Chapter 24: Blood vessels, lymphatic drainage and nerves of the thorax
The pulmonary trunk and arteries carry deoxygenated blood, but they are arteries in the sense that they transmit blood away from the heart at a relatively high pressure (20 to 30 mm of mercury) and in a pulsatile manner and in the sense that they have elastic walls like the aorta.
The pulmonary trunk extends from the conus arteriosus of the right ventricle to the concavity of the arch of the aorta, to the left of the ascending aorta, where it divides into the right and left pulmonary arteries. The point of division is approximately at the left side of the sternal angle. The pulmonary arteries and their branches are largely responsible for the normal shadows seen radiographically in the roots and hili of the lungs.
The right pulmonary artery, longer and wider than the left, passes inferior to the arch of the aorta and enters the hilus of the lung (see fig. 22-4). The left pulmonary artery is connected to the arch of the aorta by the ligamentum arteriosum (fig. 24-1), which is the fibrous remains of a prenatal vessel, the ductus arteriosus.
A pulmonary vein arises in each lobe of the lung. The right upper and middle veins usually unite, so that four veins, upper and lower on each side, enter the left atrium.
The systemic supply of the thorax is derived mainly from branches of the aorta, the chief systemic artery of the body. The aorta is divided into the ascending aorta, arch of the aorta, and descending aorta. The part of the descending aorta in the thorax is called the thoracic aorta. The aorta is an elastic artery that withstands the systolic blood pressure and provides elastic recoil. The walls of the ascending aorta and arch contain pressure receptors, which are reflexly connected to aortic depressor fibers in the vagi (slowing the hear in response to increased blood pressure).
The ascending aorta begins at the root of the aorta, where the three aortic sinuses are located. Its branches are the right and left coronary arteries. It ascends to the level of the sternal angle.
The arch of the aorta (the term aortic arch is better reserved for special embryonic vessels) runs posterior on the left side of the trachea and esophagus and superior to the left main bronchus (see figs. 22-5 and 24-1). The arch of the aorta, in proceeding backward, lies in an almost sagittal plane in the superior mediastinum, behind the lower part of the manubrium sterni (see fig. 23-21). It is visible radiographically as the aortic knob, or knuckle (see fig. 23-16). The arch is related on its inferior side to the bifurcation of the pulmonary trunk and is connected to the left pulmonary artery by the ligamentum arteriosum. The left recurrent laryngeal nerve hooks inferior to the arch. There are three main branches of the arch (situated posterior to the left brachiocephalic vein): the brachiocephalic trunk, left common carotid artery, and left subclavian artery. Immediately distal to the last-named branch, the aorta is slightly constricted (isthmus). A severe constriction (coarctation of the aorta) may occur here during development, in which case the adequacy of the collateral circulation depends on the relationship of the constriction to the opening of the ductus arteriosus (which connects the pulmonary trunk and aorta).
The brachiocephalic trunk divides behind the right sternoclavicular joint into the right subclavian and right common carotid arteries. The left common carotid and left subclavian arteries enter the neck behind the left sternoclavicular joint. Variations in the branches of the arch of the aorta may be encountered, e.g., a common origin of the brachiocephalic trunk and left common carotid artery. Sometimes an aortic ring may encircle the trachea and esophagus and press on them. The right subclavian artery may arise from the thoracic aorta and be retro-esophageal (fig. 24-1B), which is frequently stated to cause dysphagia (difficulty in swallowing).
The thoracic aorta descends in the posterior mediastinum and traverses the diaphragm to become the abdominal aorta. It begins to the left of the vertebral column, gradually moves arteriorly and to the right (where it lies behind the esophagus), and enters the abdomen in the median plane. The branches of the thoracic aorta are parietal and visceral. The parietal include several posterior intercostal arteries and the subcostal and some phrenic arteries. The visceral branches are the bronchial, esophageal, pericardial, and mediastinal.
Each brachiocephalic vein is formed by the confluence of the subclavian and internal jugular veins behind the corresponding sternoclavicular joint. The right vein descends vertically, whereas the left crosses in an oblique orientation anterior to the branches of the arch of the aorta. Near the level of the sternal angle, the two brachiocephalic veins unite to form the superior vena cava (fig. 24-1). The superior vena cava descends on the right side of the ascending aorta, receives the azygos vein, and ends in the right atrium. Rarely, a left superior vena cava may persist: it comprises parts of the left brachiocephalic vein, left superior intercostal vein, oblique vein of the left atrium, and coronary sinus (fig. 24-2B).
