The lungs are the primary organs of the respiratory system in humans and many other animals including a few fish and some snails. In mammals and most other vertebrates, two lungs are located near the backbone on either side of the heart. Their function in the respiratory system is to extract oxygen from the atmosphere and transfer it into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere, in a process of gas exchange. Respiration is driven by different muscular systems in different species. Mammals, reptiles and birds use their different muscles to support and foster breathing. In early tetrapods, air was driven into the lungs by the pharyngeal muscles via buccal pumping, a mechanism still seen in amphibians. In humans, the main muscle of respiration that drives breathing is the diaphragm. The lungs also provide airflow that makes vocal sounds including human speech possible.
Humans have two lungs, a right lung and a left lung. They are situated within the thoracic cavity of the chest. The right lung is bigger than the left, which shares space in the chest with the heart. The lungs together weigh approximately 1.3 kilograms (2.9 lb), and the right is heavier. The lungs are part of the lower respiratory tract that begins at the trachea and branches into the bronchi and bronchioles, and which receive air breathed in via the conducting zone. The conducting zone ends at the terminal bronchioles. These divide into the respiratory bronchioles of the respiratory zone which divide into alveolar ducts that give rise to the microscopic alveoli, where gas exchange takes place. Together, the lungs contain approximately 2,400 kilometres (1,500 mi) of airways and 300 to 500 million alveoli. Each lung is enclosed within a pleural sac which allows the inner and outer walls to slide over each other whilst breathing takes place, without much friction. This sac also divides each lung into sections called lobes. The right lung has three lobes and the left has two. The lobes are further divided into bronchopulmonary segments and lobules. The lungs have a unique blood supply, receiving deoxygenated blood from the heart in the pulmonary circulation for the purposes of receiving oxygen and releasing carbon dioxide, and a separate supply of oxygenated blood to the tissue of the lungs, in the bronchial circulation.
The tissue of the lungs can be affected by a number of diseases, including pneumonia and lung cancer. Chronic obstructive pulmonary disease includes chronic bronchitis and previously termed emphysema, can be related to smoking or exposure to harmful substances such as coal dust, asbestos fibres and crystalline silica dust. Diseases such as bronchitis can also affect the respiratory tract. Medical terms related to the lung often begin with pulmo-, from the Latin pulmonarius (of the lungs) as in pulmonology, or with pneumo- (from Greek πνεύμων "lung") as in pneumonia.
In embryonic development, the lungs begin to develop as an outpouching of the foregut, a tube which goes on to form the upper part of the digestive system. When the lungs are formed the fetus is held in the fluid-filled amniotic sac and so they do not function to breathe. Blood is also diverted from the lungs through the ductus arteriosus. At birth however, air begins to pass through the lungs, and the diversionary duct closes, so that the lungs can begin to respire. The lungs only fully develop in early childhood.
Diagram of the human lungs with the respiratory tract visible, and different colours for each lobe
The human lungs flank the heart and great vessels in the chest cavity
The lungs are located in the chest on either side of the heart in the rib cage. They are conical in shape with a narrow rounded apex at the top, and a broad concave base that rests on the convex surface of the diaphragm. The apex of the lung extends into the root of the neck, reaching shortly above the level of the sternal end of the first rib. The lungs stretch from close to the backbone in the rib cage to the front of the chest and downwards from the lower part of the trachea to the diaphragm. The left lung shares space with the heart, and has an indentation in its border called the cardiac notch of the left lung to accommodate this. The front and outer sides of the lungs face the ribs, which make light indentations on their surfaces. The medial surfaces of the lungs face towards the centre of the chest, and lie against the heart, great vessels, and the carina where the trachea divides into the two main bronchi. The cardiac impression is an indentation formed on the surfaces of the lungs where they rest against the heart.
The lungs are surrounded by the pulmonary pleurae. The pleurae are two serous membranes; the outer parietal pleura lines the inner wall of the rib cage and the inner visceral pleura directly lines the surface of the lungs. Between the pleurae is a potential space called the pleural cavity containing a thin layer of lubricating pleural fluid. Each lung is divided into lobes by the infoldings of the pleura as fissures. The fissures are double folds of pleura that section the lungs and help in their expansion.
|Right lung||Left lung|
The main or primary bronchi enter the lungs at the hilum and initially branch into secondary bronchi also known as lobar bronchi that supply air to each lobe of the lung. The lobar bronchi branch into tertiary bronchi also known as segmental bronchi and these supply air to the further divisions of the lobes known as bronchopulmonary segments. Each bronchopulmonary segment has its own (segmental) bronchus and arterial supply. Segments for the left and right lung are shown in the table. The segmental anatomy is useful clinically for localising disease processes in the lungs. A segment is a discrete unit that can be surgically removed without seriously affecting surrounding tissue.
The right lung has both more lobes and segments than the left. It is divided into three lobes, an upper, middle, and a lower, by two fissures, one oblique and one horizontal. The upper, horizontal fissure, separates the upper from the middle lobe. It begins in the lower oblique fissure near the posterior border of the lung, and, running horizontally forward, cuts the anterior border on a level with the sternal end of the fourth costal cartilage; on the mediastinal surface it may be traced backward to the hilum.
The mediastinal surface of the right lung is indented by a number of nearby structures. The heart sits in an impression called the cardiac impression. Above the hilum of the lung is an arched groove for the azygos vein, and above this is a wide groove for the superior vena cava and right brachiocephalic vein; behind this, and close to the top of the lung is a groove for the brachiocephalic artery. There is a groove for the esophagus behind the hilum and the pulmonary ligament, and near the lower part of the esophageal groove is a deeper groove for the inferior vena cava before it enters the heart.
The left lung is divided into two lobes, an upper and a lower, by the oblique fissure, which extends from the costal to the mediastinal surface of the lung both above and below the hilum. The left lung, unlike the right, does not have a middle lobe, though it does have a homologous feature, a projection of the upper lobe termed the "lingula". Its name means "little tongue". The lingula on the left serves as an anatomic parallel to the right middle lobe, with both areas being predisposed to similar infections and anatomic complications. There are two bronchopulmonary segments of the lingula: superior and inferior.
The mediastinal surface of the left lung has a large cardiac impression where the heart sits. This is deeper and larger than that on the right lung, at which level the heart projects to the left.
