Aug 8, Anatomy, Head and Neck, Cervical, Respiratory, Larynx, and Cricoarytenoid. In such cases, a tracheal tube or airway stents are used to bypass the obstruction and maintain the trachea open. Clipboard, Search History, and several other advanced features are temporarily unavailable. The lobes of thyroid gland which extend inferiorly until the sixth tracheal cartilage. The trachea is a D-shaped fibrocartilaginous respiratory organ. Careers. This is where the 2nd rib joins with the sternum. The conducting portion of the bronchial tree thus extends from the tracheal bifurcation to the terminal bronchiole, inclusively. The lobar bronchi then bifurcate into several segmental (tertiary) bronchi, each of which supplies a bronchopulmonary segment. These airways are located in the neck and thorax. These are connected by pores which allow air pressure to equalize between them. Aortic ach begins 8. Alveolar ducts are tubular portions with respiratory surfaces from which the alveolar sacs bud. Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2014). A helpful mnemonic for remembering the structures that lies at the level of the sternal angle is RATPLANT: Rib (2 nd), Aortic arch, Trachea birfurcation, Pulmonary trunk bifurcation, Left recurrent laryngeal (and Ligamentum arteriosum), Azygos vein, Nerves - cardiac plexus, Thoracic duct. The sternal angle, also known as the angle of Louis. 2023 A bronchopulmonary segment is a division of the lung that is separated from the rest of the lung by a septum of connective tissue, which is an advantage during surgery since a bronchopulmonary segment can be removed without affects other nearby segments. It consists of 16-20 tracheal cartilages anterolaterally and a fibromuscular wall posteriorly. In: StatPearls [Internet]. Altogether there are around 300 million alveoli in adult lungs, providing a large surface area for adequate gas exchange. The structure of its wall makes the trachea sufficiently flexible and elastic to permit the transient expansion of the esophagus during swallowing. 4. Value of tracheal bifurcation angle measurement as a radiographic sign of left atrial enlargement in dogs. 139 (5): 879-82. It is mandatory to procure user consent prior to running these cookies on your website. The trachea divides into the left and right main bronchus, which is known as the tracheal bifurcation, at the level of the sternal angle and of the fifth thoracic vertebra (or up to two vertebrae higher or lower, depending on lung volume changes due to breathing). Below is a summary of vertebral levels and associated internal or surface anatomy. The bronchioles consist of first the terminal bronchioles, then the respiratory bronchioles, and finally the alveolar sacs (which allow for gas exchange). Need a refresher on the basic anatomy of the respiratory system before diving into all things bronchi related? Learning anatomy is a massive undertaking, and we're here to help you pass with flying colours. Before Bifurcation of trachea. [2] In clinical applications, the sternal angle can be palpated at the T4 vertebral level. 2017 Jun;10(2):188-192. doi: 10.21053/ceo.2016.00185. The next set of muscles, the internal intercostals, are also oriented in an oblique fashion, orthogonally to the external intercostals. 10. Disclaimer. Other anatomic structures are present at the general level of this plane. The trachea is normally easy to follow inferi-orly until it bifurcates into right and left main bronchi. In addition, the mucous membrane will undergo a transition from ciliated pseudostratified columnar epithelium to simple cuboidal epithelium to simple squamous epithelium. [2] The carina occurs at the lower end of the trachea - usually at the level of the 4th to 5th thoracic vertebra. Required fields are marked *. Marks the level of the second costal cartilage (rib) for counting ribs or intercostal spaces. The left main bronchus enters the root of the left lung opposite to the sixth thoracic vertebra, passes underneath the aortic arch, and crosses in front of the esophagus, the thoracic duct, and the descending aorta. The amount of smooth muscle increases as the amount of cartilage decreases, and smooth muscle is also present continuously around the bronchi. In most cases, it ossifies with age. These terminal bronchioles branch even further into respiratory bronchioles, which are distinguishable by the presence of alveoli extending from their lumens. Kim Bengochea, Regis University, Denver. :interspinal line -The upper end of the trachea changes with..? and transmitted securely. Bifurcation of trachea at sternal angle What are the 3 components of the middle. The sternal angle is also called the angle of Louis, but the reason for that name was lost. Within the thorax at this point, the trachea is displaced slightly to the right by the arch of the aorta on its left. HHS Vulnerability Disclosure, Help The trachea and bronchi collectively form the tracheobronchial tree. In children, strong sutures can be used toput the sternum back together, but in all individuals above the age of 2, stainless steel wires are required to realign and close the sternum. 