lung lung the lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. 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 air and transfer it into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere, in a process of gas exchange. the pleurae, which are thin, smooth, and moist, serve to reduce friction between the lungs and chest wall during breathing, allowing for easy and effortless movements of the lungs. this article uses anatomical terminology. lungsdiagram of the human lungs with the respiratory tract visible, and different colours for each lobethe human lungs flank the heart and great vessels in the chest cavity.detailssystemrespiratory systemidentifierslatinpulmogreekπνεύμων (pneumon)meshd008168ta98a06.5.01.001ta23265anatomical terminology respiration is driven by different muscular systems in different species. mammals, reptiles and birds use their different muscles to support and foster breathing. in earlier 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, one on the left and one on the right. they are situated within the thoracic cavity of the chest. the right lung is bigger and heavier than the left, which shares space in the chest with the heart. the lungs together weigh approximately 1.3 kilograms (2.9 pounds). 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 alveolar sacs that contain the alveoli, where gas exchange takes place. alveoli are also sparsely present on the walls of the respiratory bronchioles and alveolar ducts. together, the lungs contain approximately 2,400 kilometres (1,500 miles) of airways and 300 to 500 million alveoli. each lung is enclosed within a pleural sac of two membranes called pleurae; the membranes are separated by a film of pleural fluid, which allows the inner and outer membranes to slide over each other whilst breathing takes place, without much friction. the inner pleura 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 pulmonary 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 respiratory diseases, including pneumonia and lung cancer. chronic obstructive pulmonary disease includes chronic bronchitis and emphysema, and can be related to smoking or exposure to harmful substances. a number of occupational lung diseases can be caused by 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. structure anatomy 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. both lungs have a central recession called the hilum, where the blood vessels and airways pass into the lungs making up the root of the lung. there are also bronchopulmonary lymph nodes on the hilum. 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. lobes lobes and bronchopulmonary segments right lungleft lung upper apical posterior anterior middle lateral medial lower superior medial anterior lateral posterior upper apicoposterior anterior lingula superior inferior lower superior anteriomedial lateral posterior each lung is divided into sections called lobes by the infoldings of the visceral pleura as fissures. lobes are divided into segments, and segments have further divisions as lobules. there are three lobes in the right lung and two lobes in the left lung. fissures the fissures are formed in early prenatal development by invaginations of the visceral pleura that divide the lobar bronchi, and section the lungs into lobes that helps in their expansion. the right lung is divided into three lobes by a horizontal fissure, and an oblique fissure. the left lung is divided into two lobes by an oblique fissure which is closely aligned with the oblique fissure in the right lung. in the right lung the upper horizontal fissure, separates the upper (superior) lobe from the middle lobe. the lower, oblique fissure separates the lower lobe from the middle and upper lobes. variations in the fissures are fairly common being either incompletely formed or present as an extra fissure as in the azygos fissure, or absent. incomplete fissures are responsible for interlobar collateral ventilation, airflow between lobes which is unwanted in some lung volume reduction procedures. segments 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 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. right lung the right lung has both more lobes and segments than the left. it is divided into three lobes, an upper, middle, and a lower lobe 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 back to the hilum. the lower, oblique fissure, separates the lower from the middle and upper lobes and is closely aligned with the oblique fissure in the left lung. 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 weight of the right lung varies between individuals, with a standard reference range in men of 155–720 g (0.342–1.587 lb) and in women of 100–590 g (0.22–1.30 lb). left lung the left lung is divided into two lobes, an upper and a lower lobe, 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 lung serves as an anatomic parallel to the middle lobe on the right lung, 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 weight of the left lung, by standard reference range, in men is 110–675 g (0.243–1.488 lb) in women 105–515 g (0.231–1.135 lb). illustrations chest ct (axial lung window) chest ct (coronal lung window) microanatomy cross-sectional detail of the lung the lungs are part of the lower respiratory tract, and accommodate the bronchial airways when they branch from the trachea. the bronchial airways terminate in alveoli which make up the functional tissue (parenchyma) of the lung, 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 of lymph capillaries, and they are absent in the alveoli. the lungs are supplied with the largest lymphatic drainage system of any other organ in the body. each lung is surrounded by a serous membrane of visceral pleura, which has an underlying layer of loose connective tissue attached to the substance of the lung. connective tissue thick elastic fibres from the visceral pleura (outer lining) of lung tem image of collagen fibres in a cross sectional slice of mammalian lung tissue the connective tissue of the lungs is made up of elastic and collagen fibres that are interspersed between the capillaries and the alveolar walls. elastin is the key protein of the extracellular matrix and is the main component of the elastic fibres. elastin gives the necessary