Main bronchi: There are two supplying ventilation to each lung. Gray's Anatomy (41tst ed.). Anatomy, Head and Neck, Larynx Recurrent Laryngeal Nerve. Along with branches of the pulmonary artery and veins, the main bronchi make up the roots of the lungs. PDF Anatomy, Thorax, Bronchial This marks the level of a number of other anatomical structures: It marks the point at which the costal cartilages of the second rib articulate with the sternum. [4], In a cadaveric study of preserved skeletal specimens, the sternal angle ranged from 149.0 degrees to 177.0 degrees with an average of 163.4 degrees in men and 165.0 degrees in women. Since the tracheal lumen is air filled, it appears black on CT images. Once thought to be after Antoine Louis or Wilhelm Friedrich von Ludwig, it is now believed to be after Pierre Charles Alexandre Louis.[4]. These are connected by pores which allow air pressure to equalize between them. As the volume of the lung changes with the thoracic cavity during ventilation (respiration), the entire bronchial tree will move within the lung. It presents with signs and symptoms of respiratory distress such as dyspnea, cough, stridor, wheezing and tachypnea. 1. It overlies the aortic arch on the left and the superior vena cava on the right. For details about the anatomy of the lungs, take a look at the following: Components that make up the alveolar epithelial lining are: No gas exchange takes place in the bronchi. The manubrium and proximal sternum are routinely opened upduring open-heart surgery. The sternum and manubrium are innervated by the intercostal nerves which arepart of the somatic nervous system. This is because the manubrium normally angles posteriorly on the body of the sternum, forming a raised feature referred to as the sternal angle. Vertebral levels are very commonly tested on in both undergraduate and postgraduate medical examinations. Abstracts of Presentations at the Association of Clinical Scientists 143. This information is intended for medical education, and does not create any doctor-patient relationship, and should not be used as a substitute for professional diagnosis and treatment. It is of poor diagnostic value due to the lack of sensitivity and specificity in identifying the underlying pathology. CT anatomy of the mediastinal structures at the level of the - PubMed Anatomy, Airway - PubMed Thoracic duct crosses to the left side of . To find out more, read our privacy policy. and grab your free ultimate anatomy study guide! If the bronchial cartilages are involved the condition is called bronchomalacia. 1. ADVERTISEMENT: Supporters see fewer/no ads. Fig 1 - Overview of the tracheobronchial tree. Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2014). Access free multiple choice questions on this topic. Out of these, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website. Adrian Rad BSc (Hons) The trachea and tracheobronchial tree -Begoa Rodriguez. 139 (5): 883-7. Our engaging videos, interactive quizzes, in-depth articles and HD atlas are here to get you top results faster. JAAPA. If the tracheal collapse is severe, it can even manifest with the cessation of breathing (apnea). After sixth generation, the passageways are too narrow to be supported by the cartillage, and thus are called bronchioles(small bronchi). 4. To summarize, once the trachea bifurcates into the main left and right bronchus, each bronchus segment is progressively smaller in diameter than the previous segment and subdivides from the segmental bronchus, into the large subsegmental bronchus, into the small subsegmental bronchus, and finally into the bronchioles. Learn about the sternal angle in this tutorial. The trapped particles are then removed from the trachea via the mucociliary rejection current. It marks the point at which the costal cartilages of the second rib articulate with the sternum. There was no relation of the bifurcation angle to age or gender. The amount of smooth muscle increases as the amount of cartilage decreases, and smooth muscle is also present continuously around the bronchi. It is at the level of the T4-T5 intervertebral disc. Parasympathetic supply originates from the recurrent laryngeal nerves, branches of the vagus nerve. 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 . This is the most sensitive area of the trachea for triggering the cough reflex, and can be seen on bronchoscopy. ), Last edited on 30 December 2022, at 00:10, "tracheal carina - NCI Dictionary of Cancer Terms", "Tracheal, Laryngeal, and Oropharyngeal Injuries", "Pertinent Surgical Anatomy of the Thorax and Mediastinum", "Tracheobronchial injury secondary to blunt chest trauma: Diagnosis and management", "Trachea and carina tomogram, coronal plane" at SUNY Downstate Medical Center, https://en.wikipedia.org/w/index.php?title=Carina_of_trachea&oldid=1130397191, This page was last edited on 30 December 2022, at 00:10. As it descends, the trachea is located anteriorly to the oesophagus, and inclines slightly to the right. Anatomically, the azygos vein arches over