3rd Block Anatomy - Chapter 4: Thorax PDF Study Guide

348 Chapter4 · Thorax elaviele. Parietal pleura is sensitiw, being innervated by the asymmetrical: the right main bronchus is more vertical and phrenie and intercostal neM!s. Bec:ause die lungs do not ofgreater caliber than the left. The bronchi and pulmonary complet...

348 Chapter4 · Thorax elaviele. Parietal pleura is sensitiw, being innervated by the asymmetrical: the right main bronchus is more vertical and phrenie and intercostal neM!s. Bec:ause die lungs do not ofgreater caliber than the left. The bronchi and pulmonary completely fill die pulmonary cavities, and beeause of die arteries course and branch together: the main bronchi/ proDUsion of the diaphragm and underlying abdominal viseera arteries each serve a lung, seeond-order lobar branches supply into the inferior thoracic aperture, a peripheral guttel"--ttle two left and three right lobes, and third-order segmental eostodiaphragmatic reeess-is formed. Extrapulmonary fluids branches supply the 8-1 0 bronchopulmonary segments of (exudates) aeeumulat:e in this spaee when the trunk is erect. each lung. The bronchopulmonary segment is the smallest lunp The lungs are the vital organs of respiration in resectable division of the lung. The pulmonary veins run which venous blood exchanges oxygen and carbon dioxide with independent intersegmental courses, draining adjacent a tidal airflow. Air and blood are delivered to each lung via bronchopulmonary segments. The structures of the root of its root:, consisting of a pulmonary artery and vein and a main the lung and supporting tissues (and part of the esophagus) bronchus and their branches/tributaries that enter the lung at are supplied by bronchial arteries. The lymphatic drainage its hilum. Both lungs are pyramidal, having an apex, a base, of the lungs follows a mostly predictable course, with most three surfaces, and three borders. The right lung has three of the right lung and the superior lobe of the left lung lobes dlat are separated by horizontal and oblique fissures. following ipsilateral padlways to die right lymphatic trunk The left lung has two lobes, separated by an oblique fissure, and thoracic duct. However, most of die drainage from the and features a marked cardiac: notch in its anterior border left inferior lobe passes to the right padlway. Nerve fibers of owing to die asymmetrical placement of the heart. the pulmonary plexuses are autonomic (bronchoconstric:bw! Trach bronchial tnee The tracheobronchial tree and secretomotor vagal parasympathetic fibers; inhibitory is distinguished grossly by cartilage in its walls. The and vasoconstrictive sympadletic fibers) and visceral afferent bifUrcation of die traehea (at die level of the sternal angle) is (reflex and pain). Overview of Mediastinum The superior mediastinum extends inferiorly from the superior thoracic aperture to the horizontal plane that The mediastinum (Mod. L. middle septum), occupied includes the sterruil angle anteriorly and passes approximately by the mass of tissue between the two pulmonary cavi- through thejunction (IV disc) ofT4 and T51iettebrae posteri- ties, is the central compartment of the thoracic cavity orly, often referred to as the transverse thoracic plane. The (Fig. 4.42). It is covered on each side by mediastinal pleura inferior mediastinum-between the transverse thoracic and contains all the thoracic viscera and structures except plane and the diaphragm-is further subdivided by the peri- the lungs. The mediastinum e:rtends from the superior tho- cardium into anterior. middle, and posterior parts. The peri- racic aperture to the diaphragm inferiorly and from the cardium and its contents (heart and roots of its great vessels) sternum and costal cartilages anteriorly to the bodies of constitute the middle mediastinum. Some structures, such the thoracic vertebrae posteriorly. Unlike the rigid struc- as the esophagus, pass vertically through the mediastinum ture observed in an embalmed cadaver, the mediastinum in and therefore lie in more than one mediastinal compartment. living people is a highly mobile region because it consists primarily of hollow (liquid- or air-filled) visceral structures united only by loose connective tissue, often infiltrated Pericardium with fat. The major structures in the mediastinum are also The middle mediastinum includes the pericardium, heart, surrounded by blood and lymphatic vessels, lymph nodes, and roots of its great vessels (Fig. 4.34}---ascending aorta, nerves, and fat. pulmonary trunk. and SVC-passing to and from the heart. The looseness of the connective tissue and the elasticity The pericardium is a fibroserous