The azygos system consists of veins on each side of the vertebral column, which drain the back as well as the walls of the thorax and abdomen. These veins are highly variable, but they end in the azygos, hemiazygos, and accessory hemiazygos veins (fig. 24-2). The azygos vein (Gk, a zygon, "unpaired") is formed by small vessels (such as the right subcostal and right ascending lumbar veins), and it ascends through the posterior and superior mediastinum. It arches anteriorward over the root of the right lung and ends in the superior vena cava. It receives the hemiazygos, accessory hemiazygos, and a number of posterior intercostal veins. The hemiazygos and accessory hemiazygos veins form a very variable arrangement on the left side. The hemiazygos vein arises in a manner comparable to that of the azygos. The accessory hemiazygos vein corresponds to the upper portion of the azygos vein.
The vertebral venous system consists of plexuses that drain the back, vertebrae, and structures in the vertebral canal. They communicate above with the intracranial veins and below with the portal system, and they empty into the vertebral, posterior interosseous, lumbar, and sacral veins. The veins in the vertebral plexus are valveless: blood may flow in either direction, and pressure in them is reflected in the cerebrospinal fluid. Reversed blood flow permits tumor cells to be transported from the breast, thorax, abdomen, or pelvis to the vertebrae, spinal cord, or brain. Veins of the thoracic wall (such as the thoraco-epigastric veins, which are superficially placed) connect the superior and inferior venae cavae and can provide a collateral circulation in obstruction of one of the venae cavae.
Extensive anastomoses among the caval, azygos, and vertebral systems provide multiple routes for the return of blood to the heart. In effect, the azygos and vertebral systems bypass the caval system and these veins dilate in caval obstruction.
The parietal nodes of the thorax are the parasternal, phrenic, and intercostal. The visceral nodes drain the lungs, pleurae, and mediastinum. The nodes in the roots and hili of the lungs are arranged in several groups: pulmonary along the larger bronchi, bronchopulmonary mainly at the hilus, and tracheobronchial near the bifurcation of the trachea (fig. 24-3). Lymph nodes in the roots of the lungs tend to be involved secondarily in infections, such as tuberculosis, and in tumors of the lungs and mediastinum. Their density may increase so that they become visible radiographically, especially if they become calcified. The tracheobronchial nodes drain into the tracheal (or paratracheal) nodes. Mediastinal nodes are scattered in the superior mediastinum, and they receive vessels from the thymus, pericardium, and heart. The efferents of the tracheal and mediastinal nodes form a bronchomediastinal trunk on each side of the trachea. There are also posterior mediastinal nodes, most of which drain directly to the thoracic duct.
All of the lymphatic drainage of the thorax is directed toward the bronchomediastinal trunks, thoracic duct, and descending intercostal lymphatic trunks (fig. 24-3), but the actual lymphatic trunks themselves are highly variable.
The thoracic duct extends from the abdomen to the neck, where it ends in one of the large veins (figs. 24-3 and 24-4). It begins as either a plexus or a dilatation called the cisterna chyli, passes through or near the aortic opening of the diaphragm, and ascends in the posterior mediastinum between the aorta and the azygos vein. Next it crosses obliquely to the left, posterior to and then along the left side of the esophagus. Finally it passes posteiror to the left subclavian artery, enters the neck (where it forms an arch above the level of the clavicle), and commonly ends in the left internal jugular vein (fig. 24-3). Variations are common. The thoracic duct receives the left subclavian and jugular trunks and often the left bronchomediastinal trunk.
Most of the lymph in the body reaches the venous system by way of the thoracic duct (figs. 24-4 and 24-5), but anastomoses are so extensive that no serious effects result if the thoracic duct is ligated.
On the right side, the bronchomediastinal trunk forms various combinations with the subclavian and jugular trunks. Rarely, all three unite to form a right lymphatic duct, which then empties directly into the junction of the internal jugular and subclavian veins.
The thymus is partly in the neck and partly in the thorax. It comprises one to three lobes, each of which consists of numerous lobules containing lymphocytes, which are important in the development and maintenance of the immune system. The cervical part of the thymus lies on the anterior and lateral sides of the trachea, whereas the thoracic part lies posterior to the superior portion of the sternum. The organ has a profuse blood supply and lymphatic drainage. The thymus reaches its greatest size at puberty and then begins to regress. Much of its substance is replaced by fat and fibrous tissue, but thymic tissue never disappears completely.
The thoracic nerves are described in the discussion of the thoracic wall.
The phrenic nerves supply the diaphragm. These nerves usually arise from cervical nerves 3-5. (The contribution of the C5 nerve root to the phrenic nerve often arises from the nerve to the subclavius, and it may enter the thorax separately as an accessory phrenic nerve.) After descending on the anterior surface of the anterior scalene muscle in the neck, the phrenic nerves enter the thorax and pass anterior to the roots of the lungs. Each givesrise to sensory branches to the pericardium and pleura and then divides into several branches, which pierce the diaphragm and supply that muscle, as well as part of the peritoneum, from below. The phrenic (fig. 24-6) nerves contain (1) motor fibers to the diaphragm, (2) pain fibers from the pericardium, pleura, and peritoneum, and (3) sympathetic vasomotor fibers. Pain is usually referred to the skin over the trapezius muscle that is normally innervated by cervical nerves 3 and 4. Pain sometimes refers to the region of the ear innervated by C2 and 3.