the right main bronchus from behind. In: StatPearls [Internet]. The angle of tracheal bifurcation is not related to age and gender, however, there is a weak correlation with the shape of the chest 1. Bethesda, MD 20894, Web Policies You are not required to obtain permission to distribute this article, provided that you credit the author and journal. CT assessment of tracheal carinal angle and its determinants. The angle of tracheal bifurcation: its normal mensuration. They undergo further branching to produce the secondary bronchi. Contributed Illustration by Beckie Palmer, Sternum, Angle of Louis, Manubrium. At what level the bifurcation of trachea takes place? This plane passes anteriorly through the sternal angle and posteriorly to the inferior margin of fourth thoracic vertebra. The mean angle of tracheal bifurcation is 60 (+/- 10 ), i.e. -, Benner A, Sharma P, Sharma S. StatPearls [Internet] StatPearls Publishing; Treasure Island (FL): 2022. Saeed M, Murshid KR, Rufai AA, Elsayed SE, Sadiq MS. Sternalis. 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. Bifurcation of the trachea. However in the smaller lobar and segmental bronchi cartilage is found only in crescent shapes. Reading time: 13 minutes. These nerves play a role in the contraction of the intercostal muscles as well as providing sensation to the skin. The PubMed wordmark and PubMed logo are registered trademarks of the U.S. Department of Health and Human Services (HHS). 2023 The first bronchi branch from trachea, and they are the right and left main bronchi. 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. Once you've finished editing, click 'Submit for Review', and your changes will be reviewed by our team before publishing on the site. It is roughly at the level of the bifurcation of the pulmonary trunk. The interbronchial angle is the angle between the central axis of right and left main stem bronchi 1. and grab your free ultimate anatomy study guide! There was no relation of the bifurcation angle to age or gender. Venous blood is drained to the inferior thyroid venous plexus, which empties into the brachiocephalic veins. Anatomy, Angle of Louis. 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 tracheal carina is deep to the sternal angle. 3.6. . This can sometimes allow the heart to protrude through the sternum. The trachea receives sensory innervation from the recurrent laryngeal nerve. These cookies do not store any personal information. Chen JT, Putman CE, Hedlund LW, Dahmash NS, Roberts L. Widening of the subcarinal angle by pericardial effusion. 2017 Jun;10(2):188-192. doi: 10.21053/ceo.2016.00185. Make the changes yourself here! Access over 1700 multiple choice questions. The mucosa represents the innermost layer and it is lined with pseudostratified ciliated columnar epithelium. Tracheobronchial tree. -, De Rose V, Molloy K, Gohy S, Pilette C, Greene CM. allergens, exercise) causes sudden inflammation and contraction of the smooth muscle around bronchioles (bronchospasm). Initially there are many generations of conducting bronchioles, which transport air but lack glands and are not involved in gas exchange. Your email address will not be published. Anat Histol Embryol. The lobes of thyroid gland which extend inferiorly until the sixth tracheal cartilage. Read more. This is the sternal angle. The angle of Louis is the eponymous name given to the sternal angle which is the palpable anatomical feature formed from the manubriosternal junction. Solved Which of the following takes place at the sternal - Chegg Thus, the wide range of normal values is a potential source of misinterpretation 1-3 . Last reviewed: October 25, 2022 Ann Clin Lab Sci. Bronchi will branch into smaller tubes that become bronchioles. Alveolar sacs are the blind-ended spaces from which the alveoli clusters are formed and to where they connect. [5], The carina is around the area posterior to where the aortic arch crosses to the left of the trachea. The PubMed wordmark and PubMed logo are registered trademarks of the U.S. Department of Health and Human Services (HHS). The bronchi, the main bifurcation of the trachea, are similar in structure but have complete circular cartilage rings. and transmitted securely. The angle decreases by 10 on expiration. Being a congenital anomaly, the tracheomalacia manifests in early infancy. It performs generic functions of the skeletal tissues; protection, mechanical leverage for movement, and support for other organs. Please enable it to take advantage of the complete set of features! The trachea is a D-shaped fibrocartilaginous respiratory organ. Thus absolute measurements of the angle of tracheal bifurcation (IBA/SCA) are of little diagnostic value and even grossly widened angles should not be interpreted as abnormal 1. Moore KL, Agur AMR, Dalley AF. Original Author(s): Vicky Theakston Last updated: October 31, 2020 Epub 2020 Mar 26. For example, cardiologists use this anatomical landmark as the starting point for the physical exam since the angle of Louis is 5 cm above the right atrium. [2] The carina occurs at the lower end of the trachea - usually at the level of the 4th to 5th thoracic vertebra. Descending aorta begins. MeSH The counting of ribs is essential when one is attempting to make a thoracic incision. Bifurcation of trachea at sternal angle What are the 3 components of the middle. Edinburgh: Elsevier Churchill Livingstone. If both the trachea and bronchi are involved, the disorder is called tracheobronchomalacia. Together, with the capillaries, they form the air-blood barrier. However, there is no definitive evidence of either origin, andsome speculation evensuggests it originates from another doctor, Pierre Charles Alexandre Louis. These cookies do not store any personal information. 