Trachea

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Trachea
Illu conducting passages.svg
Conducting passages
Details
Pronunciation /trəˈkə,ˈtrkiə/ [1]
Part of Respiratory tract
Artery tracheal branches of inferior thyroid artery
Vein brachiocephalic vein, azygos vein accessory hemiazygos vein
Identifiers
Latin trachea
MeSH D014132
TA98 A06.3.01.001
TA2 3213
FMA 7394
Anatomical terminology

The trachea (pl.: tracheae or tracheas), also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all animals with lungs. The trachea extends from the larynx and branches into the two primary bronchi. At the top of the trachea the cricoid cartilage attaches it to the larynx. The trachea is formed by a number of horseshoe-shaped rings, joined together vertically by overlying ligaments, and by the trachealis muscle at their ends. The epiglottis closes the opening to the larynx during swallowing.

Contents

The trachea begins to form in the second month of embryo development, becoming longer and more fixed in its position over time. It is epithelium lined with column-shaped cells that have hair-like extensions called cilia, with scattered goblet cells that produce protective mucins. The trachea can be affected by inflammation or infection, usually as a result of a viral illness affecting other parts of the respiratory tract, such as the larynx and bronchi, called croup, that can result in a cough. Infection with bacteria usually affects the trachea only and can cause narrowing or even obstruction. As a major part of the respiratory tract, when obstructed the trachea prevents air entering the lungs and so a tracheostomy may be required if the trachea is obstructed. Additionally, during surgery if mechanical ventilation is required when a person is sedated, a tube is inserted into the trachea, called intubation.

The word trachea is used to define a very different organ in invertebrates than in vertebrates. Insects have an open respiratory system made up of spiracles, tracheae, and tracheoles to transport metabolic gases to and from tissues.

Structure

Blausen 0865 TracheaAnatomy.png

An adult's trachea has an inner diameter of about 1.5 to 2 centimetres (12 to 34 in) and a length of about 10 to 11 cm (4 to 4+14 in), wider in males than females. [2] The trachea begins at the lower edge of the cricoid cartilage of the larynx [3] at the level of sixth cervical vertebra (C6) [2] and ends at the carina, the point where the trachea branches into left and right main bronchi. [2] , at the level of the fourth thoracic vertebra (T4), [2] although its position may change with breathing. [3] The trachea is surrounded by 16–20 rings of hyaline cartilage; these 'rings' are 4 millimetres high in the adult, incomplete and C-shaped. [2] Ligaments connect the rings. [3] The trachealis muscle connects the ends of the incomplete rings and runs along the back wall of the trachea. [3] Also adventitia, which is the outermost layer of connective tissue that surrounds the hyaline cartilage, contributes to the trachea's ability to bend and stretch with movement. [4]

Although trachea is a midline structure, it can be displaced normally to the right by the aortic arch. [5]

Nearby structures

The trachea passes by many structures of the neck and chest (thorax) along its course.

In front of the upper trachea lies connective tissue and skin. [2] Several other structures pass over or sit on the trachea; the jugular arch, which joins the two anterior jugular veins, sits in front of the upper part of the trachea. The sternohyoid and sternothyroid muscles stretch along its length. The thyroid gland also stretches across the upper trachea, with the isthmus overlying the second to fourth rings, and the lobes stretching to the level of the fifth or sixth cartilage. [2] The blood vessels of the thyroid rest on the trachea next to the isthmus; superior thyroid arteries join just above it, and the inferior thyroid veins below it. [2] In front of the lower trachea lies the manubrium of the sternum, the remnants of the thymus in adults. To the front left lie the large blood vessels the aortic arch and its branches the left common carotid artery and the brachiocephalic trunk; and the left brachiocephalic vein. The deep cardiac plexus and lymph nodes are also positioned in front of the lower trachea. [2]

Behind the trachea, along its length, sits the oesophagus, followed by connective tissue and the vertebral column. [2] To its sides run the carotid arteries and inferior thyroid arteries; and to its sides on its back surface run the recurrent laryngeal nerves in the upper trachea, and the vagus nerves in the lower trachea. [2]

The trachealis muscle contracts during coughing, reducing the size of the lumen of the trachea. [3]

