carbon dioxide exchange with the blood; the respiratory bronchioles and the alveolar ducts are responsible for 10% of the gas exchange.
The alveoli are responsible for the other 90%. The respiratory zone represents the 16th through the 23rd division of the respiratory tract. From the bronchi, the dividing tubes become progressively smaller with an estimated 20 to 23 divisions before ending at an alveolus; the upper respiratory tract, can refer to the parts of the respiratory system lying above the sternal angle, above the vocal folds, or above the cricoid cartilage. The larynx is sometimes included in both lower airways; the larynx is called the voice box and has the associated cartilage that produces sound. The tract consists of the nasal cavity and paranasal sinuses, the pharynx and sometimes includes the larynx; the lower respiratory tract or lower airway is derived from the developing foregut and consists of the trachea, bronchi and lungs. It sometimes includes the larynx; the lower respiratory tract is called the respiratory tree or tracheobronchial tree, to describe the branching structure of airways supplying air to the lungs, includes the trachea and bronchioles.
Trachea main bronchus lobar bronchus segmental bronchus subsegmental bronchus conducting bronchiole terminal bronchiole respiratory bronchiole alveolar duct alveolar sac alveolusAt each division point or generation, one airway branches into two or more smaller airways. The human respiratory tree may consist on average of 23 generations, while the respiratory tree of the mouse has up to 13 generations. Proximal divisions function to transmit air to the lower airways. Divisions including the respiratory bronchiole, alveolar ducts and alveoli, are specialized for gas exchange; the trachea is the largest tube in the respiratory tract and consists of tracheal rings of hyaline cartilage. It branches off into a left and a right main bronchus; the bronchi branch off into smaller sections inside the lungs, called bronchioles. These bronchioles give rise to the air sacs in the lungs called the alveoli; the lungs are the largest organs in the lower respiratory tract. The lungs are suspended within the pleural cavity of the thorax.
The pleurae are two thin membranes, one