2022 May;52(3):511-525. The right pulmonary artery lies initially below the right bronchus and then later in front of it. (2000). PMC 10. The trachea extends between the larynx and thorax, roughly occupying the midline (slightly tilted to the left). The sternum Q7 Identify structures at the level of the sternal angle. Tracheal carinal angle and left atrial size. Synonym (s): angulus sterni [TA] , Louis angle, Ludwig angle. There was only a weak correlation between bifurcation angle and height or width or the thorax. Found an error? Lower border of T4. ADVERTISEMENT: Radiopaedia is free thanks to our supporters and advertisers. Before tracheal bifurcation - General Practice notebook In addition to oxygen and carbon dioxide, environmental air also contains many potentially harmful agents such as pathogens, debris, gases and chemicals. -, Saran M, Georgakopoulos B, Bordoni B. StatPearls [Internet] StatPearls Publishing; Treasure Island (FL): 2022. Reviewer: [7][8]They later ossify in a craniocaudal direction. 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). In tracheomalacia, the tracheal cartilages become abnormally soft. In the standing position trachea bifurcation is at T6 level . Taskin V, Bates MC, Chillag SA. The sternal angle, also known as the angle of Louis. The resulting carbon dioxide filled air is transported by the tracheobronchial system towards the oral cavity and nose for excretion. PDF STRUCTURE OF THE HUMAN BODY - Loyola University Chicago A potential pitfall ("Louie's Hot Spot") in bone scan interpretation. Additionally, making an incision at the first or second rib interspace can result in damage to large, important blood vessels and the brachial plexus. In diseases such as emphysema that occurs in COPD, the alveoli are damaged or destroyed, which reduces the surface area available for effective gas exchange. -. Sternum, Jugular Notch, Manubrium, Sternal Angle, Body, Xiphoid Process, Clavicular Notch, Facets for Attachment of Costal Cartilages 1-7. The right main bronchus is shorter, wider and courses almost vertically compared to the left one. However, studies have shown that these repairs do not always lead to improvements in scoliosis and ribcage remodeling. sharing sensitive information, make sure youre on a federal Bronchi will constrict in response to inflammation and result in shortness of breath, difficulty breathing, and ultimately resulting in decreased amounts of oxygen available for cellular processes. Netter, F. (2019). The trachea bifurcates and therefore terminates, superior to the heart at the level of the sternal angle. The manubriosternal junction is the joint of the sternal body and the manubrium. [1][2][3], Thisangle of Louisis a synarthrosis, a type of joint characterized as a fibrous connection between two bones (the manubrium and the sternal body in the case of the angle of Louis) which does not allow any significant movement. As it descends, the trachea is located anteriorly to the oesophagus, and inclines slightly to the right. HOME; ABOUT; SERVICES; LOCATION; CONTACT; trachea bifurcates at the level of t4 HHS Vulnerability Disclosure, Help Importantly, the ribcage provides support for and allows ventilation through movement of the thoracic cage. Carina of trachea - Wikipedia Surfactant is a phospholipoprotein produced and distributed by type II alveolar cells, and absorbed by type I alveolar epithelial cells, with the main lipid component of the surfactant being dipalmitoylphosphatidylcholine (DPPC), which decreases surface tension of alveoli therefore making it easier for the lungs to expand. 2019 Jun;121:29-33. The bifurcation can be located anywhere between the levels of the fourth and seventh thoracic vertebrae. If the wrong rib is counted, access to the internal chest organs can be difficult. 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. These bronchi are the widest and they enter the lung. Sternal angle( Angle of Louis) - Standard of Care They are subdivided into different regions with various organs and tissues to perform specific functions. It is roughly at the level of the bifurcation of the pulmonary trunk. Denotes level of aortic arch, bifurcation of trachea, and T4-T5 intervertebral disc. 2. 