Blood and lymphatic supply

Lymph nodes of the trachea Gray622.png
Lymph nodes of the trachea

The upper part of trachea receives and drains blood through the inferior thyroid arteries and veins; [2] the lower trachea receives blood from bronchial arteries. [3] Arteries that supply the trachea do so via small branches that supply the trachea from the sides. As the branches approach the wall of the trachea, they split into inferior and superior branches, which join with the branches of the arteries above and below; these then split into branches that supply the anterior and posterior parts of the trachea. [3] The inferior thyroid arteries arise just below the isthmus of the thyroid, which sits atop the trachea. These arteries join ( anastamoses ) with ascending branches of the bronchial arteries, which are direct branches from the aorta, to supply blood to the trachea. [2] The lymphatic vessels of the trachea drain into the pretracheal nodes that lie in front of the trachea, and paratracheal lymph nodes that lie beside it. [2]

Development

In the fourth week of development of the human embryo as the respiratory bud grows, the trachea separates from the foregut through the formation of ridges which eventually separate the trachea from the oesophagus, the tracheoesophageal septum. This separates the future trachea from the oesophagus and divides the foregut tube into the laryngotracheal tube. [6] By the start of the fifth week, the left and right main bronchi have begin to form, initially as buds at the terminal end of the trachea. [6]

The trachea is no more than 4 mm in diameter during the first year of life, expanding to its adult diameter of approximately 2cm by late childhood. [2] [3] The trachea is more circular and more vertical in children compared to adults, [3] varies more in size, and also varies more in its position in relation to its surrounding structures. [2]

Microanatomy

The trachea is lined with a layer of interspersed layers of column-shaped cells with cilia. [3] The epithelium contains goblet cells, which are glandular, column-shaped cells that produce mucins, the main component of mucus. Mucus helps to moisten and protect the airways. [7] Mucus lines the ciliated cells of the trachea to trap inhaled foreign particles that the cilia then waft upward toward the larynx and then the pharynx where it can be either swallowed into the stomach or expelled as phlegm. This self-clearing mechanism is termed mucociliary clearance. [8]

The trachea is surrounded by 16 to 20 rings of hyaline cartilage; these 'rings' are incomplete and C-shaped. [2] Two or more of the cartilages often unite, partially or completely, and they are sometimes bifurcated at their extremities. The rings are generally highly elastic but they may calcify with age.

Function

The trachea's main function is to transport air to and from the lungs. It also helps to warm, humidify, and filter the air before it reaches the lungs.

The trachea is made up of rings of cartilage, which help to keep it open and prevent it from collapsing. The inside of the trachea is lined with a mucous membrane, which produces mucus to help trap dirt and dust particles. The cilia, which are tiny hairs that line the mucous membrane, help to move the mucus and trapped particles up and out of the trachea.

Clinical significance

Inflammation and infection

Inflammation of the trachea is known as tracheitis, usually due to an infection. It is usually caused by viral infections, [9] with bacterial infections occurring almost entirely in children. [10] Most commonly, infections occur with inflammation of other parts of the respiratory tract, such as the larynx and bronchi, known as croup, [10] [9] however bacterial infections may also affect the trachea alone, although they are often associated with a recent viral infection. [9] Viruses that cause croup are generally the parainfluenza viruses 1–3, with influenza viruses A and B also causing croup, but usually causing more serious infections; bacteria may also cause croup and include Staphylococcus aureus , Haemophilus influenzae , Streptococcus pneumoniae and Moraxella catarrhalis . [9] Causes of bacterial infection of the trachea are most commonly Staphylococcus aureus and Streptococcus pneumoniae . [11] In patients who are in hospital, additional bacteria that may cause tracheitis include Escherichia coli , Klebsiella pneumoniae , and Pseudomonas aeruginosa . [9]

A person affected with tracheitis may start with symptoms that suggest an upper respiratory tract infection such as a cough, sore throat, or coryzal symptoms such as a runny nose. Fevers may develop and an affected child may develop difficulty breathing and sepsis. [9] [10] Swelling of the airway can cause narrowing of the airway, causing a hoarse breathing sound called stridor, or even cause complete blockage. [10] Up to 80% of people affected by bacterial tracheitis require the use of mechanical ventilation, and treatment may include endoscopy for the purposes of acquiring microbiological specimens for culture and sensitivity, as well as removal of any dead tissue associated with the infection. Treatment in such situations usually includes antibiotics. [10]