On the same surface, immediately above the hilum, is a well-marked curved groove for the aortic arch, and a groove below it for the descending aorta. The left subclavian artery, a branch off the aortic arch, sits in a groove from the arch to near the apex of the lung. A shallower groove in front of the artery and near the edge of the lung, lodges the left brachiocephalic vein. The esophagus may sit in a wider shallow impression at the base of the lung.
The left lung (left) and right lung (right). The lobes of the lungs can be seen, and the central root of the lung is also present.
The lungs are part of the lower respiratory tract, and accommodate the bronchial airways when they branch from the trachea. The lungs include the bronchial airways that terminate in alveoli, the lung tissue in between, and veins, arteries, nerves and lymphatic vessels. The trachea and bronchi have plexuses of lymph capillaries in their mucosa and submucosa. The smaller bronchi have a single layer and they are absent in the alveoli.
All of the lower respiratory tract including the trachea, bronchi, and bronchioles is lined with respiratory epithelium. This is a ciliated epithelium interspersed with goblet cells which produce mucus, and club cells with actions similar to macrophages. Incomplete rings of cartilage in the trachea and smaller plates of cartilage in the bronchi, keep these airways open. Bronchioles are too narrow to support cartilage and their walls are of smooth muscle, and this is largely absent in the narrower respiratory bronchioles which are mainly just of epithelium. The respiratory tract ends in lobules. Each lobule consists of a respiratory bronchiole, which branches into alveolar ducts and alveolar sacs, which in turn divide into alveoli.
The epithelial cells throughout the respiratory tract secrete epithelial lining fluid (ELF), the composition of which is tightly regulated and determines how well mucociliary clearance works.:Section 4 pages 7–8 (Page 4–7ff)
Alveoli consist of two types of alveolar cell and an alveolar macrophage. The two types of cell are known as type I and type II alveolar cells (also known as pneumocytes). Types I and II make up the walls and alveolar septa. Type I cells provide 95% of the surface area of each alveoli and are flat ("squamous"), and Type II cells generally cluster in the corners of the alveoli and have a cuboidal shape. Despite this, cells occur in a roughly equal ratio of 1:1 or 6:4.
Type I are squamous epithelial cells that make up the alveolar wall structure. They have extremely thin walls that enable an easy gas exchange. These type I cells also make up the alveolar septa which separate each alveolus. The septa consist of an epithelial lining and associated basement membranes. Type I cells are not able to divide, and consequently rely on differentiation from Type II cells.
The lower respiratory tract is part of the respiratory system, and consists of the trachea and the structures below this including the lungs. The trachea receives air from the pharynx and travels down to a place where it splits (the carina) into a right and left bronchus. These supply air to the right and left lungs, splitting progressively into the secondary and tertiary bronchi for the lobes of the lungs, and into smaller and smaller bronchioles until they become the respiratory bronchioles. These in turn supply air through alveolar ducts into the alveoli, where the exchange of gases take place. Oxygen breathed in, diffuses through the walls of the alveoli into the enveloping capillaries and into the circulation, and carbon dioxide diffuses from the blood into the lungs to be breathed out.
The bronchi in the conducting zone are reinforced with hyaline cartilage in order to hold open the airways. The bronchioles have no cartilage and are surrounded instead by smooth muscle. Air is warmed to 37 °C (99 °F), humidified and cleansed by the conducting zone; particles from the air being removed by the cilia on the respiratory epithelium lining the passageways.
The lungs have a dual blood supply provided by a bronchial and a pulmonary circulation. The bronchial circulation supplies oxygenated blood to the airways of the lungs, through the bronchial arteries that leave the aorta. There are usually three arteries, two to the left lung and one to the right, and they branch alongside the bronchi and bronchioles. The pulmonary circulation carries deoxygenated blood from the heart to the lungs and returns the oxygenated blood to the heart to supply the rest of the body.
The blood volume of the lungs, is about 450 millilitres on average, about 9% of the total blood volume of the entire circulatory system. This quantity can easily fluctuate from between one-half and twice the normal volume.
The lungs are supplied by nerves of the autonomic nervous system. Input from the parasympathetic nervous system occurs via the vagus nerve. When stimulated by acetylcholine, this causes constriction of the smooth muscle lining the bronchus and bronchioles, and increases the secretions from glands. The lungs also have a sympathetic tone from norepinephrine acting on the beta 2 receptors in the respiratory tract, which causes bronchodilation.
Horizontal fissure was found to be incomplete in 25% of right lungs, or even absent in 11% of all cases. An accessory fissure was also found in 14% and 22% of left and right lungs, respectively. Oblique fissure was found to be incomplete in 21% to 47% of left lungs.
The larynx, trachea, bronchi and lungs that make up the respiratory tract, begin to form during the fourth week of embryogenesis from the lung bud which appears ventrally to the caudal portion of the foregut.
The respiratory tract has a branching structure like that of a tree. In the embryo this structure is developed in the process of branching morphogenesis, and is generated by the repeated splitting of the tip of the branch. In the development of the lungs (as in some other organs) the epithelium forms branching tubes.The lung has a left-right symmetry and each bud known as a bronchial bud grows out as a tubular epithelium that becomes a bronchus. Each bronchus branches into bronchioles. The branching is a result of the tip of each tube bifurcating. The branching process forms the bronchi, bronchioles, and ultimately the alveoli. The four genes mostly associated with branching morphogenesis in the lung are the intercellular signalling protein – sonic hedgehog (SHH), fibroblast growth factors FGF10 and FGFR2b, and bone morphogenetic protein BMP4. FGF10 is seen to have the most prominent role. FGF10 is a paracrine signalling molecule needed for epithelial branching, and SHH inhibits FGF10. The development of the alveoli is influenced by a different mechanism whereby continued bifurcation is stopped and the distal tips become dilated to form the alveoli.
At the end of the fourth week the lung bud divides into two, the right and left primary bronchial buds on each side of the trachea. During the fifth week the right bud branches into three secondary bronchial buds and the left branches into two secondary bronchial buds. These give rise to the lobes of the lungs, three on the right and two on the left. Over the following week, the secondary buds branch into tertiary buds, about ten on each side. From the sixth week to the sixteenth week, the major elements of the lungs appear except the alveoli. From week 16 to week 26, the bronchi enlarge and lung tissue becomes highly vascularised. Bronchioles and alveolar ducts also develop. By week 26 the terminal bronchioles have formed which branch into two respiratory bronchioles. During the period covering the 26th week until birth the important blood–air barrier is established. Specialised type I alveolar cells where gas exchange will take place, together with the type II alveolar cells that secrete pulmonary surfactant, appear. The surfactant reduces the surface tension at the air-alveolar surface which allows expansion of the alveolar sacs. The alveolar sacs contain the primitive alveoli that form at the end of the alveolar ducts, and their appearance around the seventh month marks the point at which limited respiration would be possible, and the premature baby could survive.