1. Let's remove the manubrium and ascending aorta to see the trachea bifurcate at the sternal angle of Louis. Cell Biology-52 - notes - second rib; intervertebral disc T 4/5 It ends at the level of the sternal angle (T5) where it divides into two main bronchi, one for each lung. Disclaimer. 2018 Nov;31(11):36-40. I would honestly say that Kenhub cut my study time in half. Bifurcation of pulmonary trunk. official website and that any information you provide is encrypted The trachea, or windpipe, is a 10-11 cm long fibrocartilaginous tube of the lower respiratory tract. Unable to process the form. The cervical part of trachea is covered anteriorly by several structures, from deep to superficial; The esophagus is located posterior to the cervical part of trachea. A complete sternal cleft can occur when the two sagittal bars of the sternum do not fuse. Sternalis, a rare anatomical variation, occurs in a single-digit percentage of the population (some estimates pointing to roughly 4%). The most commonly encountered landmarks and structures found at the various vertebral levels are shown in the table below: Medical Exam Prep would like to thank Dr. Marc Barton for permission to reproduce this extract from his book Essential Clinical Surface Anatomy. Additionally, hyaline cartilage forms an incomplete ring in the bronchi that gives them the characteristic "D"-shaped appearance in the larger bronchi, and as small plates and islands in smaller-sized bronchi. The anterior relations include, from superior to inferior; The esophagus remains posteriorly to the thoracic part of trachea. The angle of tracheal bifurcation may be widened in the following conditions due to the mechanical splaying of the bronchi 1,2: The mean angle of tracheal bifurcation is 60 (+/-10 ), i.e. The .gov means its official. In: StatPearls [Internet]. American journal of roentgenology. A review of the embryological development and associated developmental abnormalities of the sternum in the light of a rare palaeopathological case of sternal clefting. The trachea can be palpated superior to the jugular notch of sternum, all the way to the cricoid cartilage. Theinnermost set of intercostals are thin and are similar to the internal intercostal muscles in their orientation. Bronchitis is defined as inflammation of the bronchi, which can be an acute or chronic condition. The trachea is only one part of the respiratory system. Immature lungs of a preterm infant often fail to produce sufficient surfactant, which leads to respiratory problems. The Angle of Louis. 2018 Aug 10;13:2399-2407. doi: 10.2147/COPD.S165824. Reviewer: Last's anatomy, regional and applied. -The lower end is attached to..? Accessibility It is the synarthrotic joint formed by the articulation of the manubrium and the body of the sternum. (1995) AJR. Curated learning paths created by our anatomy experts, 1000s of high quality anatomy illustrations and articles. By Bibi Saint-Pol [CC BY-SA 3.0], via Wikimedia Commons, [caption id="attachment_10035" align="aligncenter" width="260"], [caption id="attachment_11583" align="aligncenter" width="219"], [caption id="attachment_11584" align="aligncenter" width="356"], [caption id="attachment_11542" align="aligncenter" width="1000"]. In most cases, it ossifies with age. The larynx is the portion of the airway between the pharynx and the trachea, contains the organs for the production of speech. 5. The inferior thyroid veins, manubrium of sternum and attachments of sternohyoid and sternothyroid muscles. It's an important structure because it marks the location of other structures in the body. 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. Reference article, Radiopaedia.org (Accessed on 04 Mar 2023) https://doi.org/10.53347/rID-57629. The sternal angle marks the location of the: Bifurcation of the trachea Second costal cartilage O . Check for errors and try again. The posterior wall of the trachea is formed by the trachealis muscle, making the cartilages appear as incomplete C-shaped rings. The free ends of these rings are supported by the trachealis muscle. Berdajs D, Znd G, Turina MI, Genoni M. Blood supply of the sternum and its importance in internal thoracic artery harvesting. Acute bronchitis is often caused by viral or bacterial infections. They are initially ciliated and graduate to the simple columnar epithelium and their lining cells no longer contain mucous producing cells. Thus, absolute measurements of the tracheal bifurcation angles are of little diagnostic value. A proposed action of sternalis is to help in the expansion of the thoracic cage. [9]Importantly in patients having internal mammary harvesting, these branches anastomose with the intercostal arteries and therefore indirectly with the posterior intercostal arteries providing a possible collateral blood flow. The right main bronchus has a larger diameter and is aligned more vertically than the left, Lobar bronchi: Two on the left and three on the right supply each of the main lobes of the lung. Robert H. Whitaker, Neil R. Borley. Roberto Grujii MD The mucociliary excretion is facilitated by the coughing reflex. 