Narrowing

An example of stridor, which develops when the trachea is narrowed or obstructed

A trachea may be narrowed or compressed, usually a result of enlarged nearby lymph nodes; cancers of the trachea or nearby structures; large thyroid goitres; or rarely as a result of other processes such as unusually swollen blood vessels. [12] Scarring from tracheobronchial injury or intubation; or inflammation associated with granulomatosis with polyangiitis may also cause a narrowing of the trachea (tracheal stenosis). [12] Obstruction invariably causes a harsh breathing sound known as stridor. [12] A camera inserted via the mouth down into the trachea, called bronchoscopy, may be performed to investigate the cause of an obstruction. [12] Management of obstructions depends on the cause. Obstructions as a result of malignancy may be managed with surgery, chemotherapy or radiotherapy. [12] A stent may be inserted over the obstruction. Benign lesions, such as narrowing resulting from scarring, are likely to be surgically excised. [12]

One cause of narrowing is tracheomalacia, which is the tendency for the trachea to collapse when there is increased external pressure, such as when airflow is increased during breathing in or out, due to decreased compliance. [13] It can be due to congenital causes, or due to things that develop after birth, such as compression from nearby masses or swelling, or trauma. [13] Congenital tracheomalacia can occur by itself or in association with other abnormalities such as bronchomalacia or laryngomalacia, and abnormal connections between the trachea and the oesophagus, amongst others. [13] Congenital tracheomalacia often improves without specific intervention; when required, interventions may include beta agonists and muscarinic agonists, which enhance the tone of the smooth muscle surrounding the trachea; positive pressure ventilation, or surgery, which may include the placement of a stent, or the removal of the affected part of the trachea. [13] In dogs, particularly miniature dogs and toy dogs, tracheomalacia, as well as bronchomalacia, [14] can lead to tracheal collapse, which often presents with a honking goose-like cough. [15]

Injury

The trachea may be injured by trauma such as in a vehicle accident, or intentionally by another wilfully inflicting damage for example as practiced in some martial arts. [16]

Intubation

Tracheal intubation refers to the insertion of a tube down the trachea. [17] This procedure is commonly performed during surgery, in order to ensure a person receives enough oxygen when sedated. The catheter is connected to a machine that monitors the airflow, oxygenation and several other metrics. This is often one of the responsibilities of an anaesthetist during surgery.

In an emergency, or when tracheal intubation is deemed impossible, a tracheotomy is often performed to insert a tube for ventilation, usually when needed for particular types of surgery to be carried out so that the airway can be kept open. The provision of the opening via a tracheotomy is called a tracheostomy. [18] Another method procedure can be carried, in an emergency situation, and this is a cricothyrotomy. [19]

Congenital disorders

Tracheal diverticulum as seen on axial CT imaging Tracheal diverticulum.png
Tracheal diverticulum as seen on axial CT imaging

Tracheal agenesis [20] is a rare birth defect in which the trachea fails to develop. The defect is usually fatal though sometimes surgical intervention has been successful.

A tracheoesophageal fistula is a congenital defect in which the trachea and esophagus are abnormally connected (a fistula ). This is because of abnormalities in the separation between the trachea and oesophagus during development. [6] This occurs in approximately 1 in 3,000 births, and the most common abnormalities is a separation of the upper and lower ends of the oesophagus, with the upper end finishing in a closed pouch. [6] Other abnormalities may be associated with this, including cardiac abnormalities, or VACTERL syndrome. [6] Such fistulas may be detected before a baby is born because of excess amniotic fluid; after birth, they are often associated with pneumonitis and pneumonia because of aspiration of food contents. [6] Congenital fistulas are often treated by surgical repair. [12] In adults, fistulas may occur because of erosion into the trachea from nearby malignant tumours, which erode into both the trachea and the oesophagus. Initially, these often result in coughing from swallowed contents of the oesophagus that are aspirated through the trachea, often progressing to fatal pneumonia; there is rarely a curative treatment. [12] A tracheo-oesophageal puncture is a surgically created hole between the trachea and the esophagus in a person who has had their larynx removed. Air travels upwards from the surgical connection to the upper oesophagus and the pharynx, creating vibrations that create sound that can be used for speech. The purpose of the puncture is to restore a person's ability to speak after the vocal cords have been removed. [21]