8600 Rockville Pike Since that time he has worked in a variety of different medical specialities. Learn the anatomy of all the remaining organs easily and efficiently using Kenhubs respiratory system quizzes and labeled diagrams! This is particularly useful when counting ribs to identify landmarks as rib one is often impalpable. sharing sensitive information, make sure youre on a federal During development, there are initially 10 segments per lung, but since the left lung only has 2 lobes, 2 pairs of bronchopulmonary segments fuse to give 8 total segments, with 4 for each lobe. [2], The carina occurs at the lower end of the trachea - usually at the level of the 4th to 5th thoracic vertebra. The thoracic part divides at the tracheal bifurcation into the right and left main bronchi. To find out more, read our privacy policy. 1970 Mar;108(3):546-9. doi: 10.2214/ajr.108.3.546. Along with branches of the pulmonary artery and veins, the main bronchi make up the roots of the lungs. This website uses cookies to improve your experience while you navigate through the website. Its main function is to transport oxygenated air from the upper respiratory tract towards the alveoli for gas exchange. They contain surfactant producing type II pneumocytes and Clara cells. The angle of Louis forms part of the ribcage. Aug 8, Anatomy, Head and Neck, Larynx Vocal Cords. Alice Ferng B.S., MD, PhD The trachea bifurcates and therefore terminates, superior to the heart at the level of the sternal angle. Unlike the lateral thorax, the manubrium and sternum have fewer nerves- and this explains why a sternotomy incision is less painful than a thoracotomy. Also, the angle varies, sometimes by 20%, in serial radiographs. Additionally, making an incision at the first or second rib interspace can result in damage to large, important blood vessels and the brachial plexus. Clinically oriented anatomy. Conducting bronchioles conduct airflow but do not contain any mucous glands or seromucous glands, Terminal bronchioles are the last division of the airway without respiratory surfaces. The trachea (windpipe) is found inferior to the thyroid cartilage and superior to division into the left and right main bronchus. Before Computed tomography-based bronchial tree three-dimensional reconstruction and airway resistance evaluation in adolescent idiopathic scoliosis. Additionally, making an incision at the first or second rib interspace can result in damage to large, important blood vessels and the brachial plexus. Common causes of congenital stridor in infants. It's important because we know that this level marks the level of the intervertebral discs which lies between thoracic vertebra T4 and T5. Microscopic anatomy of the lower respiratory tract of the grey short-tailed opossum (Monodelphis domestica). It is important to note that the right main bronchus is wider, shorter, and more vertical than the left main bronchus, and it enters the right lung at roughly the level of the fifth thoracic vertebra. The site is secure. Please enable it to take advantage of the complete set of features! MeSH Theinnermost set of intercostals are thin and are similar to the internal intercostal muscles in their orientation. The manubriosternal junction is the joint of the sternal body and the manubrium. Beyer B, Feipel V, Sholukha V, Chze L, Van Sint Jan S. In-vivo analysis of sternal angle, sternal and sternocostal kinematics in supine humans during breathing. Revise with our respiratory system quizzes and labeled diagrams. The segmental bronchi undergo further branching to form numerous smaller airways the bronchioles. Please enable it to take advantage of the complete set of features! At the level of the sternal angle, the trachea bifurcates into the right and left main bronchi. [1][2], The carina is a cartilaginous ridge separating the left and right main bronchi that is formed by the inferior-ward and posterior-ward prolongation of the inferior-most tracheal cartilage. The trachea can be palpated superior to the jugular notch of sternum, all the way to the cricoid cartilage. Descending aorta begins. Immature lungs of a preterm infant often fail to produce sufficient surfactant, which leads to respiratory problems. Necessary cookies are absolutely essential for the website to function properly. Lymphatic drainage is to the pretracheal and paratracheal (cervical, thoracic) lymph nodes that empty into the deep cervical lymph nodes. which is an imaginary plane passing from the sternal angle anteriorly to the lower border of the body of the 4th