At birth, the baby's lungs are filled with fluid secreted by the lungs and are not inflated. After birth the infant's central nervous system reacts to the sudden change in temperature and environment. This triggers the first breath, within about 10 seconds after delivery. Before birth, the lungs are filled with fetal lung fluid. After the first breath, the fluid is quickly absorbed into the body or exhaled. The resistance in the lung's blood vessels decreases giving an increased surface area for gas exchange, and the lungs begin to breathe spontaneously. This accompanies other changes which result in an increased amount of blood entering the lung tissues.
At birth the lungs are very undeveloped with only around one sixth of the alveoli of the adult lung present. The alveoli continue to form into early adulthood, and their ability to form when necessary is seen in the regeneration of the lung. Alveolar septa have a double capillary network instead of the single network of the developed lung. Only after the maturation of the capillary network can the lung enter a normal phase of growth. Following the early growth in numbers of alveoli there is another stage of the alveoli being enlarged.
The major function of the lungs is gas exchange between the lungs and the blood. The alveolar and pulmonary capillary gases equilibrate across the thin blood–air barrier. This thin membrane (about 0.5 –2 μm thick) is folded into about 300 million alveoli, providing an extremely large surface area (estimates varying between 70 and 145 m2) for gas exchange to occur.
The lungs are not capable of expanding to breathe on their own, and will only do so when there is an increase in the volume of the thoracic cavity. This is achieved by the muscles of respiration, through the contraction of the diaphragm, and the intercostal muscles which pull the rib cage upwards as shown in the diagram. During breathing out the muscles relax, returning the lungs to their resting position. At this point the lungs contain the functional residual capacity (FRC) of air, which, in the adult human, has a volume of about 2.5–3.0 litres.
During heavy breathing as in exertion, a large number of accessory muscles in the neck and abdomen are recruited, that during exhalation pull the ribcage down, decreasing the volume of the thoracic cavity. The FRC is now decreased, but since the lungs cannot be emptied completely there is still about a litre of residual air left. Lung function testing is carried out to evaluate lung volumes and capacities.
The lungs possess several characteristics which protect against infection. The respiratory tract is lined by epithelia with hair-like projections called cilia that beat rhythmically and carry mucus. This mucociliary clearance is an important defence system against air-borne infection. The dust particles and bacteria in the inhaled air are caught in the mucosal surface of the airways, and are moved up towards the pharynx by the rhythmic upward beating action of the cilia. The lining of the lung also secretes immunoglobulin A which protects against respiratory infections; goblet cells secrete mucus which also contains several antimicrobial compounds such as defensins, antiproteases, and antioxidates. A rare type of specialised cell called a pulmonary ionocyte that is suggested may regulate mucus viscosity has been described. In addition, the lining of the lung also contains macrophages, immune cells which engulf and destroy debris and microbes that enter the lung in a process known as phagocytosis; and dendritic cells which present antigens to activate components of the adaptive immune system such as T-cells and B-cells.
The size of the respiratory tract and the flow of air also protect the lungs from larger particles. Smaller particles deposit in the mouth and behind the mouth in the oropharynx, and larger particles are trapped in nasal hair after inhalation.
In addition to their function in respiration, the lungs have a number of other functions. They are involved in maintaining homeostasis, helping in the regulation of blood pressure as part of the renin–angiotensin system. The inner lining of the blood vessels secretes angiotensin-converting enzyme (ACE) an enzyme that catalyses the conversion of angiotensin I to angiotensin II. The lungs are involved in the blood's acid-base homeostasis by expelling carbon dioxide when breathing.
The lungs also serve a protective role. Several blood-borne substances, such as a few types of prostaglandins, leukotrienes, serotonin and bradykinin, are excreted through the lungs. Drugs and other substances can be absorbed, modified or excreted in the lungs. The lungs filter out small blood clots from veins and prevent them from entering arteries and causing strokes.
New research suggests a role of the lungs in the production of blood platelets.
About 20,000 protein coding genes are expressed in human cells and almost 75% of these genes are expressed in the normal lung. A little less than 200 of these genes are more specifically expressed in the lung with less than 20 genes being highly lung specific. The corresponding specific proteins are expressed within different cellular compartments such as pneumocytes in alveoli, and ciliated and mucus secreting goblet cells in the respiratory mucosa. The highest expression of lung specific proteins are different surfactant proteins, such as SFTPA1, SFTPB and SFTPC, and napsin, expressed in type II pneumocytes. Other proteins with elevated expression in the lung are the dynein protein DNAH5 in ciliated cells, and the secreted SCGB1A1 protein in mucus secreting goblet cells of the airway mucosa.
Lungs can be affected by a variety of diseases. Pulmonology is the medical speciality that deals with diseases involving the respiratory tract, and cardiothoracic surgery is the surgical field that deals with surgery of the lungs.
Inflammatory conditions of the lung tissue are pneumonia, of the respiratory tract are bronchitis and bronchiolitis, and of the pleurae surrounding the lungs pleurisy. Inflammation is usually caused by infections due to bacteria or viruses. When the lung tissue is inflamed due to other causes it is called pneumonitis. One major cause of bacterial pneumonia is tuberculosis. Chronic infections often occur in those with immunodeficiency and can include a fungal infection by Aspergillus fumigatus that can lead to an aspergilloma forming in the lung.
A pulmonary embolism is a blood clot that becomes lodged in the pulmonary arteries. The majority of emboli arise because of deep vein thrombosis in the legs. Pulmonary emboli may be investigated using a ventilation/perfusion scan, a CT scan of the arteries of the lung, or blood tests such as the D-dimer. Pulmonary hypertension describes an increased pressure at the beginning of the pulmonary artery that has a large number of differing causes. Other rarer conditions may also affect the blood supply of the lung, such as granulomatosis with polyangiitis, which causes inflammation of the small blood vessels of the lungs and kidneys.
A lung contusion is a bruise caused by chest trauma. It results in hemorrhage of the alveoli causing a build-up of fluid which can impair breathing, and this can be either mild or severe. The function of the lungs can also be affected by compression from fluid in the pleural cavity pleural effusion, or other substances such as air (pneumothorax), blood (hemothorax), or rarer causes. These may be investigated using a chest X-ray or CT scan, and may require the insertion of a surgical drain until the underlying cause is identified and treated.
