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A Bronchus is a passage or airway in the respiratory system that conducts air into the lungs. The first bronchi to branch from the trachea are the right main bronchus and the left main bronchus, also known as the primary bronchi. These are the widest and enter the lungs at each hilum, where they 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.[rx][rx] The bronchi when too narrow to be supported by cartilage are known as bronchioles. No gas exchange takes place in the bronchi.
The Bronchi (singular. bronchus) extend from the trachea (also called the “windpipe”). Together, these two structures form the tracheobronchial tree of the lungs. The trachea is the trunk of the tree located in the superior mediastinum. The bronchi are the branches of the tree within the lungs. Both the trachea and bronchi form part of the conducting zone of the respiratory system. While it is the trachea’s purpose to conduct air from the mouth and nose towards the lungs, it is the bronchi which distribute the air throughout the lungs until reaching the respiratory bronchioles and alveolar sacs (these structures pertain to the respiratory zone). The latter serves as the location for carbon dioxide and oxygen gas exchange across the wall (blood-air barrier) of pulmonary capillaries and lung alveoli.
Anatomy of The Bronchus
The bronchial structure begins at the transverse thoracic plane (also known as the sternal angle at the fourth thoracic vertebra), where the trachea bifurcates into two main bronchi, one for each lung. The main bronchi (also known as primary bronchi) enter the lungs inferior and lateral through the hila. At the bifurcation, the two main bronchi are not equally divided. The right main bronchus has a wider diameter, is shorter, and lies more vertically relative to the hilum. The left main bronchus has a smaller diameter and is more horizontal; it must pass inferior to the arch of the aorta and anterior to the esophagus and thoracic aorta to reach the left lung’s hilum.[rx]
The primary (main) bronchi then subdivide into secondary lobar bronchi. There are one secondary lobar bronchi per each lobe of the lung. Thus, the right lung has three secondary lobar bronchi, and the left lung has two secondary lobar bronchi. Following, each lobar bronchi further divides into several tertiary segmental bronchi. Each segmental bronchi supply a bronchopulmonary segment, which are the largest subdivisions of the lobe. There are ten bronchopulmonary segments in the right lung and eight through ten bronchopulmonary segments in the left lung, depending on the segment combinations.[rx]
After the tertiary segmental bronchi, the airways continue to fan out into bronchioles. Bronchioles then divide into three types: conducting, terminal, and respiratory. There are 20-25 branching generations of conducting bronchioles after the tertiary segmental bronchi. As the bronchioles become smaller in width, they become terminal bronchioles which mark the end of the conducting zone of the respiratory system. The terminal bronchioles divide further to form several generations of respiratory bronchioles, which are the narrowest airways in the lungs that give rise to alveolar ducts and alveolar sacs (respiratory bronchioles and alveoli form the respiratory zone). Each respiratory bronchiole divides into two through eleven alveolar ducts; each duct gives rise to five through six alveolar sacs.
Anatomical characteristics of the tracheobronchial tree are that it contains cartilage, smooth muscle, and mucosal lining. The trachea has C-shaped hyaline cartilage while the bronchi have hyaline cartilage plates. Cartilage is crucial for preventing airway collapse during inhalation and exhalation. Going down the bronchial tree, cartilage amount decreases, and smooth muscle increases. Specifically, the bronchioles lack cartilage in their walls and rely on smooth muscle and elastic fibers to maintain their wall integrity. The smooth muscle is also important to control airflow through contraction and dilation of the airway. Finally, the airway is covered by a mucosal lining to protect the lungs and trap foreign substances that enter the body through inhalation.[rx]
Histologically, both the trachea and bronchi have a tissue lining of pseudostratified ciliated columnar epithelium, which contains mucus-producing goblet cells. Going down the bronchial tree to the bronchioles, the epithelium changes to simple cuboidal epithelium containing club cells and no goblet cells. The club cells form surfactant which aid bronchioles to expand during inhalation and avoid bronchial collapse during exhalation. Following, the predominating cell types at the respiratory bronchioles are simple cuboidal epithelium with ciliated cells and Clara cells. Finally, the alveolar cells are simple squamous epithelium.[rx]
Blood Supply of The Bronchus
It is crucial to understand that the lungs have two types of vasculatures. First, the lungs have the pulmonary arteries, which are a low-pressure system that arises from the right ventricle and takes part in gas exchange. Second, there is a bronchial artery circulation; this arises from the left heart and is part of the systemic circulation. These arteries are a high-pressure system and provide oxygen to lung tissue. The distal-most branches of the bronchial arteries anastomose with branches of the pulmonary arteries. We will discuss the bronchial artery circulation further.
