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StatPearls . Endowment Island (FL): StatPearls Publishing; 2021 Jan-.


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Introduction

The tracheobronchial tree is composed of the trachea, the bronchi, and the bronchioles that transfer air indigenous the setting to the lungs for gas exchange. 


Structure and also Function

The trachea originates at the inferior edge the the larynx and also connects come the left and main stem bronchus. The primary function of the trachea is to permit passage that inspired and expired wait into and out of the lung. The trachea is a midline structure and lies just anterior the esophagus.

After the originates indigenous the larynx, the trachea divides into the left and also right mainstem bronchi. This junction point is referred to as the carina. The tracheal diameter is around 24 to 26 mm in adult males and also 22 to 24 mm in females. The is much more comprehensive in the proximal area 보다 the distal segment. The trachea is 16 to 20 C-shaped cartilage rings stacked one on top of another. A small thin membrane connects each of the tracheal rings. The posterior element of the trachea is surrounding to the anterior esophagus.

The trachea has 4 layers. The within mucosal layer has a lining that ciliated pseudostratified columnar epithelium with goblet cells. This cells space responsible for secreting mucus which coats the inside lining the the trachea. The mucus traps dust particles and other debris prior to they with the lung. The cilia function to propel the dust away from the lung.

The submucosa is located just below the mucosa and contains concerned tissue and also blood vessels. This layer also contains elastin and also collagen fibers, which provide the trachea with support and elasticity. The visibility of smooth muscle in the submucosa allows for the trachea to change its diameter. A great of hyaline cartilage supporting the tracheal ring surrounds the submucosa. The hyaline cartilage layer is sturdy but flexible and also prevents the fallen of the trachea throughout expiration. The outermost class is the adventitia, consist of of loosened connective organization that anchors the trachea to the nearby soft tissues.  

The trachealis muscle uncovered in the posterior wall surface allows the trachea come contract and decrease the diameter. This muscle is crucial for coughing, acquiring rid the secretions, foreign particles, saliva, or food. The key reason why the C-shaped cartilage rings are incomplete is the they allow the trachea to narrow. This activity is helpful, especially once eating food, which needs the development of the esophagus.


Embryology

Lower respiratory tract tract development begins on work 22 and also divides into five stages: embryonic, pseudoglandular, canalicular, saccular, and alveolar. Although the advancement of the respiratory tract tract begins early in fetal development, complete maturation go not happen until roughly the eighth year the life.

Embryonic stage – 3 to 6 Weeks

The embryonic stage begins at around day 22 as soon as the respiratory diverticulum creates on the ventral wall surface of the foregut endoderm. ~ one week of caudal elongation, the trachea bifurcates and also forms the right and also left primary bronchial buds. The formation of the primary bronchial sprout marks the start of branching morphogenesis. ~ the 5th week the development, the right primary bronchial bud divides right into three an additional bronchial buds, and the left primary bronchial bud divides into two second bronchial buds. The secondary bronchial buds will ultimately give rise to the lobes of every lung. In ~ the finish of the sixth week the development, the second bronchial buds additional divide to type the tertiary bronchial buds, which will become the bronchopulmonary segments in the completely developed lung. The development of the tertiary bronchial sprout is the critical round that branching the occurs throughout the embryonic stage of development.

Pseudoglandular stage – 5 to 17 Weeks                

The pseudoglandular stage starts at about the 5th week and results in the development of the bronchial tree. The tertiary bronchial buds proceed to experience branching morphogenesis till the finish of week 16. Respiratory tract epithelium differentiation starts during this stage once cilia type on the surface ar of columnar epithelial cells in the proximal airway. Differentiation climate progresses in a proximal-to-distal pattern because that the duration of this step of quick branching. At the end of this stage, the respiratory tree has arisen until the terminal bronchioles with the associated arterial system, cartilage, and also smooth muscle.  

Canalicular stage – 16 to 25 Weeks

The canalicular stage starts at roughly 16 weeks together the terminal bronchioles continue to elongate, forming an acinus created of respiratory tract bronchioles. Splanchnic mesenchyme angiogenesis neighboring the newly created acinus offers rise come a capillary network the will role as part of the blood-air barrier. Throughout this stage, type II pneumocytes begin producing surfactant and also storing the in intracellular lamellar bodies. The presence of lamellar bodies can help to distinguish form II pneumocytes from other cell types. Once secreted, surfactant is critical in reducing alveolar surface ar tension and preventing atelectasis.

Saccular stage – 24 Weeks to Birth

The saccular stage begins at about 24 weeks and demonstrates a far-ranging expansion in the gas-exchange surface area and termination the branching morphogenesis. Throughout this stage, the terminal (acinar) portions of the prayer expand, forming large airspaces lined through primarily form I pneumocytes known as saccules. Saccules are separated by primary septa written of a double-layered capillary network and also a main layer of condensed mesenchyme. By the end of this stage, the capillaries contained in the major septa invade the wall surfaces of separation, personal, instance saccules, creating the blood-air barrier.  