Sometimes as an anatomical variation one or more of the tracheal rings are formed as complete rings, rather than horseshoe shaped rings. These O rings are smaller than the normal C-shaped rings and can cause narrowing ( stenosis ) of the trachea, resulting in breathing difficulties. An operation called a slide tracheoplasty can open up the rings and rejoin them as wider rings, shortening the length of the trachea. [22] Slide tracheoplasty is said to be the best option in treating tracheal stenosis. [23]

Mounier-Kuhn syndrome is a rare congenital disorder of an abnormally enlarged trachea, characterised by absent elastic fibres, smooth muscle thinning, and a tendency to get recurrent respiratory tract infections. [24]

Replacement

From 2008, operations have experimentally replaced tracheas, with those grown from stem cells, or with synthetic substitutes, however this is regarded as experimental and there is no standardised method. [25] Difficulties with ensuring adequate blood supply to the replaced trachea is considered a major challenge to any replacement. Additionally, no evidence has been found to support the placement of stem cells taken from bone marrow on the trachea as a way of stimulating tissue regeneration, and such a method remains hypothetical. [25]

In January 2021, surgeons at Mount Sinai Hospital in New York performed the first complete trachea transplantation. The 18-hour procedure included harvesting a trachea from a donor and implanting it in the patient, connecting numerous veins and arteries to provide sufficient blood flow to the organ. [26]

Other animals

Allowing for variations in the length of the neck, the trachea in other mammals is, in general, similar to that in humans. Generally, it is also similar to the reptilian trachea. [27]

Vertebrates

In birds, the trachea runs from the pharynx to the syrinx, from which the primary bronchi diverge. Swans have an unusually elongated trachea, part of which is coiled beneath the sternum; this may act as a resonator to amplify sound. In some birds, the tracheal rings are complete, and may even be ossified. [27]

In amphibians, the trachea is normally extremely short, and leads directly into the lungs, without clear primary bronchi. A longer trachea is, however, found in some long-necked salamanders, and in caecilians. While there are irregular cartilagenous nodules on the amphibian trachea, these do not form the rings found in amniotes. [27]

The only vertebrates to have lungs, but no trachea, are the lungfish and the Polypterus , in which the lungs arise directly from the pharynx. [27]

Invertebrates

Tracheal system of dissected cockroach. The largest tracheae run across the width of the body of the cockroach and are horizontal in this image. Scale bar, 2 mm. Tracheal system of dissected cockroach.tif
Tracheal system of dissected cockroach. The largest tracheae run across the width of the body of the cockroach and are horizontal in this image. Scale bar, 2 mm.
The tracheal system branches into progressively smaller tubes, here supplying the crop of the cockroach. Scale bar, 2 mm. Cockroach tracheae supplying crop.tiff
The tracheal system branches into progressively smaller tubes, here supplying the crop of the cockroach. Scale bar, 2 mm.

The word trachea is used to define a very different organ in invertebrates than in vertebrates. Insects have an open respiratory system made up of spiracles, tracheae, and tracheoles to transport metabolic gases to and from tissues. [28] The distribution of spiracles can vary greatly among the many orders of insects, but in general each segment of the body can have only one pair of spiracles, each of which connects to an atrium and has a relatively large tracheal tube behind it. The tracheae are invaginations of the cuticular exoskeleton that branch (anastomose) throughout the body with diameters from only a few micrometres up to 0.8 mm. Diffusion of oxygen and carbon dioxide takes place across the walls of the smallest tubes, called tracheoles, which penetrate tissues and even indent individual cells. [29] Gas may be conducted through the respiratory system by means of active ventilation or passive diffusion. Unlike vertebrates, insects do not generally carry oxygen in their hemolymph. [30] This is one of the factors that may limit their size.

