The trachea begins to form in the second month of embryo development, becoming longer and more fixed in its position over time. Itsepithelium is lined withcolumn-shaped cells that have hair-like extensions calledcilia, with scatteredgoblet cells that produce protectivemucins. The trachea can be affected by inflammation or infection, usually as a result of a viral illness affecting other parts of therespiratory tract, such as the larynx and bronchi, calledcroup, that can result in a cough. Infection withbacteria usually affects the trachea only and can cause narrowing or even obstruction. As a major part of the respiratory tract, the trachea, when obstructed, prevents air from entering the lungs; thus, atracheostomy may be required. Additionally, during surgery, ifmechanical ventilation is required duringanaesthesia,a tube is inserted into the trachea: this is calledtracheal intubation.
An adult's trachea has an inner diameter of about 1.5 to 2 centimetres (1⁄2 to3⁄4 in) and a length of about10 to 11 cm (4 to4+1⁄4 in), wider in males than females.[2] The trachea begins at the lower edge of thecricoid cartilage of the larynx[3] at the level of sixthcervical 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 fourththoracic vertebra (T4),[2] although its position may change withbreathing.[3] The trachea is surrounded by 16–20 rings ofhyaline cartilage; these 'rings' are 4 millimetres high in the adult, incomplete and C-shaped.[2] Ligaments connect the rings.[3] Thetrachealis 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]
The trachea passes by many structures of theneck 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; thejugular arch, which joins the twoanterior jugular veins, sits in front of the upper part of the trachea. Thesternohyoid andsternothyroid 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 theinferior thyroid veins below it.[2] In front of the lower trachea lies the manubrium of thesternum, theremnants of thethymus in adults. To the front left lie the large blood vessels theaortic arch and its branches the leftcommon carotid artery and thebrachiocephalic trunk; and the leftbrachiocephalic vein. The deepcardiac plexus and lymph nodes are also positioned in front of the lower trachea.[2]
The upper part of trachea receives and drainsblood through theinferior 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 intoinferior 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 thebronchial arteries, which are direct branches from theaorta, to supply blood to the trachea.[2] Thelymphatic vessels of the trachea drain into thepretracheal nodes that lie in front of the trachea, andparatracheal lymph nodes that lie beside it.[2]
In the fourth week ofdevelopment of the human embryo as therespiratory bud grows, the trachea separates from theforegut through the formation of ridges which eventually separate the trachea from the oesophagus, thetracheoesophageal 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 begun 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 2 cm 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]
The trachea is lined with a layer ofinterspersed layers of column-shaped cells with cilia.[3] Theepithelium containsgoblet cells, which areglandular, column-shaped cells that producemucins, the main component ofmucus. Mucus helps to moisten and protect the airways.[7] Mucus lines theciliated 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 thestomach or expelled asphlegm. This self-clearing mechanism is termedmucociliary clearance.[8] Directly beneath this mucus layer lies the submucosa layer which is composed primarily of fibrous connective tissue and connects the mucosa to the rings of hyaline cartilage beneath.[9]
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 maycalcify withage.
Cross-section
Cross-section of the trachea, with pseudostratified ciliated columnar epithelium and goblet cells labelled
Magnified cross-section of the cartilage of the trachea
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.