Asthma, chronic bronchitis, bronchiectasis and chronic obstructive pulmonary disease (COPD) are all obstructive lung diseases characterised by airway obstruction. This limits the amount of air that is able to enter alveoli because of constriction of the bronchial tree, due to inflammation. Obstructive lung diseases are often identified because of symptoms and diagnosed with pulmonary function tests such as spirometry. Many obstructive lung diseases are managed by avoiding triggers (such as dust mites or smoking), with symptom control such as bronchodilators, and with suppression of inflammation (such as through corticosteroids) in severe cases. One common cause of COPD and emphysema is smoking, and common causes of bronchiectasis include severe infections and cystic fibrosis. The definitive cause of asthma is not yet known.
Some types of chronic lung diseases are classified as restrictive lung disease, because of a restriction in the amount of lung tissue involved in respiration. These include pulmonary fibrosis which can occur when the lung is inflamed for a long period of time. Fibrosis in the lung replaces functioning lung tissue with fibrous connective tissue. This can be due to a large variety of occupational diseases such as Coalworker's pneumoconiosis, autoimmune diseases or more rarely to a reaction to medication.
Lung cancer can either arise directly from lung tissue or as a result of metastasis from another part of the body. There are two main types of primary tumour described as either small-cell or non-small-cell lung carcinomas. The major risk factor for cancer is smoking. Once a cancer is identified it is staged using scans such as a CT scan and a sample of tissue (a biopsy) is taken. Cancers may be treated by surgically removing the tumour, radiotherapy, chemotherapy or combinations thereof, or with the aim of symptom control.Lung cancer screening is being recommended in the United States for high-risk populations.
Congenital disorders include cystic fibrosis, pulmonary hypoplasia (an incomplete development of the lungs)congenital diaphragmatic hernia, and infant respiratory distress syndrome caused by a deficiency in lung surfactant. An azygos lobe is a congenital anatomical variation which though usually without effect can cause problems in thoracoscopic procedures.
A pneumothorax (collapsed lung) is an abnormal collection of air in the pleural space that causes an uncoupling of the lung from the chest wall. The lung cannot expand against the air pressure inside the pleural space. An easy to understand example is a traumatic pneumothorax, where air enters the pleural space from outside the body, as occurs with puncture to the chest wall. Similarly, a scuba diver ascending while holding their breath with their lungs fully inflated can cause air sacs (alveoli) to burst and leak high pressure air into the pleural space.
Lung function testing is carried out by evaluating a person's capacity to inhale and exhale in different circumstances. The volume of air inhaled and exhaled by a person at rest is the tidal volume (normally 500-750mL); the inspiratory reserve volume and expiratory reserve volume are the additional amounts a person is able to forcibly inhale and exhale respectively. The summed total of forced inspiration and expiration is a person's vital capacity. Not all air is expelled from the lungs even after a forced breath out; the remainder of the air is called the residual volume. Together these terms are referred to as lung volumes.
Pulmonary plethysmographs are used to measure functional residual capacity. Functional residual capacity cannot be measured by tests that rely on breathing out, as a person is only able to breathe a maximum of 80% of their total functional capacity. The total lung capacity depends on the person's age, height, weight, and sex, and normally ranges between 4 and 6 litres. Females tend to have a 20–25% lower capacity than males. Tall people tend to have a larger total lung capacity than shorter people. Smokers have a lower capacity than nonsmokers. Thinner persons tend to have a larger capacity. Lung capacity can be increased by physical training as much as 40% but the effect may be modified by exposure to air pollution.
Other lung function tests include spirometry, measuring the amount (volume) and flow of air that can be inhaled and exhaled. The maximum volume of breath that can be exhaled is called the vital capacity. In particular, how much a person is able to exhale in one second (called forced expiratory volume (FEV1)) as a proportion of how much they are able to exhale in total (FEV). This ratio, the FEV1/FEV ratio, is important to distinguish whether a lung disease is restrictive or obstructive. Another test is that of the lung's diffusing capacity – this is a measure of the transfer of gas from air to the blood in the lung capillaries.
The lungs of birds are relatively small, but are connected to 8 or 9 air sacs that extend through much of the body, and are in turn connected to air spaces within the bones. On inhalation, air travels through the trachea of a bird into the air sacs. Air then travels continuously from the air sacs at the back, through the lungs, which are relatively fixed in size, to the air sacs at the front. From here, the air is exhaled. These fixed size lungs are called "circulatory lungs", as distinct from the "bellows-type lungs" found in most other animals.
The lungs of birds contain millions of tiny parallel passages called parabronchi. Small sacs called atria radiate from the walls of the tiny passages; these, like the alveoli in other lungs, are the site of gas exchange by simple diffusion. The blood flow around the parabronchi and their atria forms a cross-current process of gas exchange (see diagram on the right).
The air sacs, which hold air, do not contribute much to gas exchange, despite being thin-walled, as they are poorly vascularised. The air sacs expand and contract due to changes in the volume in the thorax and abdomen. This volume change is caused by the movement of the sternum and ribs and this movement is often synchronised with movement of the flight muscles.
Parabronchi in which the air flow is unidirectional are called paleopulmonic parabronchi and are found in all birds. Some birds, however, have, in addition, a lung structure where the air flow in the parabronchi is bidirectional. These are termed neopulmonic parabronchi.
The lungs of most reptiles have a single bronchus running down the centre, from which numerous branches reach out to individual pockets throughout the lungs. These pockets are similar to alveoli in mammals, but much larger and fewer in number. These give the lung a sponge-like texture. In tuataras, snakes, and some lizards, the lungs are simpler in structure, similar to that of typical amphibians.
Snakes and limbless lizards typically possess only the right lung as a major respiratory organ; the left lung is greatly reduced, or even absent. Amphisbaenians, however, have the opposite arrangement, with a major left lung, and a reduced or absent right lung.
Both crocodilians and monitor lizards have developed lungs similar to those of birds, providing an unidirectional airflow and even possessing air sacs. The now extinct pterosaurs have seemingly even further refined this type of lung, extending the airsacs into the wing membranes and, in the case of lonchodectids, tupuxuara, and azhdarchoids, the hindlimbs.