Alveolar phase – 36 Weeks to 8 Years

The alveolar stage starts at approximately 36 weeks with the illustration of alveolar protrusions in the primary septa of individual saccules. The new septations developed are described as an additional septa to differentiate them from the primary septa formed throughout branching morphogenesis. These second septa will at some point be responsible because that the division of saccules right into alveoli. The process of alveolar department begins together a burst and gradually slows throughout the first three year of life. During this time of rapid alveolar expansion, the double-layered capillary network existing in immature primary and an additional septa fuses to create a single-layered capillary network. This microvascular maturation allows for an ext efficient gas exchange.

Until age three, the driver behind lung expansion is primarily raised numbers that alveoli. After major alveolarization is complete, lung expansion is a product that both second alveolarization and also enlargement that preexisting alveoli. Lung maturation is generally complete by the eighth year the life.<1><2><3><4>


Blood Supply and also Lymphatics

The arterial it is provided of the proximal trachea consists of the tracheoesophageal branches of the inferior thyroid artery and little branches indigenous the subclavian, interior mammary, and innominate artery. The distal trachea, carina, and also bronchi are gave by the bronchial arteries, which commonly originate from the proximal to decrease aorta. Prior to entering the trachea, every vessel branches superiorly and inferiorly throughout several tracheal rings, producing a lateral longitudinal anastomosis. From this anastomosis, transverse intercartilaginous arteries enter the space between the tracheal rings and also course one of two people anteriorly or posteriorly. The anterior transverse intercartilaginous arteries will travel between the rings and eventually end in the submucosal capillary plexus when they reach the midline. This plexus will certainly serve together the blood supply because that the tracheal cartilage while the membranous portions of the trachea space supplied straight by secondary branches the the tracheoesophageal arteries.

The dual blood it is provided of the lungs consists of the high-pressure bronchial arteries and the low-pressure pulmonary arteries. The bronchial arteries carry out 1% of complete blood circulation to the lungs and supply the non-respiratory portions of the lungs, visceral pleura, intrapulmonary blood ship walls, and lymphatic system. The pulmonary arteries administer the other 99% that the complete blood flow to the lung and bear the obligation for transferring deoxygenated blood from the right ventricle to the gas-exchange portions of the lung.

Venous drainage the the bronchi wake up in component through the bronchial veins into the azygous and hemiazygous systems. The remaining blood drains into the pulmonary veins together they go back to the left heart.<5><6><7><8>


Nerves

The trachea obtain parasympathetic innervation indigenous branches that the vagus nerve (CNX). After exiting the skull, the bilateral vagus nerves take trip along the posterolateral aspect of the typical carotid arteries until entering the remarkable mediastinum. The appropriate recurrent laryngeal nerve branches first, overcome underneath the ideal subclavian artery and also travels superiorly along the appropriate tracheoesophageal groove. The left recurrent laryngeal nerve branches under the aortic arch, lateral come the ligamentum arteriosum, and travels superiorly in the left tracheoesophageal groove. As these nerves ascend, they provide off branches that carry out parasympathetic, somatic, and also motor innervation come the trachea. Both the left and also right recurrent laryngeal nerves then go into the larynx with the inferior constrictor muscle of the pharynx. Damage to the recurrent laryngeal nerves throughout surgery can cause vocal cord paralysis, resulting in hoarseness or finish loss of voice. Forgiveness innervation of the trachea is noted by the middle cervical ganglia bilaterally.   

Various anatomical variations of the recurrent laryngeal nerve have actually been identified and can pose challenges during surgery. A rare but serious sport is the non-recurrent appropriate laryngeal nerve that passes directly from the vagus nerve towards the larynx. This anomaly usually occurs in the visibility of an aberrant right subclavian artery the arises directly from the arch the the aorta after ~ the left subclavian artery. In this case, the right laryngeal nerve does no pass underneath the appropriate subclavian artery, make it prone to injury during surgical dissection.<9><8><10><11>


Muscles

The C-shaped cartilages the the trachea are associated posteriorly by a membranous wall, and a longitudinal piece of smooth muscle dubbed the trachealis muscle the is in contact with the anterior esophagus. The trachealis muscle functions to constrict the airway by pulling the cartilages together, allowing for raised expiratory force during coughing. Smooth muscle included in the walls of the lower conducting prayer is essential for bronchoconstriction.<11><8><12>


Physiologic Variants

Tracheoesophageal Fistula

A congenital tracheoesophageal fistula (TE fistula) occurs when an aberrant connection between the trachea and the esophagus forms in utero. This developmental abnormality generally presents in association with esophageal atresia and also appears to an outcome from wrong septation of the foregut. In a majority of cases, the proximal esophagus develops a blind pouch through an associated distal car fistula. Various other variants including esophageal atresia with proximal te fistula, dare fistula without esophageal atresia, and esophageal atresia through both proximal and distal car fistulas may likewise occur.