First, the bronchial arteries come off the descending aorta and supply about 1% of lung blood flow. They supply circulation to the upper esophagus, then pass posteriorly to the main bronchi down the brachial tree supplying oxygenated blood to the non-respiratory conducting tissues of the lungs. It also irrigates the visceral pleura, the intrapulmonary blood vessel walls, and the lymphatic system. The bronchial artery’s origin varies between person to person; generally, the single right bronchial artery rises from the third or fourth intercostal artery while the two left bronchial artery arises directly from the aorta. The right bronchial artery may also branch from a superior intercostal artery or a left superior bronchial artery.[rx]
The bronchial vein receives blood from the bronchi and drains back into the systemic circulation. Nevertheless, it only clears a portion of the blood brought by the bronchial arteries. The blood that remains is carried away by the pulmonary veins. Blood returning from visceral pleura, peripheral lung regions, and distal root components drain into the pulmonary veins. Additionally, the bronchial veins may receive drainage from small vessels from the tracheobronchial lymph nodes. The right bronchial vein drains through the azygous vein and the left bronchial vein into the accessory hemizygous vein or the left superior intercostal vein.
The lymphatic vessels pick up the fluid that leaks into the bronchial tree, pulmonary vessels, and connective tissue septa and returns it to the circulatory system. It is important to state that the lymphatic vessels are not in the walls of the pulmonary alveoli. The lung’s lymphatic plexus originate from two different structures: the superficial subpleural and the deep lymphatic plexuses. Deep into the visceral pleura, is the location of the superficial lymphatic plexus. It drains visceral pleura and lung parenchyma into the bronchopulmonary (hilar) lymph nodes; these are located in the lung’s hilum.[rx][rx]
Located in the bronchi’s submucosa and in the peribronchial connective tissue lies the deep lymphatic plexus. Its role is to drain fluid from the root of the lung into the intrinsic pulmonary lymph nodes, which are in the lobar bronchi. Lymph vessel drainage continues to follow the pulmonary vessels and bronchi towards the hilum, where collected fluid also exits into the bronchopulmonary (hilar) lymph nodes. Interestingly, various lymphatics of the left lower lobe drain into the right superior tracheobronchial nodes.[rx]
As noted, both superficial and deep lymphatic plexuses drain from the hilum into the superior and inferior (cranial) tracheobronchial lymph nodes (located at the trachea’s bifurcation). Then the lymph fluid drains into the tracheal (paratracheal) nodes and later into the right and left bronchomediastinal nodes and trunks. The right bronchomediastinal trunk converges with other lymphatic trunks to form the right lymphatic duct. The left bronchomediastinal trunk converges into the thoracic duct. These thoracic and right lymphatic ducts exit the lymph fluid into the venous angles: left subclavian vein and right subclavian vein, respectfully.[rx]
Nerves Supply of Bronchus
The nerve supply to the lung arises from both the phrenic nerve and the pulmonary plexus. The phrenic nerve originates from the neck on third through fifth cervical nerves and travels down between the lung (anterior to the root of the lung) and heart until reaching the diaphragm. Thus, it innervates the fibrous pericardium, the central part of the mediastinal and diaphragmatic pleurae, and the diaphragm for motor and sensory functions.[rx]
The pulmonary plexus arises from the vagus nerve (cranial nerve X). It divides into the anterior pulmonary plexus (lies anteriorly to the lung root) and the posterior pulmonary plexus (lies behind the root of the lung). The plexus is considered an extension of the fibers of the cardiac plexus, which travels with the pulmonary arteries into the lungs. Specifically, the plexus networks have parasympathetic, sympathetic, and visceral afferent fibers.[rx]
The parasympathetic fibers that travel to the pulmonary plexus are composed of the presynaptic fibers of CN X. These fibers are the motor input to the bronchial tree’s smooth muscle (results in bronchoconstriction), are inhibitory to the pulmonary vessels causing them to vasodilate, and finally are secretomotor by causing the bronchial tree glands to increase secretions. The sympathetic fibers of the pulmonary plexus are postsynaptic fibers whose cell bodies are in the paravertebral sympathetic ganglia of sympathetic trunks. These sympathetic fibers are bronchodilators by inhibiting the bronchial muscles, vasoconstrictors, and inhibit the bronchial tree’s alveolar glands. Lastly, the visceral afferent fibers of the pulmonary plexus can conduct reflexes along the parasympathetic fibers that control function (reflexive) or can conduct painful stimuli (nociceptive), which accompanies the sympathetic fibers through the sympathetic trunk.
It is essential to point out that the nerves of the parietal pleura derive from the intercostal and phrenic nerves. Intercostal nerves also innervate the costal pleura and the peripheral diaphragmatic pleura.[rx]
Muscles Attachment of Bronchus
The bronchi of the lungs contain smooth muscle that contract or dilate to control the narrowing of bronchial airways. Specifically, the bronchioles rely on smooth muscle and elastic fibers to maintain their wall integrity.[rx] These smooth muscles also secrete inflammatory mediators, thus responsible for bronchial inflammation.[rx]
It is essential to understand that the lungs do not have muscles to expand and contract during respiration. Instead, they rely on external muscles for respiration. The muscles activated during inspiration are the diaphragm (most important), intercostal muscles, sternocleidomastoid, levator costarum, serratus anterior, scalenus, pectoralis major and minor, and finally the serratus posterior superior muscles. The muscles of expiration, which aid the lungs to expel air, involves the muscles of the anterior abdominal wall, internal intercostal muscles, and serratus posterior inferior muscles.[rx]