|Nerve||Pulmonary branches of vagus nerve|
A bronchus (plural bronchi, adjective bronchial), also known as a main or primary bronchus, is a passage of airway in the respiratory tract that conducts air into the lungs. The bronchus branches into smaller tubes, which in turn become bronchioles. No gas exchange takes place in the bronchi.
The trachea (windpipe) divides into two main or primary bronchi, the left and the right, at the level of the sternal angle and of the fifth thoracic vertebra or up to two vertebrae higher or lower, depending on breathing, at the anatomical point the carina of trachea. The right main bronchus is wider, shorter, and more vertical than the left main bronchus.It enters the right lung at approximately the fifth thoracic vertebra. The right main bronchus subdivides into three secondary bronchi (also known as lobar bronchi), which deliver air to the three lobes of the right lung.The azygos vein arches over it from behind; and the right pulmonary artery lies at first below and then in front of it. About 2 cm from its commencement it gives off a branch, the eparterial bronchus, to the upper lobe of the right lung. Eparterial refers to its position above the right pulmonary artery. The right bronchus now passes below the artery, and is known as the hyparterial branch; it divides into two branches for the middle and lower lobes.
The left main bronchus is smaller in caliber but longer than the right, being 5 cm long. It enters the root of the left lung opposite the sixth thoracic vertebra. It passes beneath the aortic arch, crosses in front of the esophagus, the thoracic duct, and the descending aorta, and has the left pulmonary artery lying at first above, and then in front of it. The left bronchus has no eparterial branch, and therefore it has been supposed by some that there is no upper lobe to the left lung, but that the so-called upper lobe corresponds to the middle lobe of the right lung.
The left main bronchus divides into two secondary or lobar bronchi to deliver air to the two lobes of the left lung. The secondary bronchi divide further into tertiary bronchi, (also known as segmental bronchi), each of which supplies a bronchopulmonary segment. A bronchopulmonary segment is a division of a lung separated from the rest of the lung by a septum of connective tissue. This property allows a bronchopulmonary segment to be surgically removed without affecting other segments. Initially, there are ten segments per lung, but during development with the left lung having just two lobes, two pairs of segments fuse to give eight, four for each lobe. The segmental bronchi divide into many smaller bronchioles which divide into terminal bronchioles, each of which then gives rise to several respiratory bronchioles, which go on to divide into two to eleven alveolar ducts. There are five or six alveolar sacs associated with each alveolar duct. The alveolus is the basic anatomical unit of gas exchange in the lung.
The hyaline cartilage forms an incomplete ring in the bronchi, giving them a "D"-shaped appearance in the larger bronchi and as small plates and islands in the smaller bronchi. Smooth muscle is present continuously around the bronchi.
The cartilage and mucous membrane of the primary bronchi are similar to those in the trachea. As the branching continues through the bronchial tree, the amount of hyaline cartilage in the walls decreases until it is absent in the bronchioles. As the cartilage decreases, the amount of smooth muscle increases. The mucous membrane also undergoes a transition from ciliated pseudostratified columnar epithelium to simple cuboidal epithelium to simple squamous epithelium.
In 0.1 to 5% of people there is a right superior lobe bronchus arising from the main stem bronchus prior to the carina. This is known as a tracheal bronchus, and seen as an anatomical variation. It can have multiple variations and, although usually asymptomatic, it can be the root cause of pulmonary disease such as a recurrent infection. In such cases resection is often curative 
The alveolar ducts and alveoli consist primarily of simple squamous epithelium, which permits rapid diffusion of oxygen and carbon dioxide. Exchange of gases between the air in the lungs and the blood in the capillaries occurs across the walls of the alveolar ducts and alveoli.
Bronchitis is defined as inflammation of the bronchi, which can either be acute or chronic. Acute bronchitis is usually caused by viral or bacterial infections. Most sufferers of chronic bronchitis also suffer from chronic obstructive pulmonary disease (COPD), and is usually associated with smoking or long-term exposure to irritants.
While the left mainstem bronchus departs from the trachea at an angle, the right mainstem bronchus is almost a vertical continuation of the trachea. This anatomy predisposes the right lung to several problems:
- If food, liquids, or foreign bodies are aspirated, they often will lodge in the right mainstem bronchus. Aspiration pneumonia may result.
- If the endotracheal tube used for intubation is inserted too far, it usually lodges in the right mainstem bronchus. This allows ventilation of the right lung, but leaves the left lung useless.
In asthma, the constriction of the bronchi can result in a difficulty in breathing giving shortness of breath; this can lead to a lack of oxygen reaching the body for cellular processes. In this case an asthma puffer can be used to rectify the problem. The puffer is a broncho-dilator, which serves to soothe the constricted bronchi and to once again expand the airways. This effect occurs quite quickly.
- Maton, Anthea; Jean Hopkins; Charles William McLaughlin; Susan Johnson; Maryanna Quon Warner; David LaHart; Jill D. Wright (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. ISBN 0-13-981176-1.[page needed]
- Brodsky, JB; Lemmens, JM (2003). "Left Double-Lumen Tubes: Clinical Experience With 1,170 Patients". Journal of Cardiothoracic and Vascular Anesthesia 17 (3): 289–98. doi:10.1016/S1053-0770(03)00046-6. PMID 12827573.
- Shih, Fu-Chieh; Wei-Jing Lee, Hung-Jung Lin (2009-03-31). "Tracheal bronchus". Canadian Medical Association Journal 180 (7): 783–783. doi:10.1503/cmaj.080280. ISSN 0820-3946.
- Barat, Michael; Horst R. Konrad (1987-03-04). "Tracheal bronchus". American Journal of Otolaryngology 8 (2): 118–122. doi:10.1016/S0196-0709(87)80034-0. ISSN 0196-0709. Retrieved 2011-05-17.
- Sacristán Bou, L. and Peña Blas, F. Bronchial Atresia in Lung Diseases - Selected State of the Art Reviews (2012). ISBN 978-953-51-0180-2. Published under CC BY 3.0 license.
- Moore, Keith L. and Arthur F. Dalley. Clinically Oriented Anatomy, 4th ed. (1999). ISBN 0-7817-5936-6