membrane that covers of the lungs and parietal pleura on each side of the medias- the heart and the beginning of its great vessels (Figs. 4.33B tinum enable it to accommodate movement as well as vol- and 4.43). The pericardium is a closed sac composed of two ume and pressure changes in the thoracic cavity, for example, layers. The tough external layer, the fibrous pericardium, those resulting from movements of the diaphragm, thoracic is continuous with the central tendon of the diaphragm wall, and tracheobronchial tree during respiration, contrac- (Fig. 4.32). The internal surface ofthe fibrous pericardium is tion (beating) of the heart and pulsations ofthe great arteries, lined with a glistening serous membrane, the parietal layer and passage of ingested substances through the esophagus. of serous pericardium. This layer is reflected onto the The connective tissue becomes more fibrous and rigid with heart at the great vessels (aorta. pulmonary trunk and veins, age; hence, the mediastinal structures become less mobile. and superior and inferior venae cavae) as the visceral layer The mediastinum is divided into superior and inferior parts of serous pericardium. The serous perlcardium is com- for descriptive purposes (Fig. 4.42). posed mainly of mesothelium, a single layer of flattened cells  Visan. ofTboru:ic: Cavity 349 Diaphragm FIGURE 4.42. Subdivisions and levels ofmediutm~m~. The subdivisions ofthe mediastinum are demonstrated as ifdJe person were in the supine position. The level ofthe viscera relative to the subdivisions as defined by thoracic cage landmarb depends on the individual's position because the soft tissue ofthe mediastinum sags with the pull ofgravity. forming an epitheliwn that lines both the internal surface of continuous inferiorly with the central tendon of the the fibrous pericardium and the external surface of the heart. diaphragm (Fig. 4.43C, D). The fibrous pericardium is 'The inferiorwall (Hoar) ofthe fibrous pericardial sac is firmly continuous superiorly with the tunica adventitia (peri- attached and confluent (partially blended) centrally with the vascular connective tissue) of the great vessels entering central rendon ofthe diaphragm. The site ofcontinuity has been and leaving the heart and with the pretracheallayer of referred to as the pericardiacopbrenic ligament; however, deep cervical fascia. the fibrous pericardiwn and central tendon are not separate attached anteriorly to the posterior surface of the sternum st:ru.ctures that fused together secondarily, nor are they sepa- by the sternopericardialligaments, which are highly rable by dissection. As a result ofthe attachments just described, variable in their development. the heart is relatively well rethered in place inside this fibrous bound posteriorly by loose connective tissue to structures pericardia\ sac. The pericardium is influenced by movements in the posterior mediastinum. of the heart and great vessels, the sternum, and the diaphragm. Parietal layer} VISCeral Serous pericardium , ___._,_ layer Abrous pericardium Parietal pericardium } Serous VIsceral } pericardium pericardium (epicardium) Myocardium Heart Endocardium {A} {B) (C) (D) I Partcardlal cavHy Diaphragm ligament (central tendon) Diagrammatic right Iatini views FIGURE 4.43. Peric:ardium and heart. A. The heart occupies the middle mediastinum and is endosed by peri(:ardium, composed oftwo parts. The tough, outer fibrous peri(:ardium stabilizes the heart and helps prevent it &om overdilating. Between the fibrou5 pericardium and the heart is a "collapsed~ sac, the serous peri(:ardium. The embryonic heart invaginaw the wall ofthe serous sac (B) and soon practically obliterates the pericardia! cavity (C), leaving only a potential space between the layer.; ofserous pericardium. C and D. The pericardiaoophrenic ligament is the cantinuityofthe fibrous pericardium with the central tendon ofthe diaphragm. 350 Chapter4 · Thorax Internal jugular vein Phrenic nerve------~ Subclavian v e i n - - - - -..-...-------S~bclavian vein Brachiocephalic ttunk-----' ---:------um brachlocephallc vein Right brachiocephalic vein.-----11.4arJubJriOS'ter~aaljoint Phrenic n e r v e - - - - - -.-,; (forms sternal angle) Superior vena cava----....,-=.... (SVC) ~~,..------~na costal cartilage ~~;-~----Internal thoracic artefy ::s,.-----=:oo- - Phrenic nerve Right dome of diaphragm...--Left dome of diaphragm Xiphisternal joint Anterior vr.ws FIGURE 4.44. P.