A tracheal tube may contain ridge-like circumferential rings of taenidia in various geometries such as loops or helices. Taenidia provide strength and flexibility to the trachea. In the head, thorax, or abdomen, tracheae may also be connected to air sacs. Many insects, such as grasshoppers and bees, which actively pump the air sacs in their abdomen, are able to control the flow of air through their body. In some aquatic insects, the tracheae exchange gas through the body wall directly, in the form of a gill, or function essentially as normal, via a plastron. Note that despite being internal, the tracheae of arthropods are lined with cuticular tissue and are shed during moulting (ecdysis). [29]

Additional images

Related Research Articles

<span class="mw-page-title-main">Lung</span> Primary organ of the respiratory system

The lungs are the central organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of the heart. Their function in the respiratory system is to extract oxygen from the air and transfer it into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere, in a process of gas exchange. The pleurae, which are thin, smooth, and moist, serve to reduce friction between the lungs and chest wall during breathing, allowing for easy and effortless movements of the lungs.

<span class="mw-page-title-main">Tracheal intubation</span> Placement of a tube into the trachea

Tracheal intubation, usually simply referred to as intubation, is the placement of a flexible plastic tube into the trachea (windpipe) to maintain an open airway or to serve as a conduit through which to administer certain drugs. It is frequently performed in critically injured, ill, or anesthetized patients to facilitate ventilation of the lungs, including mechanical ventilation, and to prevent the possibility of asphyxiation or airway obstruction.

Tracheomalacia is a condition or incident where the cartilage that keeps the airway (trachea) open is soft such that the trachea partly collapses especially during increased airflow. This condition is most commonly seen in infants and young children. The usual symptom is stridor when a person breathes out. This is usually known as a collapsed windpipe.

<span class="mw-page-title-main">Brachiocephalic artery</span> Artery of the mediastinum

The brachiocephalic artery, brachiocephalic trunk, or innominate artery is an artery of the mediastinum that supplies blood to the right arm, head, and neck.

<span class="mw-page-title-main">Tracheotomy</span> Temporary surgical incision to create an airway into the trachea

Tracheotomy, or tracheostomy, is a surgical airway management procedure which consists of making an incision (cut) on the anterior aspect (front) of the neck and opening a direct airway through an incision in the trachea (windpipe). The resulting stoma (hole) can serve independently as an airway or as a site for a tracheal tube or tracheostomy tube to be inserted; this tube allows a person to breathe without the use of the nose or mouth.

<span class="mw-page-title-main">Respiratory tract</span> Organs involved in transmission of air to and from the point where gases diffuse into tissue

The respiratory tract is the subdivision of the respiratory system involved with the process of conducting air to the alveoli for the purposes of gas exchange in mammals. The respiratory tract is lined with respiratory epithelium as respiratory mucosa.

<span class="mw-page-title-main">Bronchus</span> Airway in the respiratory tract

A bronchus is a passage or airway in the lower respiratory tract that conducts air into the lungs. The first or primary bronchi to branch from the trachea at the carina 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 each hilum. 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. The bronchi, when too narrow to be supported by cartilage, are known as bronchioles. No gas exchange takes place in the bronchi.

<span class="mw-page-title-main">Epiglottis</span> Leaf-shaped flap in the throat that prevents food from entering the windpipe and the lungs

The epiglottis is a leaf-shaped flap in the throat that prevents food and water from entering the trachea and the lungs. It stays open during breathing, allowing air into the larynx. During swallowing, it closes to prevent aspiration of food into the lungs, forcing the swallowed liquids or food to go along the esophagus toward the stomach instead. It is thus the valve that diverts passage to either the trachea or the esophagus.

Stridor is a high-pitched extra-thoracic breath sound resulting from turbulent air flow in the larynx or lower in the bronchial tree. It is different from a stertor which is a noise originating in the pharynx.

<span class="mw-page-title-main">Tracheitis</span> Inflammation of the trachea

Tracheitis is an inflammation of the trachea. Although the trachea is usually considered part of the lower respiratory tract, in ICD-10 tracheitis is classified under "acute upper respiratory infections".