An example ofstridor, which develops when the trachea is narrowed or obstructed
A trachea may benarrowed or compressed, usually a result of enlarged nearby lymph nodes; cancers of the trachea or nearby structures; large thyroidgoitres; or rarely as a result of other processes such asunusually swollen blood vessels.[13] Scarring fromtracheobronchial injury or intubation; or inflammation associated withgranulomatosis with polyangiitis may also cause a narrowing of the trachea (tracheal stenosis).[13] Obstruction invariably causes a harsh breathing sound known asstridor.[13] A camera inserted via the mouth down into the trachea, calledbronchoscopy, may be performed to investigate the cause of an obstruction.[13] Management of obstructions depends on the cause. Obstructions as a result of malignancy may be managed with surgery,chemotherapy orradiotherapy.[13] A stent may be inserted over the obstruction. Benign lesions, such as narrowing resulting from scarring, are likely to be surgically excised.[13]
One cause of narrowing istracheomalacia, 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 decreasedcompliance.[14] 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.[14] Congenital tracheomalacia can occur by itself or in association with other abnormalities such asbronchomalacia orlaryngomalacia, andabnormal connections between the trachea and the oesophagus, amongst others.[14] Congenital tracheomalacia often improves without specific intervention; when required, interventions may includebeta agonists andmuscarinic agonists, which enhance the tone of the smooth muscle surrounding the trachea;positive pressure ventilation, or surgery, which may include the placement of astent, or the removal of the affected part of the trachea.[14] In dogs, particularly miniature dogs andtoy dogs, tracheomalacia, as well asbronchomalacia,[15] can lead totracheal collapse, which often presents with a honking goose-like cough.[16]
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 somemartial arts.[17]
Tracheal intubation refers to the insertion of atube down the trachea.[18] This procedure is commonly performed duringsurgery, 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 ananaesthetist during surgery.
In an emergency, or when tracheal intubation is deemed impossible, atracheotomy 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 atracheostomy.[19] Another method procedure can be carried, in an emergency situation, and this is acricothyrotomy.[20]
Tracheal agenesis[21] is a rarebirth defect in which the trachea fails to develop. The defect is usually fatal though sometimes surgical intervention has been successful.
Atracheoesophageal fistula is a congenital defect in which the trachea and esophagus are abnormally connected (afistula). 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, orVACTERL syndrome.[6] Such fistulas may be detected before a baby is born because ofexcess amniotic fluid; after birth, they are often associated with pneumonitis and pneumonia because ofaspiration of food contents.[6] Congenital fistulas are often treated by surgical repair.[13] 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.[13] Atracheo-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.[22]
Sometimes as ananatomical variation one or more of the tracheal rings are formed as complete rings, rather than horseshoe shaped rings. TheseO rings are smaller than the normal C-shaped rings and can cause narrowing (stenosis) of the trachea, resulting in breathing difficulties. An operation called aslide tracheoplasty can open up the rings and rejoin them as wider rings, shortening the length of the trachea.[23] Slide tracheoplasty is said to be the best option in treating tracheal stenosis.[24]
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.[25]
From 2008, operations have experimentally replaced tracheas, with those grown fromstem cells, or with synthetic substitutes, however this is regarded as experimental and there is no standardised method.[26] 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 frombone marrow on the trachea as a way of stimulating tissue regeneration, and such a method remains hypothetical.[26]
In January 2021, surgeons atMount 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.[27]
Allowing for variations in the length of the neck, the trachea in othermammals is, in general, similar to that in humans. Generally, it is also similar to thereptilian trachea.[28]
Inbirds, the trachea runs from thepharynx to thesyrinx, 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 beossified.[28]
Inamphibians, 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-neckedsalamanders, and incaecilians. While there are irregular cartilagenous nodules on the amphibian trachea, these do not form the rings found inamniotes.[28]
The onlyvertebrates to have lungs, but no trachea, are thelungfish and thePolypterus, in which the lungs arise directly from the pharynx.[28]
Tracheal system of dissectedcockroach. 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 thecrop of the cockroach. Scale bar, 2 mm.
Ininsects, the wordtrachea describes a different organ than in vertebrates. Insects have an openrespiratory system made up ofspiracles, deepairways (tracheae) andtracheoles to conduct air directly to and from metabolic tissues.[29] The distribution of spiracles can vary greatly among the manyorders of insects, but in general eachbody segment can have only one pair of spiracles, each of which connects to an air cavity calledatrium and has a relatively large tracheal tube behind it. The tracheae are invaginations of the cuticularexoskeleton that branch (anastomose) throughout the body with diameters from only a few micrometres up to 0.8 mm.Diffusion of oxygen andcarbon dioxide takes place across the walls of the smallest tubes, called tracheoles, which penetrate tissues and even indent individual cells.[30] Gas may be conducted through the respiratory system by means of activeventilation or passive diffusion. Unlike vertebrates, insects do not generally carry oxygen in theirhemolymph.[31]This is one of the factors that may limit their size.