-icvdial M~: in relation to nemum and phrenic nerws. This dissection exposes the pericardia! sac posterior to the body of the sternum from just superior to the stemal angle to the level of the xiphisternal joint. The pericardia! sac: (and therefore the heart) lies approximately one third to the right of the midsternalline and two thirds to the left (ii\N't). Note: Compare the anterior thoracic wall features (sternum, costal cartilages, internal thoracic arteries) of this figure to Figure 4.1 90. The heart and roots of the great vessels within the peri- (1) where the aorta and pulmonary trunk leave the heart and cardial sac lie posterior to the sternum, costal cartilages, and (2) where the superior vena cava (SVC), inferior vena cava anterior ends of the 3rd--5th ribs on the left side (Fig. 4.44). (IVC), and pulmonary veins enter the heart. The transverse The heart and pericardial sac are situated obliquely, approxi- perlcardial sinus is a transversely running passage within mately two thirds to the left and one third to the right of the pericardial cavity between these two groups of vessels the median plane. If you tum your face to the left about 45° and the reflections of serous pericardium around them. without rotating your shoulders, the rotation of your head The reflection of the serous pericardium around the second approximates that of the heart relative to the trunk. group of vessels defines the obl4que perlcardialsinus. The The fibrous pericardium protects the heart against sud- pericardial sinuses form during development of the heart as den overffiling because it is so unyielding and closely related a consequence ofthe folding ofthe primordial heart tube. As to the great vessels that pierce it superiorly. The ascending the heart tube folds, its venous end moves posterosuperiorly aorta carries the pericardium superiorly beyond the heart to (Fig. 4.45) so that the venous end of the tube lies adjacent the level of the sternal angle. to the arterial end, separated only by the transverse peri- The pericardial eavity is the potential space between cardia! sinus (Fig. 4.46). Thus, the transverse sinus is poste- opposing layers of the parietal and visceral layers of serous rior to the intrapericardial parts of the pulmonary trunk and pericardium. It normally contains a thin film of fluid ascending aorta, anterior to the SVC, and superior to the that enables the heart to move and beat in a frictionless atria of the heart. envirownent. A3 the veins of the heart develop and expand, a peri- The wcerallayer of serous pericardium forms the ept- cardia! reflection surrounding them fonns the oblique cardmm, the outermost of three layers of the heart wall. It pericardia! sinus. a wide pocket-like recess in the peri- extends onto the beginning of the great vessels, becoming cardia! cavity posterior to the base (posterior aspect) of continuous with the parietal layer of serous pericardium the heart, formed by the left atrium (Figs. 4.45 and 4.46).  V'aKera ofTIIorac:ic: Cavity 3S1. Primordial arterial and venous ends ol the developing heart are brought together, forming Plfmordlll hN.It tube In pertcardlll aac Prtmordlal tranaverae pertcardlllelnue (T) Heartloopa ventrally.. tnlnSVIII"Se l*lcardlll sinus. Arterial end Venous end Pericardia! cavity Posterior view of adult heart Lataral 'VIews of embryonic heart Veins expand and pericardia! renec:aon Is canted out around them to form obllqu. p!ll'lcardlll 1lnue. FIGURE 4.45. Dewlopmentofheartand ptric:anf111m. The longitudinal embryonic heart tube invaginates the double-layered pericardia! sac (somewhat lib placing a wiener in a hot dog bun). The primordial heart tube then "loopJ" ventrally, bringing the primordial arterial and venouJ ends of the heart together and cruting the primordial transverse pericardial sinus (7) between them. With growth oft:fte embryo, the veins expand and spread apart, infi!riorly and laterally. The pericardium reflected around them fi:mns the boundaries ofthe oblique pericanlial sinus (IVC, inferior vena cava;.SVC, superior vena cava). The oblique sinus is bounded laterally by the pericardia! inferiorly and will admit several fingers; however, they can- reflections surrounding the pulmonary veins and IVC not pass around any of these structures because the sinus is and posteriorly by the pericardium overlying the anterior a blind sac (cul-de-sac). aspect of the esophagus. The oblique sinus can be entered The arterial supply of the pericardium (Fig. 4.47} is mainly from a slender branch of the internal thoracic artery, the pericardiacophrenic artery, that often accompanies or Left brachio- cephalic vein Internal thoracic artery ~--Arch of aorta ~~iiii~-Ph!'411liC nerve * Obllq pelfcardlal alnua Branches of Anterior view musculophrenic ancllntemal FIGURE 4.46. Interior of pericardia~ sac. To remove the heart from t:fte thoracic arteries