Reptilian lungs typically receive air via expansion and contraction of the ribs driven by axial muscles and buccal pumping. Crocodilians also rely on the hepatic piston method, in which the liver is pulled back by a muscle anchored to the pubic bone (part of the pelvis) called the diaphragmaticus, which in turn creates negative pressure in the crocodile's thoracic cavity, allowing air to be moved into the lungs by Boyle's law. Turtles, which are unable to move their ribs, instead use their forelimbs and pectoral girdle to force air in and out of the lungs.
The lungs of most frogs and other amphibians are simple and balloon-like, with gas exchange limited to the outer surface of the lung. This is not very efficient, but amphibians have low metabolic demands and can also quickly dispose of carbon dioxide by diffusion across their skin in water, and supplement their oxygen supply by the same method. Amphibians employ a positive pressure system to get air to their lungs, forcing air down into the lungs by buccal pumping. This is distinct from most higher vertebrates, who use a breathing system driven by negative pressure where the lungs are inflated by expanding the rib cage. In buccal pumping, the floor of the mouth is lowered, filling the mouth cavity with air. The throat muscles then presses the throat against the underside of the skull, forcing the air into the lungs.
Due to the possibility of respiration across the skin combined with small size, all known lungless tetrapods are amphibians. The majority of salamander species are lungless salamanders, which respirate through their skin and tissues lining their mouth. This necessarily restrict their size: all are small and rather thread-like in appearance, maximising skin surface relative to body volume. Other known lungless tetrapods are the Bornean flat-headed frog and Atretochoana eiselti, a caecilian.
The lungs of amphibians typically have a few narrow internal walls (septa) of soft tissue around the outer walls, increasing the respiratory surface area and giving the lung a honey-comb appearance. In some salamanders even these are lacking, and the lung has a smooth wall. In caecilians, as in snakes, only the right lung attains any size or development.
The lungs of lungfish are similar to those of amphibians, with few, if any, internal septa. In the Australian lungfish, there is only a single lung, albeit divided into two lobes. Other lungfish and Polypterus, however, have two lungs, which are located in the upper part of the body, with the connecting duct curving around and above the esophagus. The blood supply also twists around the esophagus, suggesting that the lungs originally evolved in the ventral part of the body, as in other vertebrates.
Some invertebrates have lung-like structures that serve a similar respiratory purpose as, but are not evolutionarily related to, vertebrate lungs. Some arachnids, such as spiders and scorpions, have structures called book lungs used for atmospheric gas exchange. Some species of spider have four pairs of book lungs but most have two pairs. Scorpions have spiracles on their body for the entrance of air to the book lungs.
The coconut crab is terrestrial and uses structures called branchiostegal lungs to breathe air. They cannot swim and would drown in water, yet they possess a rudimentary set of gills. They can breathe on land and hold their breath underwater. The branchiostegal lungs are seen as a developmental adaptive stage from water-living to enable land-living, or from fish to amphibian.
Pulmonates are mostly land snails and slugs that have developed a simple lung from the mantle cavity. An externally located opening called the pneumostome allows air to be taken into the mantle cavity lung.
The lungs of today's terrestrial vertebrates and the gas bladders of today's fish are believed to have evolved from simple sacs, as outpocketings of the esophagus, that allowed early fish to gulp air under oxygen-poor conditions. These outpocketings first arose in the bony fish. In most of the ray-finned fish the sacs evolved into closed off gas bladders, while a number of carp, trout, herring, catfish, and eels have retained the physostome condition with the sack being open to the esophagus. In more basal bony fish, such as the gar, bichir, bowfin and the lobe-finned fish, the bladders have evolved to primarily function as lungs. The lobe-finned fish gave rise to the land-based tetrapods. Thus, the lungs of vertebrates are homologous to the gas bladders of fish (but not to their gills).
Bronchitis is inflammation of the bronchi (large and medium-sized airways) in the lungs. Symptoms include coughing up mucus, wheezing, shortness of breath, and chest discomfort. Bronchitis is divided into two types: acute and chronic. Acute bronchitis is also known as a chest cold.Acute bronchitis usually has a cough that lasts around three weeks. In more than 90% of cases the cause is a viral infection. These viruses may be spread through the air when people cough or by direct contact. Risk factors include exposure to tobacco smoke, dust, and other air pollution. A small number of cases are due to high levels of air pollution or bacteria such as Mycoplasma pneumoniae or Bordetella pertussis. Treatment of acute bronchitis typically involves rest, paracetamol (acetaminophen), and NSAIDs to help with the fever.Chronic bronchitis is defined as a productive cough that lasts for three months or more per year for at least two years. Most people with chronic bronchitis have chronic obstructive pulmonary disease (COPD). Tobacco smoking is the most common cause, with a number of other factors such as air pollution and genetics playing a smaller role. Treatments include quitting smoking, vaccinations, rehabilitation, and often inhaled bronchodilators and steroids. Some people may benefit from long-term oxygen therapy or lung transplantation.Acute bronchitis is one of the most common diseases. About 5% of adults are affected and about 6% of children have at least one episode a year. In 2010, COPD affected 329 million people or nearly 5% of the global population. In 2013, it resulted in 2.9 million deaths, a change from 2.4 million deaths in 1990.Cardiothoracic surgery
Cardiothoracic surgery (also known as thoracic surgery) is the field of medicine involved in surgical treatment of organs inside the thorax (the chest)—generally treatment of conditions of the heart (heart disease) and lungs (lung disease). In most countries, cardiac surgery (involving the heart and the great vessels) and general thoracic surgery (involving the lungs, esophagus, thymus, etc.) are separate surgical specialties; the exceptions are the United States, Australia, New Zealand, and some EU countries, such as the United Kingdom and Portugal.Chronic obstructive pulmonary disease
Chronic obstructive pulmonary disease (COPD) is a type of obstructive lung disease characterized by long-term breathing problems and poor airflow. The main symptoms include shortness of breath and cough with sputum production. COPD is a progressive disease, meaning it typically worsens over time. Eventually everyday activities, such as walking or getting dressed, become difficult. Chronic bronchitis and emphysema are older terms used for different types of COPD. The term "chronic bronchitis" is still used to define a productive cough that is present for at least three months each year for two years.Tobacco smoking is the most common cause of COPD, with factors such as air pollution and genetics playing a smaller role. In the developing world, one of the common sources of air pollution is poorly vented heating and cooking fires. Long-term exposure to these irritants causes an inflammatory response in the lungs, resulting in narrowing of the small airways and breakdown of lung tissue. The diagnosis is based on poor airflow as measured by lung function tests. In contrast to asthma, the airflow reduction does not improve much with the use of a bronchodilator.Most cases of COPD can be prevented by reducing exposure to risk factors. This includes decreasing rates of smoking and improving indoor and outdoor air quality. While treatment can slow worsening, no cure is known. COPD treatments include smoking cessation, vaccinations, respiratory rehabilitation, and often inhaled bronchodilators and steroids. Some people may benefit from long-term oxygen therapy or lung transplantation. In those who have periods of acute worsening, increased use of medications and hospitalization may be needed.As of 2015, COPD affected about 174.5 million (2.4%) of the global population. It typically occurs in people over the age of 40. Males and females are affected equally commonly. In 2015, it resulted in 3.2 million deaths, up from 2.4 million deaths in 1990. More than 90% of these deaths occur in the developing world. The number of deaths is projected to increase further because of higher smoking rates in the developing world, and an aging population in many countries. It resulted in an estimated economic cost of $2.1 trillion in 2010.Crouching Tiger, Hidden Dragon
Crouching Tiger, Hidden Dragon (simplified Chinese: 卧虎藏龙; traditional Chinese: 臥虎藏龍) is a 2000 wuxia film directed by Ang Lee and written by Wang Hui-ling, James Schamus and Tsai Kuo Jung, based on the Chinese novel by Wang Dulu. The film features an international cast of Chinese actors, including Chow Yun-fat, Michelle Yeoh, Zhang Ziyi and Chang Chen.
