Thorax Flashcards
(12 cards)
Diaphragm
The diaphragm, a curved musculotendinous sheet attached to the circumference of the thoracic outlet and to the upper lumbar vertebrae, forms the floor of the thoracic cavity, separating it from the abdominal cavity. The diaphragm is relatively flat centrally and domed peripherally, rising higher on the right side than on the left, an asymmetry that reflects the relative densities of the underlying liver and gastric fundus, respectively. Fro- its -ost superior point on each side, the diaphragm slopes inferiorly to its costovertebral attachments; this slope is most marked posteriorly, where the space between the diaphragm and the posterior wall of the thorax is very narrow. The domes are higher in the supine position.
MEDIASTINUM
The mediastinum lies between the right and left pleural sacs in and near the median sagittal plane of the chest. It extends from the sternum anteriorly to the vertebral column posteriorly. A horizontal plane passing through the manubriosternal joint, and the intervertebral disc between the fourth and fifth thoracic vertebrae separates the mediastinum into superior and inferior portions.
Superior mediastinum
The superior mediastinum lies between the manubrium sterni anteriorly and the upper thoracic vertebrae posteriorly, and is limited laterally by the pleurae. Its inferior boundary is a slightly oblique plane that passes backwards from the manubriosternal joint to the lower part of the body of the fourth thoracic vertebra. It contains the lower ends of sternohyoid, sternothyroid and longus colli on each side; the aortic arch, brachiocephalic trunk and thoracic portions of the left common carotid and left subclavian arteries; the brachiocephalic veins and upper half of the superior vena cava; the left highest intercostal vein; the vagus, cardiac, phrenic and left recurrent laryngeal nerves; and the trachea, oesophagus, thoracic duct, thymic remnants and lymph nodes.
Inferior mediastinum
The inferior mediastinum is divided into three parts: anterior, in front of the pericardium; middle, containing the pericardium and its contents; and posterior, behind the pericardium.
Anterior mediastinum
The anterior mediastinum exists only on the left side where the left pleura diverges from the mid-sternal line. It is bounded anteriorly by the sternum, laterally by the pleurae, and posteriorly by the pericardium. It is narrow above but widens a little below. Its anterior wall is for-ed by the left transversus thoracis and the fifth to seventh left costal cartilages. It contains a quantity of loose areolar tissue, lymphatic vessels that ascend from the convex surface of the liver, two or three anterior mediastinal lymph nodes, and the small mediastinal branches of the internal thoracic artery
Middle mediastinum
The middle mediastinum is the broadest part of the interpleural space; it contains the heart enclosed in the pericardium; the ascending aorta; the lower half of the superior vena cava with the azygos vein opening into it; the bifurcation of the trachea and the right and left principal bronchi; the pulmonary artery dividing into its
two branches; the right and left pulmonary veins and phrenic nerves; and some bronchial lymph nodes
Posterior mediastinum
The posterior mediastinum is an irregular triangular space, bounded anteriorly by the pericardium above and the posterior surface of the diaphragm below, posteriorly by the vertebral column from the lower border of the fourth to the twelfth thoracic vertebrae, and laterally by the mediastinal pleura. It contains the thoracic part of the descending aorta, the azygos and hemiazygos veins, the vagus and splanchnic nerves, oesophagus, thoracic duct and lymph nodes.
ARTERIES
The skin of the thorax is supplied by direct cutaneous vessels and musculocutaneous perforators, which reach the skin primarily via the intercostal muscles, pectoralis major, latissimus dorsi and trapezius. Branches from the thoraco-acromial axis, lateral thoracic, internal thoracic, anterior and posterior intercostal, thoracodorsal, transverse cervical, dorsal scapular and circumflex scapular arteries are the major contributing vessels.
Muscles of the thoracic wall receive their blood supply from the internal thoracic artery (either directly or via the musculophrenic artery), the superior intercostal artery (from the costocervical trunk), superior thoracic artery (fro- the axillary artery), descending thoracic aorta, and the subcostal artery. Additional contributions come from vessels that supply the proximal muscles of the upper limb, namely: suprascapular, superficial cervical, thoraco-acromial, lateral thoracic and subscapular arteries.
VEINS
The intercostal veins accompany the similarly named arteries in the intercostal spaces. The s-mall anterior intercostal veins are tributaries of the internal thoracic and musculophrenic veins; the internal thoracic veins drain into the appropriate brachiocephalic vein. The posterior intercostal veins drain backwards, and most drain directly or indirectly into the azygos vein on the right and the hemiazygos or hemiazygos veins on the left. The azygos veins exhibit great variation in their origin, course, tributaries, anastomoses and termination.