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; Asthma attacks are acute exacerbations of the condition whereby a trigger (e.g. The bronchial structure begins at the transverse thoracic plane (also known as the sternal angle at the fourth thoracic vertebra), where the trachea bifurcates into two main bronchi, one for each . Below is a summary of vertebral levels and associated internal or surface anatomy. The right main bronchus has a larger diameter and is . 4. Epub 2011 Sep 15. eCollection 2018. The carinal bifurcation of the trachea into the left and right main bronchi. This site needs JavaScript to work properly. Complete fusion of the angle of Louis generally occurs atapproximately 30 years of age. Bronchi: Anatomy, function and histology | Kenhub 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 sublaryngeal airway is also known as. It consists of connective tissue that contains mucus glands, smooth muscle, vessels, nerves and lymphatics. This site needs JavaScript to work properly. As a result, they cannot maintain the trachea open and it continuously collapses during inspiration and expiration. 2. Coppola V, Vallone G, Coscioni E, Coppola M, Maraziti G, Alfinito M, Di Benedetto G. Br J Radiol. Seyfer AE, Shriver CD, Miller TR, Graeber GM. [4] The azygos vein crosses right to the trachea above the carina. Kim Bengochea, Regis University, Denver. 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 and bronchi are lined by ciliated pseudostratified columnar epithelium, interspersed by goblet cells, which produce mucus. It is located approximately 7 cm below the upper margin of the manubrium. 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. 2022 Jul 25. All rights reserved. Commonly used is the lecithin-sphingomyelin ratio ("L/S ratio"), where a L/S ratio of less than 2:1 means that the fetal lungs may be surfactant deficient, which can result in infant respiratory distress syndrome (IRDS) and ultimately neonatal death. Secondary bronchi will further subdivide into the tertiary bronchi, which are also called the segmental bronchi, each of which supplies a bronchopulmonary segment. 95% of the patients have normal values between 40-80 . Sternal angle (of Louis) Bifurcation of trachea Arch of aorta begins and ends Azygous vein enters superior vena cava: T5-8: Body of sternum: T6: Upper border of liver: T7: Inferior angle of the scapula: T8: Caval opening in the diaphragm Phrenic nerves traverse diaphragm: T8/9: Xiphisternal junction: T9: Xiphoid Superior epigastric vessels . 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. The left pulmonary artery is found lying initially above the left main bronchus, then later in front of it. Onoe R, Yamashiro T, Handa H, Azagami S, Matsuoka S, Inoue T, Miyazawa T, Mineshita M. Int J Chron Obstruct Pulmon Dis. It marks the point at which the costal cartilages of the second rib articulate with the sternum. 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. Other anatomic structures are present at the general level of this plane. The angle also marks a number of other features: The angle is in the form of a secondary cartilaginous joint (symphysis). 00 8 1 N 3 5 6 Uvula 4 CASA CA . In between these runs the neurovascular bundle. The site is secure. 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. In such cases, a tracheal tube or airway stents are used to bypass the obstruction and maintain the trachea open. Read more. 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. Careers. An incomplete fusion can cause a sternal foramen to be left within the sternum. Atlas of Human Anatomy (7th ed.). Where does the trachea divide. The pericardium extends from just superior to the angle of Louis to the level of the xiphisternal joint. . ISBN:1451119453. 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. (Carina not labeled; the ridge that separates the left and right bronchus. The sternal angle is also called the angle of Louis, but the reason for that name was lost. This article incorporates public domain material from Dictionary of Cancer Terms. 3. Trachea Flashcards | Quizlet It is divided into two parts: The trachea and bronchi collectively form the tracheobronchial tree. In contrast, the left main bronchus is smaller in size, but longer in length (~5 cm, as opposed to 2-3 cm long) than the right main bronchus. Aug 8, Anatomy, Head and Neck, Cervical, Respiratory, Larynx, and Cricoarytenoid. [11]It commonly originates from the lower costal cartilages as tendinous fibers and runs superiorly parallel to the sternal body, inserting into the sternal angle. Anterior surface of sternum and costal cartilages. The blood supply to the sternum has been investigated through contrast cross-sectional imaging both in vivo and in cadavers. On the right side, the trachea is related to the right, On the left side, the trachea is neighboured by the aortic arch, left recurrent laryngeal nerve, left common carotid artery and left. Vertebral levels (anatomical landmarks) - Radiopaedia 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.