A multinational venture, the film was made on a US$17 million budget, and was produced by Asian Union Film & Entertainment, China Film Co-Productions Corporation, Columbia Pictures Film Production Asia, Edko Films, Good Machine International, and Zoom Hunt Productions. With dialogue in Mandarin, subtitled for various markets, Crouching Tiger, Hidden Dragon became a surprise international success, grossing $213.5 million worldwide. It grossed US$128 million in the United States, becoming the highest-grossing foreign-language film produced overseas in American history.Crouching Tiger, Hidden Dragon has won over 40 awards, and was nominated for 10 Academy Awards, including Best Picture, and won Best Foreign Language Film (Taiwan), Best Art Direction, Best Original Score and Best Cinematography, receiving the most nominations ever for a non-English language film at the time (Roma has since tied this record). The film also won four BAFTAs and two Golden Globe Awards, one for Best Foreign Film. Along with its awards success, Crouching Tiger continues to be hailed as one of the greatest and most influential martial arts films. The film has been praised for its story, direction, and cinematography, and for its martial arts sequences.Cystic fibrosis
Cystic fibrosis (CF) is a genetic disorder that affects mostly the lungs, but also the pancreas, liver, kidneys, and intestine. Long-term issues include difficulty breathing and coughing up mucus as a result of frequent lung infections. Other signs and symptoms may include sinus infections, poor growth, fatty stool, clubbing of the fingers and toes, and infertility in most males. Different people may have different degrees of symptoms.CF is inherited in an autosomal recessive manner. It is caused by the presence of mutations in both copies of the gene for the cystic fibrosis transmembrane conductance regulator (CFTR) protein. Those with a single working copy are carriers and otherwise mostly normal. CFTR is involved in production of sweat, digestive fluids, and mucus. When the CFTR is not functional, secretions which are usually thin instead become thick. The condition is diagnosed by a sweat test and genetic testing. Screening of infants at birth takes place in some areas of the world.There is no known cure for cystic fibrosis. Lung infections are treated with antibiotics which may be given intravenously, inhaled, or by mouth. Sometimes, the antibiotic azithromycin is used long term. Inhaled hypertonic saline and salbutamol may also be useful. Lung transplantation may be an option if lung function continues to worsen. Pancreatic enzyme replacement and fat-soluble vitamin supplementation are important, especially in the young. Airway clearance techniques such as chest physiotherapy have some short-term benefit, but long-term effects are unclear. The average life expectancy is between 42 and 50 years in the developed world. Lung problems are responsible for death in 80% of people with cystic fibrosis.CF is most common among people of Northern European ancestry and affects about one out of every 3,000 newborns. About one in 25 people is a carrier. It is least common in Africans and Asians. It was first recognized as a specific disease by Dorothy Andersen in 1938, with descriptions that fit the condition occurring at least as far back as 1595. The name "cystic fibrosis" refers to the characteristic fibrosis and cysts that form within the pancreas.Iron lung
A negative pressure ventilator, also known as iron lung (colloquialism) or pulmotor (generic trademark), is a nearly-obsolete mechanical respirator which enables a person to breathe on his or her own in a normal manner, when muscle control is lost, or the work of breathing exceeds the person's ability, as may result from certain diseases (e.g. poliomyelitis, botulism) and certain poisons (e.g. barbiturates, tubocurarine).
Examples of the device include both the Drinker respirator, the Emerson respirator, and the Both (or Emerson-Drinker) respirator. The negative form of pressure ventilation (decreasing surrounding pressure to induce inhalation then repressurizing to 1 bar (15 psi; 750 mmHg)) has been almost entirely superseded by positive pressure ventilation (forcing air into the lungs with a pressure greater than 1 bar then allowing the body to naturally exhale before repeating) or negative pressure cuirass ventilation.Jackie Chan
Datuk Chan Kong-sang (Chinese: 陳港生; born 7 April 1954), known professionally as Jackie Chan, is a Hong Kong martial artist, actor, film director, producer, stuntman, and singer. He is known for his acrobatic fighting style, comic timing, use of improvised weapons, and innovative stunts, which he typically performs himself, in the cinematic world. He has trained in wushu or kungfu and hapkido, and has been acting since the 1960s, appearing in over 150 films.