In cases of esophageal atresia with a distal car fistula, polyhydramnios is regularly a detect on prenatal imaging because of the i can not qualify of the child to sloop down amniotic fluid. ~ birth, this inability to swallow causes excessive salivation regurgitation ~ feeds. Infants additionally classically current with respiratory distress and choking. Urgent surgical intervention is required, and also the post-operative survival price is now greater than 90%.

Tracheomalacia

Tracheomalacia consists of a range of different conditions identified by weak of the trachea. This weakness, whether congenital or acquired, reasons increased collapsibility of the airway. Throughout normal breathing, pressure alters within the thoracic cavity reason the trachea to adjust shape. These pressure changes, in the visibility of reduced tracheal integrity, have the right to lead come partial or full airway collapse. This airway collapse usually becomes exacerbated as soon as pressure is increased, such together coughing or Valsalva. Symptom vary based upon the degree of trachea collapsibility and breathing patterns. If bronchial weakness/collapse is present, the problem is dubbed tracheobronchomalacia. Bronchoscopy is right now the gold typical of diagnosis for these conditions.<13><14><15>


Surgical Considerations

Operations entailing the tracheobronchial tree need an intimate understanding of common anatomy, common and uncommon variants, and surrounding anatomical structures. The surgeon need to be all set to secure the patient’s airway if complications arise.

When performing a tracheostomy, the cannula is normally inserted between the 2nd and 3rd tracheal rings. The tracheostomy should not be made in the an initial ring together it dangers subglottic stenosis. Performing a tracheostomy listed below the 3rd tracheal rings increases the risks of a tracheoinnominate artery fistula. In patients v a tracheostomy and a strict feeding tube under the esophagus, over there is constantly a peril of creating a tracheoesophageal fistula. One serious complication of this procedure is airway fire, which occurs if very oxygenated air current in the trachea becomes exposed to the electrocautery. Because of this possible complication, influenced oxygen have to remain below 30%, and a scalpel should be offered to enter the trachea.<9><6>


Clinical Significance

Infantile respiratory Distress Syndrome

Infantile respiratory distress syndrome, or IRDS, is often seen in premature birth births as result of either insufficient production that pulmonary surfactant or abnormal lung development. Due to the fact that surfactant production starts at 20 weeks gestation and continues with birth, the threat of IRDS is inversely proportional come gestational age. Decreased surfactant concentration boosts surface stress in the alveoli, leading to alveolar fallen on expiration and increased work of breathing. The inefficient ventilation and also gas exchange results in hypoxia and hypercarbia. Clinical indicators of respiratory tract distress in a newborn include labored breathing, tachypnea, and also nasal flaring. Notice recognition and also treatment the this syndrome are essential to protect against respiratory arrest.<16><17>

Asthma

Asthma is a usual inflammatory condition of the airway that can range from gentle respiratory compromise to life-threatening airway closure. That is classically thought about a disease of the conducting airways and generally spares the lung parenchyma. Although the etiology is not well understood, influenced individuals usually have a genetic predisposition to an allergy the is exacerbated by a range of environmental components that cause inflammation. The hallmarks the asthma incorporate bronchial hyperresponsiveness, chronic inflammation, and also reversible airway obstruction.  Symptoms incorporate coughing, wheezing, chest tightness, and also shortness the breath. Symptom generally start in childhood, however the visibility of symptoms early on in life does no guarantee disease persistence into adulthood. Diagnosis is made when airway obstruction shows to it is in reversible v pulmonary function testing. Treatment is based on symptom severity and requires routine monitoring and adjustment to optimize an illness control. Today, the gold traditional of treatment has inhaled corticosteroids for maintenance treatment supplemented through a short-acting beta-agonist offered as required for the quick relief of symptoms. Without sufficient recognition and also treatment, mortality for this condition is high.

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Other Issues

Alpha-1 Antitrypsin Deficiency

Alpha-1 Antitrypsin deficiency is one autosomal recessive genetic problem that mainly affects the lungs and also the liver. Lessened serum alpha-1 antitrypsin protein levels characterize the disease. The liver to produce this enzyme, and also it is responsible for the deactivation that proteolytic enzymes released during immune responses in lung tissue, an especially neutrophilic elastase. Since lung organization has constant exposure to airborne pathogens, the capacity to downregulate the immune response is an important for the maintenance of healthy lung parenchyma. Poor serum alpha-1 antitrypsin levels result in unchecked inflammation responses in the lungs and also the eventual destruction of lung tissue, top to steady COPD and panacinar emphysema.  Symptoms of a homozygous recessive carrier of the disease generally manifest at an early stage in adulthood. Additional risk factors, such together smoking, can accelerate the an illness process and further diminish life expectancy.<21>