thoracic vertebra posteriorly; laterally by the pleurae; The free ends of these rings are supported by the trachealis muscle. The first two nerves supply the proximal sternum and manubrium. After sixth generation, the passageways are too narrow to be supported by the cartillage, and thus are called bronchioles(small bronchi). The sternum Q7 Identify structures at the level of the sternal angle. Kirum GG, Munabi IG, Kukiriza J, Tumusiime G, Kange M, Ibingira C, Buwembo W. Anatomical variations of the sternal angle and anomalies of adult human sterna from the Galloway osteological collection at Makerere University Anatomy Department. The trachea is only one part of the respiratory system. Robert H. Whitaker, Neil R. Borley. It is divided into two parts: The trachea and bronchi collectively form the tracheobronchial tree. Any cookies that may not be particularly necessary for the website to function and is used specifically to collect user personal data via analytics, ads, other embedded contents are termed as non-necessary cookies. Anatomy, Angle of Louis. Since the tracheal lumen is air filled, it appears black on CT images. 1988 Nov;6(4):639-69. The Angle of Louis. Such diffused involvement happens during Williams-Campbell syndrome. 164 (5): 1089-92. FOIA This plane passes anteriorly through the sternal angle and posteriorly to the inferior margin of fourth thoracic vertebra. Now that you've learned everything about the trachea, it's time to put that knowledge to the test with the following quiz! -, Benner A, Sharma P, Sharma S. StatPearls [Internet] StatPearls Publishing; Treasure Island (FL): 2022. National Library of Medicine If the wrong rib is counted, access to the internal chest organs can be difficult. The tracheal bifurcation houses a sagittally-oriented cartilaginous ridge called the carina. 2. The inferior articular surface is located on the superior border of the body of sternum. Instant Anatomy. Unable to process the form. The affected area can span only a few cartilages or affect the entire trachea. 3.6. . The angle decreases by 10 on expiration. Main bronchi: There are two supplying ventilation to each lung. Wani TM, Bissonnette B, Engelhardt T, Buchh B, Arnous H, AlGhamdi F, Tobias JD. The sternal angle marks the location of the: Bifurcation of the trachea Second costal cartilage O . The trachea, or windpipe, is a 10-11 cm long fibrocartilaginous tube of the lower respiratory tract. Therefore, it is more susceptible to foreign body obstructions. respiratory system quizzes and labeled diagrams. http://creativecommons.org/licenses/by-nc-nd/4.0/. Being a congenital anomaly, the tracheomalacia manifests in early infancy. The combination of sweeping movements by the cilia and mucus from the goblet cells forms the functional mucociliary escalator. Denotes level of aortic arch, bifurcation of trachea, and T4-T5 intervertebral disc. The PubMed wordmark and PubMed logo are registered trademarks of the U.S. Department of Health and Human Services (HHS). This category only includes cookies that ensures basic functionalities and security features of the website. These studies have revealed the presence ofa perforator artery into the sternum at each intercostal level as a branch from the internal mammary arteries. Asthma is a chronic inflammatory disorder of the airways, characterised by hypersensitivity, reversible outflow obstruction and bronchospasm. On this Wikipedia the language links are at the top of the page across from the article title. and grab your free ultimate anatomy study guide! The right main bronchus has a larger diameter and is . American journal of roentgenology. government site. The carina is the sagittally-oriented cartilaginous ridge at the bifurcation of the trachea and is an important reference point in chest imaging. LWW. -, De Rose V, Molloy K, Gohy S, Pilette C, Greene CM. There is remodelling of the small airways, causing increased smooth muscle thickness around the bronchioles, damaged epithelium and a thickened basement membrane. The second histological layer is the submucosa. 3. The mucosa represents the innermost layer and it is lined with pseudostratified ciliated columnar epithelium. Berdajs D, Znd G, Turina MI, Genoni M. Blood supply of the sternum and its importance in internal thoracic artery harvesting. -the head's position -adjacent organs with a connective tissue, the movement is very small (up to 1-2 cm) The length of the trachea is 10-12 cm and it is 1-2 cm longer in males -> 2 parts Careers. Unable to process the form. Edinburgh: Elsevier Churchill Livingstone. We also use third-party cookies that help us analyze and understand how you use this website. Abstracts of Presentations at the Association of Clinical Scientists 143. Segmental bronchi continue their branching until they reach the final, sixt generation of bronchi. 