Chan is one of the most recognizable and influential cinematic personalities in the world, gaining a widespread following in both the Eastern and Western hemispheres, and has received stars on the Hong Kong Avenue of Stars, and the Hollywood Walk of Fame. He has been referenced in various pop songs, cartoons, and video games. He is an operatically trained vocalist and is also a Cantopop and Mandopop star, having released a number of albums and sung many of the theme songs for the films in which he has starred. He is also a globally known philanthropist, and has been named as one of the top 10 most charitable celebrities by Forbes magazine. In 2004, film scholar Andrew Willis stated that Chan was "perhaps" the "most recognised star in the world". In 2015, Forbes estimated his net worth to be $350 million, and as of 2016, he was the second-highest paid actor in the world.Lung cancer
Lung cancer, also known as lung carcinoma, is a malignant lung tumor characterized by uncontrolled cell growth in tissues of the lung. This growth can spread beyond the lung by the process of metastasis into nearby tissue or other parts of the body. Most cancers that start in the lung, known as primary lung cancers, are carcinomas. The two main types are small-cell lung carcinoma (SCLC) and non-small-cell lung carcinoma (NSCLC). The most common symptoms are coughing (including coughing up blood), weight loss, shortness of breath, and chest pains.The vast majority (85%) of cases of lung cancer are due to long-term tobacco smoking. About 10–15% of cases occur in people who have never smoked. These cases are often caused by a combination of genetic factors and exposure to radon gas, asbestos, second-hand smoke, or other forms of air pollution. Lung cancer may be seen on chest radiographs and computed tomography (CT) scans. The diagnosis is confirmed by biopsy which is usually performed by bronchoscopy or CT-guidance.Avoidance of risk factors, including smoking and air pollution, is the primary method of prevention. Treatment and long-term outcomes depend on the type of cancer, the stage (degree of spread), and the person's overall health. Most cases are not curable. Common treatments include surgery, chemotherapy, and radiotherapy. NSCLC is sometimes treated with surgery, whereas SCLC usually responds better to chemotherapy and radiotherapy.Worldwide in 2012, lung cancer occurred in 1.8 million people and resulted in 1.6 million deaths. This makes it the most common cause of cancer-related death in men and second most common in women after breast cancer. The most common age at diagnosis is 70 years. Overall, 17.4% of people in the United States diagnosed with lung cancer survive five years after the diagnosis, while outcomes on average are worse in the developing world.Non-small-cell lung carcinoma
Non-small-cell lung carcinoma (NSCLC) is any type of epithelial lung cancer other than small cell lung carcinoma (SCLC). NSCLC accounts for about 85% of all lung cancers. As a class, NSCLCs are relatively insensitive to chemotherapy, compared to small cell carcinoma. When possible, they are primarily treated by surgical resection with curative intent, although chemotherapy has been used increasingly both pre-operatively (neoadjuvant chemotherapy) and post-operatively (adjuvant chemotherapy).Pleurisy
Pleurisy, also known as pleuritis, is inflammation of the membranes that surround the lungs and line the chest cavity (pleurae). This can result in a sharp chest pain with breathing. Occasionally the pain may be a constant dull ache. Other symptoms may include shortness of breath, cough, fever or weight loss, depending on the underlying cause.The most common cause is a viral infection. Other causes include pneumonia, pulmonary embolism, autoimmune disorders, lung cancer, following heart surgery, pancreatitis, chest trauma, and asbestosis. Occasionally the cause remains unknown. The underlying mechanism involves the rubbing together of the pleurae instead of smooth gliding. Other conditions that can produce similar symptoms include pericarditis, heart attack, cholecystitis, and pneumothorax. Diagnosis may include a chest X-ray, electrocardiogram (ECG), and blood tests.Treatment depends on the underlying cause. Paracetamol (acetaminophen) and ibuprofen may be used to decrease pain. Incentive spirometry may be recommended to encourage larger breaths. About one million people are affected in the United States each year. Descriptions of the condition date from at least as early as 400 BC by Hippocrates.Pneumonia
Pneumonia is an inflammatory condition of the lung affecting primarily the small air sacs known as alveoli. Typically symptoms include some combination of productive or dry cough, chest pain, fever, and trouble breathing. Severity is variable.Pneumonia is usually caused by infection with viruses or bacteria and less commonly by other microorganisms, certain medications and conditions such as autoimmune diseases. Risk factors include other lung diseases such as cystic fibrosis, COPD, and asthma, diabetes, heart failure, a history of smoking, a poor ability to cough such as following a stroke, or a weak immune system. Diagnosis is often based on the symptoms and physical examination. Chest X-ray, blood tests, and culture of the sputum may help confirm the diagnosis. The disease may be classified by where it was acquired with community, hospital, or health care associated pneumonia.Vaccines to prevent certain types of pneumonia are available. Other methods of prevention include handwashing and not smoking. Treatment depends on the underlying cause. Pneumonia believed to be due to bacteria is treated with antibiotics. If the pneumonia is severe, the affected person is generally hospitalized. Oxygen therapy may be used if oxygen levels are low.Pneumonia affects approximately 450 million people globally (7% of the population) and results in about 4 million deaths per year. Pneumonia was regarded by William Osler in the 19th century as "the captain of the men of death". With the introduction of antibiotics and vaccines in the 20th century, survival improved. Nevertheless, in developing countries, and among the very old, the very young, and the chronically ill, pneumonia remains a leading cause of death. Pneumonia often shortens suffering among those already close to death and has thus been called "the old man's friend".Pneumothorax
A pneumothorax is an abnormal collection of air in the pleural space between the lung and the chest wall. Symptoms typically include sudden onset of sharp, one-sided chest pain and shortness of breath. In a minority of cases the amount of air in the chest increases when a one-way valve is formed by an area of damaged tissue, leading to a tension pneumothorax. This condition can cause a steadily worsening oxygen shortage and low blood pressure. Unless reversed by effective treatment, it can result in death. Very rarely both lungs may be affected by a pneumothorax. It is often called a collapsed lung, although that term may also refer to atelectasis.A primary pneumothorax is one that occurs without an apparent cause and in the absence of significant lung disease, while a secondary pneumothorax occurs in the presence of existing lung disease. Smoking increases the risk as do lung diseases including COPD, asthma, and tuberculosis. A pneumothorax can also be caused by physical trauma to the chest (including a blast injury) or as a complication of a healthcare intervention, in which case it is called a traumatic pneumothorax.Diagnosis of a pneumothorax by physical examination alone can be difficult (particularly in smaller pneumothoraces). A chest X-ray, computed tomography (CT) scan, or ultrasound is usually used to confirm its presence. Other conditions that can result in similar symptoms include a hemothorax (buildup of blood in the pleural space), pulmonary embolism, and heart attack. A large bulla may look similar on a chest X-ray.A small spontaneous pneumothorax will typically resolve without treatment and requires only monitoring. This approach may be most appropriate in people who have no underlying lung disease. In a larger pneumothorax, or if there is shortness of breath, the air may be removed with a syringe or a chest tube connected to a one-way valve system. Occasionally, surgery may be required if tube drainage is unsuccessful, or as a preventive measure, if there have been repeated episodes. The surgical treatments usually involve pleurodesis (in which the layers of pleura are induced to stick together) or pleurectomy (the surgical removal of pleural membranes). About 17–23 cases of pneumothorax occur per 100,000 people per year. They are more common in men than women.Pulmonary alveolus