<span class="mw-page-title-main">Carina of trachea</span> Ridge of cartilage separating the openings of the main bronchi

The carina of trachea is a ridge of cartilage at the base of the trachea separating the openings of the left and right main bronchi.

<span class="mw-page-title-main">Lung bud</span> Embryological precursor of the respiratory system

The lung bud sometimes referred to as the respiratory bud forms from the respiratory diverticulum, an embryological endodermal structure that develops into the respiratory tract organs such as the larynx, trachea, bronchi and lungs. It arises from part of the laryngotracheal tube.

<span class="mw-page-title-main">Subglottic stenosis</span> Medical condition

Subglottic stenosis is a congenital or acquired narrowing of the subglottic airway. It can be congenital, acquired, iatrogenic, or very rarely, idiopathic. It is defined as the narrowing of the portion of the airway that lies between the vocal cords and the lower part of the cricoid cartilage. In a normal infant, the subglottic airway is 4.5-5.5 millimeters wide, while in a premature infant, the normal width is 3.5 millimeters. Subglottic stenosis is defined as a diameter of under 4 millimeters in an infant. Acquired cases are more common than congenital cases due to prolonged intubation being introduced in the 1960s. It is most frequently caused by certain medical procedures or external trauma, although infections and systemic diseases can also cause it.

<span class="mw-page-title-main">Branchial cleft cyst</span> Medical condition

A branchial cleft cyst or simply branchial cyst is a cyst as a swelling in the upper part of neck anterior to sternocleidomastoid. It can, but does not necessarily, have an opening to the skin surface, called a fistula. The cause is usually a developmental abnormality arising in the early prenatal period, typically failure of obliteration of the second, third, and fourth branchial cleft, i.e. failure of fusion of the second branchial arches and epicardial ridge in lower part of the neck. Branchial cleft cysts account for almost 20% of neck masses in children. Less commonly, the cysts can develop from the first, third, or fourth clefts, and their location and the location of associated fistulas differs accordingly.

<span class="mw-page-title-main">Laryngotracheal stenosis</span> Medical condition

Laryngotracheal stenosis refers to abnormal narrowing of the central air passageways. This can occur at the level of the larynx, trachea, carina or main bronchi. In a small number of patients narrowing may be present in more than one anatomical location.

<span class="mw-page-title-main">Tracheobronchial injury</span> Damage to the tracheobronchial tree

Tracheobronchial injury is damage to the tracheobronchial tree. It can result from blunt or penetrating trauma to the neck or chest, inhalation of harmful fumes or smoke, or aspiration of liquids or objects.

Double aortic arch is a relatively rare congenital cardiovascular malformation. DAA is an anomaly of the aortic arch in which two aortic arches form a complete vascular ring that can compress the trachea and/or esophagus. Most commonly there is a larger (dominant) right arch behind and a smaller (hypoplastic) left aortic arch in front of the trachea/esophagus. The two arches join to form the descending aorta which is usually on the left side. In some cases the end of the smaller left aortic arch closes and the vascular tissue becomes a fibrous cord. Although in these cases a complete ring of two patent aortic arches is not present, the term ‘vascular ring’ is the accepted generic term even in these anomalies.

<span class="mw-page-title-main">Tracheobronchomalacia</span> Medical condition

Tracheobronchomalacia (TBM) is a condition characterized by flaccidity of the tracheal support cartilage which leads to tracheal collapse. This condition can also affect the bronchi. There are two forms of this condition: primary TBM and secondary TBM. Primary TBM is congenital and starts as early as birth. It is mainly linked to genetic causes. Secondary TBM is acquired and starts in adulthood. It is mainly developed after an accident or chronic inflammation.

Tracheal agenesis is a rare birth defect with a prevalence of less than 1 in 50,000 in which the trachea fails to develop, resulting in an impaired communication between the larynx and the alveoli of the lungs. Although the defect is normally fatal, occasional cases have been reported of long-term survival following surgical intervention.

<span class="mw-page-title-main">Tracheoinnominate fistula</span> Medical condition

Tracheoinnominate fistula is an abnormal connection (fistula) between the innominate artery and the trachea. A TIF is a rare but life-threatening iatrogenic injury, usually the sequela of a tracheotomy.

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