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lung
The lungs are the primary organs of respiration 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 musculoskeletal systems 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 primary muscle that drives breathing is the diaphragm.
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 lung, as the left lung shares space in the chest with the heart. The lungs together weigh approximately , with the right lung weighing more than the left. 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. These divide until air reaches microscopic alveoli, which is where the process of gas exchange takes place. Together, the lungs contain approximately of airways and 300 to 500 million alveoli. The lungs are enclosed within a sac called the pleural sac which allows the inner and outer walls to slide over each other whilst breathing takes place, without much friction. This sac encloses each lung and also divides each lung into sections called lobes. The right lung has three lobes, whilst the left lung only has two. The lobes are further divided into bronchopulmonary segments and lobules. The lungs have a unique blood supply, receiving deoxygenated blood sent from the heart for the purposes of receiving oxygen (the pulmonary circulation) and a separate blood supply of oxygenated blood (the bronchial circulation). The tissue of the lungs can be affected by a number of 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.
In fetal life, the lungs develop as an outpouching of the foregut, a tube which goes on to form the upper part of the digestive system. At this time, the fetus is situated in the amniotic sac and so the lungs 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.
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.
==Structure==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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