Lung

• 1. Anatomical Structure And Function

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  The human lungs sit within the thoracic cavity on either side of the heart. They weigh approximately 1.3 kilograms (2.9 lb) together, with the right lung being heavier than the left. The right lung contains three lobes while the left lung has two. This difference exists because the left lung shares space in the chest with the heart. A specific indentation called the cardiac notch accommodates this organ on the left side.

  Air enters through the trachea and branches into bronchi and bronchioles. These airways divide until they reach microscopic alveoli where gas exchange takes place. Together the lungs contain approximately 2,400 kilometers (1,500 mi) of airways. There are also between 300 to 500 million alveoli inside each pair of lungs. Each lung is enclosed within a pleural sac of two pleurae which allows the inner and outer walls to slide over each other without much friction during breathing.

  The tissue of the lungs can be affected by several respiratory diseases including pneumonia and lung cancer. Chronic diseases such as chronic obstructive pulmonary disease and emphysema can be related to smoking or exposure to harmful substances. Diseases such as bronchitis can also affect the respiratory tract. Medical terms related to the lung often begin with pulmo- from the Latin pulmonarius or with pneumo- from Greek πνεύμων meaning lung.

• 2. Developmental Biology And Growth

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  In embryonic development the lungs begin to develop as an outpouching of the foregut. This tube 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 so they do not function to breathe. Blood is diverted from the lungs through the ductus arteriosus until birth occurs.

  At birth however air begins to pass through the lungs and the diversionary duct closes so that the lungs can begin to respire. The baby's lungs are filled with fluid secreted by the lungs and are not inflated before delivery. After the first breath the fluid is quickly absorbed into the body or exhaled within about ten seconds after delivery.

  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.

• 3. Pathology And Clinical Disorders

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  Pulmonology is the medical speciality that deals with respiratory diseases involving the lungs and respiratory system. Cardiothoracic surgery deals with surgery of the lungs including lung volume reduction surgery lobectomy pneumectomy and lung transplantation. Inflammation and infection conditions include pneumonia bronchitis and bronchiolitis as well as pleurisy affecting the pleurae surrounding the lungs.

  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 hypertension describes an increased pressure at the beginning of the pulmonary artery that has a large number of differing causes. A lung contusion is a bruise caused by chest trauma resulting in hemorrhage of the alveoli causing a build-up of fluid which can impair breathing.

  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 from a biopsy is taken.

• 4. Evolutionary Origins Across Species

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  The lungs of today's terrestrial vertebrates and the gas bladders of today's fish are believed to have evolved from simple sacs. These outpocketings arose in the bony fish allowing early fish to gulp air under oxygen-poor conditions. In most ray-finned fish the sacs evolved into closed off gas bladders while a number of carp trout herring catfish and eels retained the physostome condition with the sac being open to the oesophagus.

  In more basal bony fish such as the gar bichir bowfin and the lobe-finned fish the sacs 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. Lungs are found in three groups of fish: the coelacanths the bichirs and the lungfish.

  The coelacanth has a nonfunctional and unpaired vestigial lung surrounded by a fatty organ. Bichirs have a pair which are hollow unchambered sacs where gas-exchange occurs on very flat folds that increase their inner surface area. The lungs of lungfish show more resemblance to tetrapod lungs with an elaborate network of parenchymal septa dividing them into numerous respiration chambers.

• 5. Comparative Anatomy In Vertebrates

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  Birds possess relatively small lungs connected to eight or nine air sacs extending through much of the body. On inhalation air travels through the trachea into the air sacs then continuously from the back air sacs through the fixed-size lungs to front air sacs before exhalation. These fixed size lungs are called circulatory lungs distinct from bellows-type lungs found in most other animals.

  Reptilian lungs typically receive air via expansion and contraction of 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 creating negative pressure in the thoracic cavity allowing air to be moved into the lungs. Turtles use forelimbs and pectoral girdle to force air in and out since they cannot move their ribs.

  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 lungs are inflated by expanding the rib cage. The floor of the mouth lowers filling the mouth cavity with air while throat muscles press the throat against the underside of the skull forcing air into the lungs.

• 6. Medical Diagnostics And Testing

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  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, 750 mL. The inspiratory reserve volume and expiratory reserve volume are additional amounts a person is able to forcibly inhale and exhale respectively.

  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 age height weight and sex and normally ranges between four and six litres.

  Other lung function tests include spirometry measuring the amount 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.