sac, the eight vessels pien:ing the sac: were severed. The oblique pericardia! Antwlor vltw sinus is c:i~umscribed by five veins. The superior vena cava (SVC), pulmonary trunk, and especiallythe aorta have intraperic:a.n:lial portions. The peak of the pericardia! sac: occurs at the junction ofthe ascending aorm. and the arch FIGURE 4.47. Armrial supply and wnoua drainap of l"'f'1nrdium. ofthe aorta. Thetransvcne pericardia! sinus is bounded anteriorly by the The arteries ofthe pericardium derive primarily from the internal serous pericardium covering the posterior aspect ofthe pulmonary trunk and thoracic arteries with minor contributions from their musculophrenic ascending aorta, posteriorly by that covering the SVC, and infi!riorty by the branches and the thoracic aorta. The veins are tributaries of the visceral pericardium covering the atria ofthe heart (IYC, inferiorvenac:ava). brachioeephalic veins. 352 Chapter4 · Thorax (A)SWMQ (B)6WMk8 (C)8W'Mb FIGURE 4.48. TI'&I'IISYeru Hd.:ion d!rougfl an embryo, cranial to the Hptum tnniMtlum,lhowin.c dewlopment offibroid pericatdium and relocluion of phrenic nerve. Exuberant growth of the lungs into the primordial pleura cavities (pleuroperitonea! canals) cleaves the pleuroperic:ardial folds &om the bocfywall, creating the pleuropericardial membranes. The membranes include the phrenic nerve and become the fibrous pericardium that encloses t:fte heart and separates the pleural and pericardia! cavities. at least parallels the phrenic nerve to the diaphragm. Smaller The innervation of the pericardium by the phrenic nerves contributions of blood come from the and the course of these somatic neroes between the heart and the lungs make little sense unless the development ofthe musculophrenic artery, a terminal branch of the internal fibrous pericardium is considered. A membrane (pleuroperi- thoracic artery-. cardial membrane) that includes the phrenic nerve is split or bronchial, esophageal, and superior phrenic arteries, separated from the developing body wall by the developing branches of the thoracic aorta. pleural cavities, which extend to accommodate the rapidly coronary arteries (visceral layer of serous pericardium only), the first branches of the aorta. growing lungs (Fig. 4.48). The lungs develop within the peri- cardioperitoneal canals that run on both sides of the foregut, The "¥enous drainage of the pericardium is from the connecting the thoracic and abdominal cavities on each side of the septum transversum. The canals (primordial pleural pericardiacophrenic t)elns, tributaries of the brachioce- cavities) are too small to accommodate the rapid growth of phalic (or internal thoracic) veins. the lungs, and they begin to invade the mesenchyme of the variable tributaries ofthe azygos t>enOUS system (discussed body wall posteriorly, laterally, and anteriorly, splitting it into 1ater in this chapter). two layers: an outer layer that becomes the definitive tho- The nerve supply of the pericardium is from the racic wall (ribs and intercostal muscles) and an inner or deep layer (the pleuropericardial membranes) that contains the phrenic nerves (C3-C5), primary source of sensory fibers; phrenic nerves and forms the fibrous pericardium (Moore pain sensations conveyed by these nerves are commonly et al., 2016). Thus, the pericardial sac can be a source of pain referred to the skin (C3-C5 dermatomes) of the ipsllateral just as the rib cage or parietal pleura can be, although the supraclavicular region (top ofthe shoulder ofthe same side). pain tends to be referred to dermatomes of the body wall- OOgt-1$ neroes, function uncertain. areas from which we more commonly receive sensation. sympathetic trunks, vasomotor. CLINICAL BOX MEDIASTINUM OVERVIEW AND the position of the person (i.e., gravity). When a person is PERICARDIUM supine or when a cadaver is being dissected, the viscera are positioned higher (more superior) relative to the subdivi- Levels of Viscera Relative to sions of the mediastinum than when the person is upright (Figs. 4.42 and B4.19A). In other words, gravity pulls the Mediastinal Divisions viscera downward when we are vertical. The division between the superior and inferior Anatomical descriptions traditionally describe the level of ~'"'"""15"'1' mediastinum (transoerBethoracicpltme) is defined the viscera as if the person were in the supine position-that _ in terms of bony body wall structures and is mostly is, lying face upward in bed or on the operating or dissection independent of gravitational effects. The level of the viscera table. In this position, the abdominal viscera spread horizon- relative to the subdivisions of the mediastinum