72. Formed of a cartilaginous skeleton of nine cartilages, it includes the important organs of the epiglottis and the vocal folds (vocal cords) which are the opening to the glottis. Then take a look at the next study unit and quiz. Reference article, Radiopaedia.org (Accessed on 04 Mar 2023) https://doi.org/10.53347/rID-57629. The angle between the manubrium and the body of the sternum at the manubriosternal junction. The sternal angle is a crucial clinical marker for locating numerous other anatomical . This joint is also known as the manubriosternal angle (angle of Louis) that serves as an important anatomical landmark. Thus, absolute measurements of the tracheal bifurcation angles are of little diagnostic value. Widening of the tracheal bifurcation on chest radiographs: value as a sign of left atrial enlargement. HHS Vulnerability Disclosure, Help official website and that any information you provide is encrypted All content published on Kenhub is reviewed by medical and anatomy experts. It is at the level of the T4-T5 intervertebral disc. This is the sternal angle. 2 nd costal cartilage. The bifurcation can be located anywhere between the levels of the fourth and seventh thoracic vertebrae. The information we provide is grounded on academic literature and peer-reviewed research. Is our article missing some key information? But opting out of some of these cookies may affect your browsing experience. Copyright A wide range of normal values was found. Another recent ratio used is the surfactant/albumin (S/A) ratio, where a S/A ratio of less than 35 indicates immature lungs, between 35-55 is indeterminate, and more than 55 indicates mature surfactant production (which correlates with a L/S ratio of 2.2 or greater). Wang J, Zhou B, Yang X, Zhou C, Ling T, Hu B, Song Y, Liu L. Eur Spine J. Parasympathetic supply originates from the recurrent laryngeal nerves, branches of the vagus nerve. An official website of the United States government. Epub 2011 Sep 15. It is roughly at the level of the bifurcation of the trachea. Bookshelf The structure of bronchi are very similar to that of the trachea, though differences are seen in the shape of their cartilage. In this article we will look at the anatomical position, structure and neurovascular supply of the airways; as well as considering their clinical relevance. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); We use cookies on our website to give you the most relevant experience by remembering your preferences and repeat visits. Learn about the sternal angle in this tutorial. Angle of Louis. ADVERTISEMENT: Supporters see fewer/no ads. Are you curious to find out more about the histology of the lungs? You also have the option to opt-out of these cookies. The airway can be subdivided into the upper and lower airway, each of which has numerous subdivisions as follows. Associated veins have been identified following a similar course. Reviewer: The trachea marks the beginning of the tracheobronchial tree. The right pulmonary artery lies initially below the right bronchus and then later in front of it. A bronchus, which is also known as a main or primary bronchus, represents the airway in the respiratory tract that conducts air into the lungs. 2. Contributed Illustration by Beckie Palmer, Sternum, Angle of Louis, Manubrium. Segmentum apicoposterius pulmonis sinistri. We found that the bifurcation of the trachea lay at the plane in 41% of subjects, that the plane passed through the concavity of the arch of the aorta in 49% of subjects, and that, although there was notable individual variation, the manubriosternal plane passed through the upper part of the fifth thoracic vertebra in 53% of cases. Projection of bifurcation of trachea -ant. At the time the article was created Joseph Anderson had no recorded disclosures. The tracheal carina is deep to the sternal angle. Trachea ends; primary bronchi begin T5-9 Body of sternum Heart 2011 - 2012 In the standing position trachea bifurcation is at T6 level . Philadelphia, PA: Lippincott Williams & Wilkins. Essential Clinical Surface Anatomy is available for purchasehere. Would you like email updates of new search results? Author: Epub 2020 Mar 26. The alveolus is the basic anatomic unit of gas exchange. van der Merwe AE, Weston DA, Oostra RJ, Maat GJ. Tracheal bifurcation angle. Dr. Marc Barton qualified from Imperial College School of Medicine in 2001. It's an important structure because it marks the location of other structures in the body. In between these runs the neurovascular bundle. Most commonly it is located at the level of the sternal angle and vertebra T5. If the bronchial cartilages are involved the condition is called bronchomalacia. FOIA (1982) AJR. In tracheomalacia, the tracheal cartilages become abnormally soft. It travels inferiorly into the superior mediastinum,bifurcating at the