The lungs are divided up by fissures into a predictable arrangement of lobes:
The lobes are characterised by a discrete connection with the first subdivision of the bronchial tree after the level of the principal bronchi to both lungs - these are the lobar bronchi. In a similar manner, the vascular, nerve and lymphatic supply from the hila to each lobe has minimal connection with other lobes. This makes the lobes relatively independent funtional units within the lung. Indeed, pathology may be confined to one lobe and corrective surgery may be facilitated by the clear demarcation between lobes produced by the fissures.
Further, the lobes are subdivided into bronchopulmonary segments which are volumes of lung with an organized arrangement relative to each other within a lobe and similar functional independence as the lobes. With further subdivision of bronchi, each segment is divided into secondary lobules.
The lobar fissures are often incomplete, making a connection between two apposed lobes. Conversely, more than the expected number of lobes may be produced by new fissures e.g. the azygous lobe of the lung. The additional lobes are termed 'accessory lobes'.
The equivalent of the right middle lobe on the left side is termed the lingula. It is occasionally considered as a lobe but it is small in comparison and rarely separated from the left superior lobe by a fissure.
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