elasticity and resilience required for the persistent stretching involved in breathing, known as lung compliance. it is also responsible for the elastic recoil needed. elastin is more concentrated in areas of high stress such as the openings of the alveoli, and alveolar junctions. the connective tissue links all the alveoli to form the lung parenchyma which has a sponge-like appearance. the alveoli have interconnecting air passages in their walls known as the pores of kohn. respiratory epithelium main article: respiratory epithelium 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 mucin the main component of mucus, ciliated cells, basal cells, and in the terminal bronchioles–club cells with actions similar to basal cells, and macrophages. the epithelial cells, and the submucosal glands throughout the respiratory tract secrete airway surface liquid (asl), the composition of which is tightly regulated and determines how well mucociliary clearance works. pulmonary neuroendocrine cells are found throughout the respiratory epithelium including the alveolar epithelium, though they only account for around 0.5 percent of the total epithelial population. pnecs are innervated airway epithelial cells that are particularly focused at airway junction points. these cells can produce serotonin, dopamine, and norepinephrine, as well as polypeptide products. cytoplasmic processes from the pulmonary neuroendocrine cells extend into the airway lumen where they may sense the composition of inspired gas. bronchial airways in the bronchi there are incomplete tracheal rings of cartilage and smaller plates of cartilage that keep them open.: 472 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.: 472 the absence of cartilage in the terminal bronchioles gives them an alternative name of membranous bronchioles. a lobule of the lung enclosed in septa and supplied by a terminal bronchiole that branches into the respiratory bronchioles. each respiratory bronchiole supplies the alveoli held in each acinus accompanied by a pulmonary artery branch. respiratory zone the conducting zone of the respiratory tract ends at the terminal bronchioles when they branch into the respiratory bronchioles. this marks the beginning of the terminal respiratory unit called the acinus which includes the respiratory bronchioles, the alveolar ducts, alveolar sacs, and alveoli. an acinus measures up to 10 mm in diameter. a primary pulmonary lobule is that part of the acinus that includes the alveolar ducts, sacs, and alveoli but does not include the respiratory bronchioles. the unit described as the secondary pulmonary lobule is the lobule most referred to as the pulmonary lobule or respiratory lobule.: 489 this lobule is a discrete unit that is the smallest component of the lung that can be seen without aid. the secondary pulmonary lobule is likely to be made up of between 30 and 50 primary lobules. the lobule is supplied by a terminal bronchiole that branches into respiratory bronchioles. the respiratory bronchioles supply the alveoli in each acinus and is accompanied by a pulmonary artery branch. each lobule is enclosed by an interlobular septa. each acinus is incompletely separated by an interlobular septa. the respiratory bronchiole gives rise to the alveolar ducts that lead to the alveolar sacs, which contain two or more alveoli. the walls of the alveoli are extremely thin allowing a fast rate of diffusion. the alveoli have interconnecting small air passages in their walls known as the pores of kohn. alveoli main article: pulmonary alveolus alveoli and their capillary networks 3d medical illustration showing different terminating ends of bronchioles 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 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. type ii are larger and they line the alveoli and produce and secrete epithelial lining fluid, and lung surfactant. type ii cells are able to divide and differentiate to type i cells. the alveolar macrophages have an important role in the immune system. they remove substances which deposit in the alveoli including loose red blood cells that have been forced out from blood vessels. microbiota main article: lung microbiota there is a large presence of microorganisms in the lungs known as the lung microbiota that interacts with the airway epithelial cells; an interaction of probable importance in maintaining homeostasis. the microbiota is complex and dynamic in healthy people, and altered in diseases such as asthma and copd. for example significant changes can take place in copd following infection with rhinovirus. fungal genera that are commonly found as mycobiota in the microbiota include candida, malassezia, saccharomyces, and aspergillus. respiratory tract main article: respiratory tract the lungs as main part of respiratory tract 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 primary 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. estimates of the total surface area of lungs vary from 50 to 75 square metres (540 to 810 sq ft); although this is often quoted in textbooks and the media being "the size of a tennis court", it is actually less than half the size of a singles court. 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, in a process called mucociliary clearance. pulmonary stretch receptors in the smooth muscle of the airways initiate a reflex known as the hering–breuer reflex that prevents the lungs from over-inflation, during forceful inspiration. blood supply main article: pulmonary circulation 3d rendering of a high-resolution ct scan of the thorax. the anterior thoracic wall, the airways and the pulmonary vessels anterior to the root of the lung have been digitally removed in order to visualize the different levels of the pulmonary circulation. 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. also, in the event of blood loss through hemorrhage, blood from the lungs can partially compensate by automatically transferring to the systemic circulation. nerve supply 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 adrenoceptors in the respiratory tract, which causes bronchodilation. the action of breathing takes place because of nerve signals sent by the respiratory center in the brainstem, along the phrenic nerve from the cervical plexus to the diaphragm. variation the lobes of the lung are subject to anatomical variations. a horizontal interlobar 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. an oblique fissure was found to be incomplete in 21% to 47% of left lungs. in some cases a fissure is absent, or extra, resulting in a right lung with only two lobes, or a left lung with three lobes. a variation in the airway branching structure has been found specifically in the central airway branching. this variation is associated with the development of copd in adulthood. development the development of the human lungs arise from the laryngotracheal groove and develop to maturity over several weeks in the foetus and for several years following birth. 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. lungs during development, showing the early branching of the primitive bronchial buds the respiratory tract has a branching structure, and is also known as the respiratory 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