level of the sternal angle (forming the right and left main bronchi). The right branch continues behind the ascending aorta for a total length of 2.5 cm before branching to 3 secondary bronchi. If the tracheal collapse is severe, it can even manifest with the cessation of breathing (apnea). Tracheobronchial Angle Measurements in Children: An Anthropometric Retrospective Study With Multislice Computed Tomography. Each main bronchus divides into increasingly smaller intrapulmonary bronchi that supply the lungs; The trachea is related to many anatomical structures as it travels between the neck and mediastinum. -. Accessibility Using in-vivo spiral-CT data, the movement in the joint during forced breathing has been measured at approximately 4.4 degrees.[6]. The airway, or respiratory tract, describes the organs of the respiratory tract that allow airflow during ventilation. Emerg Med Clin North Am. At the time the article was last revised Luke Plant had no recorded disclosures. Moore KL, Agur AMR, Dalley AF. Question: Which of the following takes place at the sternal angle (MACA) Arch of the aorta Esophgus enters the anterior mediastinum Thoracic duct crosses left and posterior to the esophagus Bifurcation of the trachea Articulation of rib 2 . Bethesda, MD 20894, Web Policies Also, the horizontal plane that passes through the joint and the articular disc between the fourth and fifth thoracic vertebrae divides the mediastinum into superior and inferior parts. The outermost intercostal muscles (external intercostals) have fibers running in an oblique direction. 2015 Sep;59(9):533-41. doi: 10.4103/0019-5049.165849. American journal of roentgenology. Immune cells and their secretions (lysozyme, IgA antibodies, lymphocytes) prevent harmful microorganisms from reaching the lower airways. (Sternal angle not labeled, but visible at second, https://en.wikipedia.org/w/index.php?title=Sternal_angle&oldid=1119012903, Creative Commons Attribution-ShareAlike License 3.0, Carina of the trachea is deep to the sternal angle, Passage of the thoracic duct from right to left behind esophagus, Loop of left recurrent laryngeal nerve around aortic arch, This page was last edited on 30 October 2022, at 07:21. This can sometimes allow the heart to protrude through the sternum. Identification of the second rib and thus the second intercostal space inferiorly is also useful when auscultating heart sounds. Your email address will not be published. Normal tracheal bifurcation angle: a reassessment. It performs generic functions of the skeletal tissues; protection, mechanical leverage for movement, and support for other organs. Bifurcation of trachea at sternal angle What are the 3 components of the middle. Clinically Oriented Anatomy (7th ed.). Learning anatomy is a massive undertaking, and we're here to help you pass with flying colours. [3][4] This is in line with the sternal angle, but the carina may raise or descend up to two vertebrae higher or lower with breathing. Together, with the capillaries, they form the air-blood barrier. The interbronchial angle is the angle between the central axis of right and left main stem bronchi 1. The thoracic part divides at the tracheal bifurcation into the right and left main bronchi. Bronchioles lack supporting cartilage skeletons and have a diameter of around 1 mm. Anatomy, Head and Neck, Larynx Recurrent Laryngeal Nerve. The tracheal bifurcation angle was measured in 100 normal adult patients. The carina lies to the left of the midline, and runs antero-posteriorly (front to back). Unauthorized use of these marks is strictly prohibited. Call Us: (02) 9223 2502 . The sternum can protrude in pectus carinatum (known as pigeon breast due to its similarity to an avian shape of the ribcage). It is a palpable clinical landmark in surface anatomy. These airways are located in the neck and thorax. Ann Clin Lab Sci. Alveolar is the final portion of the airway and is lined with a single-cell layer of pneumocytes and in proximity to capillaries. They undergo further branching to produce the secondary bronchi. It marks the point at which the costal cartilages of the second rib articulate with the sternum. A clinically useful feature of the (manubriosternal) joint is that it can be palpated easily. Vertebral levels are very commonly tested on in both undergraduate and postgraduate medical examinations. The medical information on this site is provided as an information resource only, and is not to be used or relied on for any diagnostic or treatment purposes. He worked as a GP partner from 2006 until 2008 and more recently as a higher specialist trainee in Emergency Medicine. The tracheal bifurcation angle can have a wide range of normal values in patients and can vary significantly in serial radiographs. 