A pulmonary alveolus (plural: alveoli, from Latin alveolus, "little cavity") is a hollow cavity found in the lung parenchyma, and is the basic unit of ventilation. Lung alveoli are the ends of the respiratory tree, branching from either alveolar sacs or alveolar ducts, which like alveoli are both sites of gas exchange with the blood as well. Alveoli are particular to mammalian lungs. Different structures are involved in gas exchange in other vertebrates. The alveolar membrane is the gas exchange surface. Carbon dioxide rich blood is pumped from the rest of the body into the capillaries that surround the alveoli where, through diffusion, carbon dioxide is released and oxygen is absorbed.Pulmonary edema
Pulmonary edema is fluid accumulation in the tissue and air spaces of the lungs. It leads to impaired gas exchange and may cause respiratory failure. It is due to either failure of the left ventricle of the heart to remove blood adequately from the pulmonary circulation (cardiogenic pulmonary edema), or an injury to the lung parenchyma or vasculature of the lung (noncardiogenic pulmonary edema). Treatment is focused on three aspects: firstly improving respiratory function, secondly, treating the underlying cause, and thirdly avoiding further damage to the lung. Pulmonary edema, especially acute, can lead to fatal respiratory distress or cardiac arrest due to hypoxia. It is a cardinal feature of congestive heart failure. The term edema is from the Greek οἴδημα (oídēma, "swelling"), from οἰδέω (oidéō, "I swell").Pulmonary fibrosis
Pulmonary fibrosis (literally "scarring of the lungs") is a respiratory disease in which scars are formed in the lung tissues, leading to serious breathing problems. Scar formation, the accumulation of excess fibrous connective tissue (the process called fibrosis), leads to thickening of the walls, and causes reduced oxygen supply in the blood. As a consequence patients suffer from perpetual shortness of breath.In some patients the specific cause of the disease can be diagnosed, but in others the probable cause cannot be determined, a condition called idiopathic pulmonary fibrosis. There is no known cure for the scars and damage in the lung due to pulmonary fibrosis.Pulmonology
Pulmonology is a medical speciality that deals with diseases involving the respiratory tract. The term is derived from the Latin word pulmō, pulmōnis ("lung") and the Greek suffix -λογία, -logia ("study of"). Pulmonology is synonymous with pneumology (from Greek πνεύμων ("lung") and -λογία), respirology and respiratory medicine.
Pulmonology is known as chest medicine and respiratory medicine in some countries and areas. Pulmonology is considered a branch of internal medicine, and is related to intensive care medicine. Pulmonology often involves managing patients who need life support and mechanical ventilation. Pulmonologists are specially trained in diseases and conditions of the chest, particularly pneumonia, asthma, tuberculosis, emphysema, and complicated chest infections.Qianlong Emperor
The Qianlong Emperor (25 September 1711 – 7 February 1799) was the sixth emperor of the Manchu-led Qing dynasty, and the fourth Qing emperor to rule over China proper. Born Hongli, the fourth son of the Yongzheng Emperor, he reigned officially from 11 October 1735 to 8 February 1796. On 8 February, he abdicated in favour of his son, the Jiaqing Emperor—a filial act in order not to reign longer than his grandfather, the illustrious Kangxi Emperor. Despite his retirement, however, he retained ultimate power as the Emperor Emeritus (or Retired Emperor) until his death in 1799; he thus was one of the longest-reigning de facto rulers in the history of the world, and dying at the age of 87, one of the longest-lived. As a capable and cultured ruler inheriting a thriving empire, during his long reign the Qing Empire reached its most splendid and prosperous era, boasting a large population and economy. As a military leader, he led military campaigns expanding the dynastic territory to the largest extent by conquering and sometimes destroying Central Asian kingdoms. This turned around in his late years: the Qing empire began to decline with corruption and wastefulness in his court and a stagnating civil society.
A British valet who accompanied his diplomat master to the Qing court in 1793 described the emperor:
The Emperor is about five feet ten inches in height, and of a slender but elegant form; his complexion is comparatively fair, though his eyes are dark; his nose is rather aquiline, and the whole of his countenance presents a perfect regularity of feature, which, by no means, announce the great age he is said to have attained; his person is attracting, and his deportment accompanied by an affability, which, without lessening the dignity of the prince, evinces the amiable character of the man. His dress consisted of a loose robe of yellow silk, a cap of black velvet with a red ball on the top, and adorned with a peacock's feather, which is the peculiar distinction of mandarins of the first class. He wore silk boots embroidered with gold, and a sash of blue girded his waist.Respiratory disease
Respiratory disease is a medical term that encompasses pathological conditions affecting the organs and tissues that make gas exchange difficult in higher organisms, and includes conditions of the upper respiratory tract, trachea, bronchi, bronchioles, alveoli, pleura and pleural cavity, and the nerves and muscles of breathing. Respiratory diseases range from mild and self-limiting, such as the common cold, to life-threatening entities like bacterial pneumonia, pulmonary embolism, acute asthma and lung cancer.The study of respiratory disease is known as pulmonology. A doctor who specializes in respiratory disease is known as a pulmonologist, a chest medicine specialist, a respiratory medicine specialist, a respirologist or a thoracic medicine specialist.
Respiratory diseases can be classified in many different ways, including by the organ or tissue involved, by the type and pattern of associated signs and symptoms, or by the cause of the disease.Respiratory failure
Respiratory failure results from inadequate gas exchange by the respiratory system, meaning that the arterial oxygen, carbon dioxide or both cannot be kept at normal levels. A drop in the oxygen carried in blood is known as hypoxemia; a rise in arterial carbon dioxide levels is called hypercapnia. Respiratory failure is classified as either Type I or Type II, based on whether there is a high carbon dioxide level. The definition of respiratory failure in clinical trials usually includes increased respiratory rate, abnormal blood gases (hypoxemia, hypercapnia, or both), and evidence of increased work of breathing.The normal partial pressure reference values are: oxygen PaO2 more than 80 mmHg (11 kPa), and carbon dioxide PaCO2 lesser than 45 mmHg (6.0 kPa).
General anatomy: systems and organs, regional anatomy, planes and lines, superficial axial anatomy, superficial anatomy of limbs
Anatomy of the lower respiratory tract