The
trachea (windpipe) divides at the
carina into two main or
primary bronchi, the left bronchus and the right bronchus. The carina of the trachea is located at the level of the
sternal angle and the fifth
thoracic vertebra (at rest). The
right main bronchus is wider, shorter, and more vertical than the left main bronchus, its mean length is 1.09 cm. It enters the
root of 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 oxygen to the three
lobes of the right lung—the superior, middle and inferior lobe. 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 to the superior lobe of the right lung, which is also called the
eparterial bronchus.
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 which divides into the two lobar bronchi to 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 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 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 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 as
subsegmental bronchi. These branch 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 main bronchi have relatively large
lumens that are lined by
respiratory epithelium. This cellular lining has cilia departing towards the mouth which removes dust and other small particles. There is a
smooth muscle layer below the epithelium arranged as two ribbons of muscle that spiral in opposite directions. This smooth muscle layer contains
seromucous glands, which secrete
mucus, 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. The bronchial wall normally has a thickness of 10% to 20% of the total bronchial diameter.
Microanatomy and much smaller
microvilli on non-ciliated bronchiolar epithelium The
cartilage and
mucous membrane of the main bronchus (primary bronchi) are similar to those in the trachea. They are lined with
respiratory epithelium, which is classified as
ciliated pseudostratified columnar epithelium.
Variation 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
cardiac 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. An accessory cardiac bronchus is usually an asymptomatic condition but may be associated with persistent infection or
hemoptysis. 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 aerated
lung parenchyma. ==Function==