Heart
The projection of the cardiac borders on to the anterior thoracic wall forms a trapezoid. The upper border slopes gently from the second left costal cartilage to the third right costal cartilage. The right border is a gently curved line, convex to the right, running from the third to the sixth right costal cartilages, usually 1–2 cm lateral to the sternal edge. The inferior or acute border runs leftwards from the sixth right costal cartilage to the cardiac apex,
located approximately 9 cm lateral to the midline, often in the left fifth intercostal space or level with the fifth or sixth rib. The -ost inferolateral point at which a pulsation is visible and palpable is called the cardiac apex beat and is usually palpable near the cardiac apex. The left (‘obtuse’) border is convex laterally and extends supero-medially from the cardiac apex to meet the second left costal cartilage approximately 1 cm from the left sternal edge. An oblique line joining the sternal end of the third left and sixth right costal cartilages represents the anterior part of the coronary sulcus/atrioventricular groove, which separates the right atrium from the right ventricle. The pulmonary trunk is delineated by two parallel lines drawn perpendicular to the ends of the pulmonary valve line, up to the level of the left second intercostal space. The orifice of the aortic valve is located below and to the right of the orifice of the pulmonary valve, and is represented by a line approximately 2.5 cm long running inferolaterally and to the right from the medial end of the left third intercostal space. Two parallel lines, drawn perpendicular to the ends of the valve line and slanting up towards the right half of the manubriosternal joint, outline the location of the ascending aorta. The direction of blood flow through a valve carries the valve sound
with it and thus the sites of cardiac valve auscultation do not correspond to the surface anatomy of the valves. Convenient sites at which to auscultate using a stethoscope are: the sternal end of the left second intercostal space (pulmonary area); the sternal end of the right second intercostal space (aortic area); near the cardiac apex (mitral area); and over the left lower sternal border, at the level of the fifth intercostal space (tricuspid area).
Great vessels
The aortic arch lies mainly posterior to the manubrium. The concavity of the arch sits approximately 1 cm inferior to the sternal plane, level with the upper half of the fifth thoracic vertebra. Starting at the aortic valve, the ascending aorta curves anteriorly, superiorly and to the right, and becomes the aortic arch posterior to the right half of the manubrium at the level of the right second costal cartilage. It continues to ascend to the right side of the manubrium sterni, then arches to the left across or over the sternal plane and descends such that the aortic
knuckle protrudes just to the left of the manubrium sterni around the first intercostal space. The brachiocephalic trunk arises approximately posterior to the centre point of the manubrium sterni and ascends to the right sternoclavicular joint. The left and right brachiocephalic veins are formed posterior to the sternoclavicular joints; the right brachiocephalic vein descends almost vertically, whereas the left brachiocephalic vein passes posterior to the superior portion of the manubriusterni. The superior vena cava forms posterior to the right first costal cartilage and descends vertically to enter the right atrium at the level of the right third costal cartilage. The azygos vein enters the superior vena cava approximately 2 cm inferior to the sternal plane at the level of the lower
part of the fifth thoracic vertebra.
Right atrium
The further development of the right atrium is characterized by the incorporation of the sinus venosus into the right part of the primary atrium. This process is under control of the T-box transcription factor Tbx18. At about 4 weeks of development, the sinuatrial junction of the looping primary heart tube is positioned symmetrically in the midline. The left and right common cardinal veins drain directly into the cavity of the primary atrium. The atrial
myocardium extends to the margins of the pericardial cavity and, strictly speaking, the sinus venosus is not yet formed because the systemic venous tributaries are embedded within the mesenchyme of the septum
transversus. During subsequent development, the pericardial cavity expands to enclose the terminal segments of the systemic venous tributaries; at the same time, their walls differentiate as myocardium. They can now be
termed the left and right horns of the sinus venosus; each horn receives the union of the corresponding umbilical vein, vitelline radicles and common cardinal vein. Concomitantly, the constriction between the left horn and the atrium becomes more pronounced. As the dorsal wall of the left atrium is formed from additions of mediastinal myocardium, the left horn becomes incorporated into the developing left atrioventricular junction, its orifice draining to the newly formed right atrium. At the same time, the left-sided venous tributaries diminish in size; the left common cardinal vein forms the oblique vein of the left atrium, and the left sinus horn forms the coronary sinus, maintaining its own myocardial wall as it becomes incorporated into the atrioventricular junction. The right sinus horn increases rapidly in size with growth of the liver. The vitello-umbilical blood flow enters the right
horn through a wide but short hepatocardiac channel, which becomes the cranial end of the inferior vena cava. The right horn also receives the right common cardinal vein, draining the blood from the right side of the body. Later, when transverse connections are established between the cardinal veins, the blood from the left side of the body also reaches the heart via the veins draining the right side. As these changes take place, the right sinus horn, including the proximal parts of the superior and inferior cardinal veins, becomes incorporated into the right atrium, forming the smooth-walled systemic venous sinus, also known as the sinus venarum. The right sinus horn opens into the right atrium through its dorsal and caudal walls. The sinuatrial orifice becomes elongated and slit-like, guarded by two muscular folds: the left and right sinuatrial (venous) valves. These two valves meet cranially and become continuous with a fold that projects from the atrial roof: the septum spurium. The valves also meet caudally, and merge with the inferior atrioventricular cushion. With ongoing development, the cranial part of the right sinuatrial valve loses its fold-like form, but its position is indicated in the adult heart by the site of the crista terminalis of the right atrium. Its caudal part forms the valve of the coronary sinus, also known as the Thebesian valve, and most of the valve of the inferior vena cava (Eustachian valve). The union of the two valvular remnants then passes through the tissue that separates the orifice of the coronary sinus from the fossa ovalis. This area is known as the ‘sinus septum’ but, in reality, this ‘septum’ is no more than a muscular fold in the dorsal wall of the right atrium. The continuation of the venous valves persists as the tendon of Todaro, an important landmark for the location of the atrioventricular node in the definitive heart. The left venous valve blends with the right side of the atrial septum; there is usually no trace of it in the postnatal heart.