8600 Rockville Pike Get instant access to this gallery, plus: Introduction to the musculoskeletal system, Nerves, vessels and lymphatics of the abdomen, Nerves, vessels and lymphatics of the pelvis, Infratemporal region and pterygopalatine fossa, Meninges, ventricular system and subarachnoid space. The right main bronchus is shorter, wider and courses almost vertically compared to the left one. Each alveolar duct has 5 or 6 associated alveolar sacs. Your email address will not be published. As a result, they cannot maintain the trachea open and it continuously collapses during inspiration and expiration. Onoe R, Yamashiro T, Handa H, Azagami S, Matsuoka S, Inoue T, Miyazawa T, Mineshita M. Int J Chron Obstruct Pulmon Dis. Feeling ready for a recap test? Pectoralis major has its origin across the anterior surface of the sternum and the sternocostal articulations of the superior ribs, and therefore, includes the sternal angle. The trachea extends from inferior end of larynx at C6 vertebra and ends at the level of sternal angle (T4-T5 IV disc) where it divides into left and right main bronchi Answer link Jane Feb 24, 2018 T 6 Range is T 5 T 7 Explanation: In a cadaver the level of bifurcation of trachea is at the lower border of T 4 vertebra , range is T 3 T 5 Read more. This bifurcation normally occurs at about the sternal angle, where the body of the sternum However, studies have shown that these repairs do not always lead to improvements in scoliosis and ribcage remodeling. This category only includes cookies that ensures basic functionalities and security features of the website. Accessibility These cookies do not store any personal information. Adrian Rad BSc (Hons) Get instant access to this gallery, plus: Introduction to the musculoskeletal system, Nerves, vessels and lymphatics of the abdomen, Nerves, vessels and lymphatics of the pelvis, Infratemporal region and pterygopalatine fossa, Meninges, ventricular system and subarachnoid space, respiratory system quizzes and labeled diagrams, Anterolaterally: 16-20 tracheal cartilages, Air transport in and out of the lungs; protection. The angle of Louis also marks the site of bifurcation of the trachea into the right and left main bronchi and corresponds with the upper border of the atria of the heart. The sternum develops at the same time as the rest of the ribcage from mesenchymal bands or bars which develop chondritic tissues as they move ventrally and medially forming cartilaginous shapes of the adult bones. There are 10 bronchopulmonary segments in the right lung (3 in the superior lobe, 2 in the middle lobe, 5 in the inferior lobe), and 8 segments in the left lung (4 in the upper lobe, 4 in the lower lobe). It is roughly at the level of the bifurcation of the pulmonary trunk. The carina of trachea (also: "tracheal carina"[1]) is a ridge[1] of cartilage[2] at the base of the trachea separating the openings of the left and right main bronchi. 0. Please note that some texts differ slightly on certain levels and levels may be altered by patient positioning (supine versus erect) and patient body habitus. Anatomy and physiology of respiratory system relevant to anaesthesia. (Carina not labeled; the ridge that separates the left and right bronchus. At the bifurcation of the primary bronchi, a ridge of cartilage called thecarina runs anteroposteriorly between theopenings of the two bronchi. Tracheostomies and continuous night-time ventilatory support may also be required. 5. Ligamentum arteriosum attaches to the aortic arch deep to the angle of Louis. (This level varies from the vertebral bodies of T4 to T6; see Chapter 1.) Vertebral levels (anatomical landmarks). The larynx is the portion of the airway between the pharynx and the trachea, contains the organs for the production of speech. Many vertebral levels are associated with key anatomical landmarks. The trachea extends between the larynx and thorax, roughly occupying the midline (slightly tilted to the left). The left branch continues for 5 cm before branching to 2 secondary bronchi. As it descends, the trachea is located anteriorly to the oesophagus, and inclines slightly to the right. In the child, the trachea is smaller, more deeply placed, and more movable than in the adult. The https:// ensures that you are connecting to the Last's anatomy, regional and applied. Many different sternal anomalies can occur following abnormal development. Aortic arch ends. They later ossify in a craniocaudal direction. 95% of the patients have normal values between 40-80 . Standring, S. (2016). The tracheal cartilages are composed of hyaline cartilage and interconnected by fibroelastic tissue. Sympathetic supply is provided by the sympathetic trunks. vein joining superior vena cava. It marks the level of the transverse thoracic plane which divides the mediastinum into the superior and inferior mediastinum.

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