Abronchus (/ˈbrɒŋkəs/BRONG-kəs;pl.:bronchi,/ˈbrɒŋkaɪ/BRONG-ky) is a passage or airway in thelower respiratory tract that conductsair into thelungs. The first or primary bronchi to branch from thetrachea at thecarina are the right main bronchus and the left main bronchus. These are the widest bronchi, and enter the right lung, and the left lung at eachhilum. The main bronchi branch into narrower secondary bronchi or lobar bronchi, and these branch into narrower tertiary bronchi or segmental bronchi. Further divisions of the segmental bronchi are known as 4th order, 5th order, and 6th order segmental bronchi, or grouped together as subsegmental bronchi.[1][2]The bronchi, when too narrow to be supported by cartilage, are known asbronchioles. Nogas exchange takes place in the bronchi.
Thetrachea (windpipe) divides at thecarina into two main orprimary bronchi, the left bronchus and the right bronchus. The carina of the trachea is located at the level of thesternal angle and the fifththoracic vertebra (at rest).
Theright main bronchus is wider, shorter, and more vertical than the left main bronchus,[3] its mean length is 1.09 cm.[4] It enters theroot of the right lung at approximately the fifth thoracic vertebra. The right main bronchus subdivides into threesecondary bronchi (also known aslobar bronchi), which deliver oxygen to the threelobes of the right lung—the superior, middle and inferior lobe. Theazygos 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 to the superior lobe of the right lung, which is also called theeparterial bronchus.Eparterial refers to its position above the rightpulmonary artery. The right bronchus now passes below the artery, and is known as thehyparterial branch which divides into the two lobar bronchi to the middle and lower lobes.
Theleft main bronchus is smaller in caliber but longer than the right, being 5 cm long. It enters theroot of the left lung opposite the sixth thoracic vertebra. It passes beneath theaortic arch, crosses in front of theesophagus, thethoracic duct, and thedescending aorta, and has the left pulmonary artery lying at first above, and then in front of it. The left bronchus has noeparterial 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 theright lung. The left main bronchus divides into two secondary bronchi or lobar bronchi, to deliver air to the two lobes of the left lung—the superior and the inferior lobe.
The secondary bronchi divide further intotertiary bronchi, (also known assegmental bronchi), each of which supplies abronchopulmonary segment. A bronchopulmonary segment is a division of a lung separated from the rest of the lung by aseptum ofconnective tissue. This property allows a bronchopulmonary segment to be surgically removed without affecting other segments. Initially, there are ten segments in each 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 tertiary bronchi divide further in another three branchings known as 4th order, 5th order and 6th order segmental bronchi which are also referred to assubsegmental bronchi. These branch into many smallerbronchioles which divide intoterminal bronchioles, each of which then gives rise to severalrespiratory bronchioles, which go on to divide into two to elevenalveolar ducts. There are five or sixalveolar sacs associated with each alveolar duct. Thealveolus is the basic anatomical unit of gas exchange in the lung.
The main bronchi have relatively largelumens that are lined byrespiratory epithelium. This cellular lining has cilia departing towards the mouth which removes dust and other small particles. There is asmooth muscle layer below the epithelium arranged as two ribbons of muscle that spiral in opposite directions. This smooth muscle layer containsseromucous glands, which secretemucus, in its wall.Hyaline cartilage is present in the bronchi, surrounding the smooth muscle layer. In the main bronchi, the cartilage forms C-shaped rings like those in the trachea, while in the smaller bronchi, hyaline cartilage is present in irregularly arranged crescent-shaped plates and islands. These plates give structural support to the bronchi and keep the airway open.[5]
The bronchial wall normally has a thickness of 10% to 20% of the total bronchial diameter.[6]
As 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 ciliated cuboidal epithelium, to simplesquamous epithelium in the alveolar ducts and alveoli[7][8]
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 atracheal bronchus, and seen as ananatomical variation.[9] 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 casesresection is often curative.[10][11]
Thecardiac bronchus has a prevalence of ≈0.3% and presents as an accessory bronchus arising from the bronchus intermedius between the upper lobar bronchus and the origin of the middle and lower lobar bronchi of the right main bronchus.[12]
An accessory cardiac bronchus is usually an asymptomatic condition but may be associated with persistent infection orhemoptysis.[13][14] In about half of observed cases the cardiac bronchus presents as a short dead-ending bronchial stump, in the remainder the bronchus may exhibit branching and associated aeratedlung parenchyma.
The bronchi function to carryair that is breathed in through to the functional tissues of the lungs, called alveoli. Exchange of gases between the air in the lungs and the blood in thecapillaries occurs across the walls of the alveolar ducts and alveoli. Thealveolar ducts and alveoli consist primarily of simplesquamous epithelium, which permits rapid diffusion ofoxygen andcarbon dioxide.
Bronchial wall thickness (T) and bronchial diameter (D).
Bronchial wall thickening, as can be seen onCT scan, generally (but not always) impliesinflammation of the bronchi (bronchitis).[15] Normally, the ratio of the bronchial wall thickness and the bronchial diameter is between 0.17 and 0.23.[16]
The left main bronchus departs from the trachea at a greater angle than that of the right main bronchus. The right bronchus is also wider than the left and these differences predispose the right lung toaspirational problems. If food, liquids, or foreign bodies are aspirated, they will tend to lodge in the right main bronchus.Bacterial pneumonia andaspiration pneumonia may result.
If atracheal tube used forintubation is inserted too far, it will usually lodge in the right bronchus, allowing ventilation only of the right lung.
In asthma, the constriction of the bronchi can result in difficulty in breathing givingshortness of breath; this can lead to alack of oxygen reaching the body for cellular processes. In this case, aninhaler can be used to rectify the problem. The inhaler administers abronchodilator, which serves to soothe the constricted bronchi and to re-expand the airways. This effect occurs quite quickly.
Bronchial atresia is a rare congenital disorder that can have a varied appearance. A bronchial atresia is a defect in the development of the bronchi, affecting one or more bronchi – usually segmental bronchi and sometimes lobar. The defect takes the form of a blind-ended bronchus. The surroundingtissue secretes mucus normally but builds up and becomes distended.[17] This can lead toregional emphysema.[18]
The collected mucus may form a mucoid impaction or abronchocele, or both. Apectus excavatum may accompany a bronchial atresia.[17]
^Netter, Frank H. (2014).Atlas of Human Anatomy Including Student Consult Interactive Ancillaries and Guides (6th ed.). Philadelphia, Penn.: W B Saunders Co. p. 200.ISBN978-1-4557-0418-7.
^Maton, Anthea; Jean Hopkins; Charles William McLaughlin; Susan Johnson; Maryanna Quon Warner; David LaHart; Jill D. Wright (1993).Human Biology and Health. wood Cliffs, New Jersey, USA: Prentice Hall.ISBN0-13-981176-1.[page needed]
^Robinson, CL; Müller, NL; Essery, C (January 1989). "Clinical significance and measurement of the length of the right main bronchus".Canadian Journal of Surgery.32 (1):27–8.PMID2642720.
^Saladin, K (2012).Anatomy & physiology : the unity of form and function (6th ed.). McGraw-Hill. p. 862.ISBN9780073378251.
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