A tracheal tube may contain ridge-like circumferential rings oftaenidia in variousgeometries such as loops orhelices. Taenidia provide strength and flexibility to the trachea. In the head, thorax, orabdomen, tracheae may also be connected to air sacs. Many insects, such asgrasshoppers andbees, 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 agill, or function essentially as normal, via aplastron. Note that despite being internal, the tracheae of arthropods are lined with cuticular tissue and are shed during moulting (ecdysis).[30]
^abcdefghijklmnopqStandring S (2016). "Trachea and bronchi". In Standring S (ed.).Gray's anatomy : the anatomical basis of clinical practice (41st ed.). Philadelphia. pp. 965–969.ISBN9780702052309.OCLC920806541.{{cite book}}: CS1 maint: location missing publisher (link)
^Dennie CJ, Coblentz CL (April 1993). "The trachea: normal anatomic features, imaging and causes of displacement".Canadian Association of Radiologists Journal.44 (2):81–9.PMID8462036.
^abcdefSadley TW (2019).Langman's medical embryology (14th ed.). Philadelphia: Wolters Kluwer. pp. 223–229.ISBN9781496383907.
^Antunes MB, Cohen NA (February 2007). "Mucociliary clearance--a critical upper airway host defense mechanism and methods of assessment".Current Opinion in Allergy and Clinical Immunology.7 (1):5–10.doi:10.1097/aci.0b013e3280114eef.PMID17218804.S2CID9551913.
^Ayyalasomayajula, Venkat, and Bjørn Skallerud. "Microstructure and Mechanics of the Bovine Trachea: Layer Specific Investigations through SHG Imaging and Biaxial Testing." Journal of the Mechanical Behavior of Biomedical Materials, vol. 134, 1 Oct. 2022, p. 5,https://doi.org/10.1016/j.jmbbm.2022.105371.
^abcdefTristram D (2019). "Laryngitis, Tracheitis, Epiglottitis, and Bronchiolitis: Sore Throat, Change in Voice, Feverora Wheezing Infant in Respiratory Distress".Introduction to Clinical Infectious Diseases. Springer International Publishing. pp. 75–85.doi:10.1007/978-3-319-91080-2_7.ISBN978-3-319-91080-2.S2CID86826724.
^abcdefghRalston SH, Penman ID, Strachan MW, Hobson RP, eds. (2018).Davidson's principles and practice of medicine (23rd ed.). Elsevier.ISBN978-0-7020-7028-0.
^Maggiore AD (January 2014). "Tracheal and airway collapse in dogs".The Veterinary Clinics of North America. Small Animal Practice.44 (1):117–127.doi:10.1016/j.cvsm.2013.09.004.PMID24268337.
^Santiago-Rosado, Livia M.; Sigmon, David F.; Lewison, Cheryl S. (2022)."Tracheal Trauma".StatPearls. StatPearls Publishing.PMID29763191. Retrieved23 October 2022.
^Menon B, Aggarwal B, Iqbal A (January 2008). "Mounier-Kuhn syndrome: report of 8 cases of tracheobronchomegaly with associated complications".Southern Medical Journal.101 (1):83–87.doi:10.1097/SMJ.0b013e31815d4259.PMID18176298.
^Wasserthal, Lutz T. (1998). Chapter 25: The Open Hemolymph System of Holometabola and Its Relation to the Tracheal Space. In "Microscopic Anatomy of Invertebrates". Wiley-Liss, Inc.ISBN0-471-15955-7.
^abTriplehorn C, Johnson NF, Borror DJ (2005).Borror and DeLong's introduction to the study of insects (7th ed.). Belmont, CA: Thompson Brooks/Cole. pp. 28–29.ISBN978-0030968358.OCLC55793895.
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