depends on tally, pushing the mediastinal structures superiorly. However,  V"ascera of Thoracic: Cavity 3S3 (* Transverse thoracic plane) Superior ~---Anterior mediastinum mediastinum Manubrium Sternal angle ~~--Anterior mediastinum Sternum ==-:;;=:::::~~~~ P081erlor Middle mediastinum mediastinum Xiphoid process Diaphragm Posterior (A) Supine poetuon (B) Standing position mediastinum FIGURE 84.19. Posdion of thoruic viscera in (A) supine and (B) s1anding position.. when the individual is standing or sitting ered., the levels superior to the jugular notch of the manubrium, into the of the viscera are as shown in Figure B4.19B. This occurs potential space anterior to the trachea. During mediastinos- because the soft structures in the mediastinum, especially copy. surgeons can view or biopsy mediastinal lymph nodes the pericardium and its contents, the heart and great vessels, to determine if cancer cells have metastasized to them (e.g., and the abdominal viscera supporting them, sag inferiorly from a bronchogenic carcinoma). The mediastinum can also under the influence of gravity. be explored and biopsies taken through an anterior thora- In the supine position (Fig. B4.19A), the cotomy (removing part of a costal cartilage; see the Clinical Box "Thoracotomy, Intercostal Space Incisions, and Rib arch of the aorta lies superior to the transverse thoracic Excision," earlier in this chapter). plane. bifurcation of the trachea is tran.sected by the transverse thoracic plane. Widening of Mediastinum central tendon of the diaphragm (or the diaphragmatic Radiologists and emergency physicians some- surface or inferior extent of the heart) lies at the level of times observe widening of the mediastinum the xiphisternal junction and vertebra '1'9. when viewing chest radiographs. Any structure When standing Qt" sitting upright (Fig. B4.19B), the in the mediastinum may contribute to pathological wid- ening. It is often observed after trauma resulting from arch of the aorta is transected by the transverse thoracic a head-on collision, for example, which produces hem- plane. orrhage into the mediastinum from lacerated great ves- tracheal bifurcation lies inferior to the transverse thoracic sels, such as the aorta or SVC. Frequently. malignant plane. lymphoma (cancer oflymphatic tissue) produces massive central tendon of the diaphragm may fall to the level of enlargement of mediastinal lymph nodes and widening of the middle of the xiphoid process and~TlO IV discs. the mediastinum. Hypertrophy (enlargement) ofthe heart This vertical movement of mediastinal structures must be (often occurring due to congestioe heart faaure, in which considered during physical and radiological examinations in venous blood returns to the heart at a rate that exceeds the erect and supine positions. In addition, when lying on cardiac output) is a common cause of widening of the infe- one's side, the mediastinum sags toward the lower side under rior mediastinum. the pull of gravity. Surgical Significance of Transverse Mediastinoscopy and Mediastinal Pericardial Sinus Biopsies The transverse pericardial sinus is especially impor- Using an endoscope (mediastinoscope), surgeons.....,..,..._._ tant to cardiac surgeons. After the pericardia! sac is lo....;'IO....,..... can see much of the mediastinum and conduct ~ opened anteriorly, a finger can be passed through minor surgical procedures. They insert the endo- the transverse pericardial sinus posterior to the ascending scope through a small incision at the root of the neck, just aortaandpulmonarytrunk(Fig. B4.20). Bypassing a surgical 3S4 Chapter4 · Thorax Cardiac Tamponade The fibrous pericardium is a tough, inelastic, closed sac that contains the heart, normally the only occupant other than a thin lubricating layer of pericardia! fluid. If extensive pericardia! effusion e:Dsts, the compromised volume of the sac does not allow full expan- sion of the heart, limiting the amount of blood the heart can receive, which in twn reduces cardiac output Cardiac tam- ponade {heart compression) is a potentially lethal condition because heart volume is increasingly compromised by the fluid outside the heart but inside the pericardia! cavity. Blood in the pericardia! cavity, hemopericardium, likewise produces cardiac tamponade. Hemopericardium may result from perforation of a weakened area of heart muscle owing FIGURE 14.20. Tr&I'IIIYIInHI pericanlial inu1. to a previous myocardial infarction (MI) or heart attack, from bleeding into the pericardia! cavity after cardiac opera- tions, or from stab wounds. This situation is especially lethal clamp or a ligature around these large vessels, inserting the be