The vagi descend through the neck and enter the thorax, where they contribute to the pulmonary plexuses and then form the esophageal plexus. They then continue as anterior and posterior vagal trunks, which pass through the esophageal opening of the diaphragm. Each vagus has a recurrent laryngeal branch and a variable number of cardiac branches in the neck and thorax.
The right recurrent laryngeal nerve arises as the vagus crosses anterior to the subclavian artery, hooks around that vessel, and ascends between the trachea and esophagus. The left recurrent laryngeal nerve arises as the vagus crosses the left side of the arch of the aorta, hooks around the inferior side of the arch (to the left of the ligamentum arteriosum), and then ascends on the right side of the arch between the trachea and esophagus. The left recurrent laryngeal nerve is liable to damage from disorders of the aorta (e.g., aneurysms) or of the mediastinum (e.g., tumors), resulting in hoarseness. It is believed that both recurrent laryngeal nerves owe their adult relationships to their embryonic arrangement caudal to the sixth aortic arches.
The vagi contain (1) parasympathetic fibers (e.g., to the heart), (2) sensory fibers (many of which are concerned with cardiovascular and pulmonary reflexes; others, in the mucosa of the bronchial tree, cause coughing), and (3) motor fibers to the pharynx and larynx (fig. 24-7), in the head and neck.
The sympathetic trunks (figs. 24-8 and 32-5) descend through the neck and enter the thorax, where they lie anterior to the necks of the ribs (see figs. 20-2, 20-3, and 20-5). The thoracic part of each trunk has about a dozen ganglia, the first of which is often fused with the inferior cervical ganglion to form the stellate ganglion. The sympathetic trunks enter the abdomen by piercing the crura of the diaphragm or by passing posterior to the medial arcuate ligaments. The trunks and ganglia are connected with the thoracic ventral rami by rami communicantes, which convey preganglionic and postganglionic fibers. Preganglionic fibers from segments T1 to 6 of the spinal cord supply the heart, coronary vessels, and bronchial tree. Apart from cardiac and pulmonary branches, the main visceral branches are the three splanchnic nerves. The greater, lesser and least splanchnic nerves (from superior to inferior) traverse the lower thoracic ganglia, pierce the crura of the diaphragm, and end in ganglia and plexuses (celiac and renal) in the abdomen. The sympathetic trunks and their branches contain pain fibers from the thoracic and abdominal viscera and from blood vessels. These sensory fibers traverse the sympathetic trunks and rami communicantes to reach the spinal nerves, dorsal roots, and spinal cord.
Many branches of the vagi and sympathetic trunks form plexuses in the thorax, e.g., the cardiac, pulmonary, esophageal, and aortic plexuses.
Clemens, H. J., Die Venensysteme der menschlichen Wirbelsiiule, de Gruyter, Berlin, 1961. Well-illustrated account of vertebral venous system.
Jacobsson, S.-I., Clinical Anatomy and Pathology of the Thoracic Duct, Almquist and Wiksell, Stockholm, 1972. An excellent, concise monograph on anatomy.
Pontes, A. de P., Arterias supra-aorticas, University of Brazil, Rio de Janeiro, 1963. A study of the arch of the aorta and variations in its branches.
24-1 How does (a) the right pulmonary artery differ from the left and (b) the left bronchus differ from the right?
24-2 What is the ligamentum arteriosum?
24-3 Which structure hooks around the arch of the aorta?
24-4 What is a retro-esophageal right subclavian artery?
24-5 Which regions of the body are drained by the thoracic duct?
24-6 What is the origin of the phrenic nerves?
Figure 24-1 A, The three arteries (1, 2, 3) usually given off by the arch of the aorta. Also shown are the brachiocephalic and superior caval veins in outline (as if transparent) and the trachea, esophagus, and phrenic and vagus nerves. Note the different course of the recurrent laryngeal nerves on the two sides, the left nerve being related to the ligamentum arteriosum. Cf. fig. 50-17. B, An instance in which a retro-esophageal right subclavian artery (asterisk) arises as the last branch of the arch of the aorta.
Figure 24-2 A, Scheme of the main connections of the azygos, caval, and vertebral systems of veins. Connections of the azygos and hemiazygos veins with the posterior intercostal veins also occur. B, The main veins of the thorax. An interrupted blue line indicates the course of a left superior vena cava (a rare anomaly) on its way to the coronary sinus.
Figure 24-3 The visceral lymph nodes and collecting trunks of the thorax.
Figure 24-4 The thoracic duct. It commonly crosses the vertebral column at T.V.5 or 6. (Based on Davis.)
Figure 24-5 All except the shaded area is drained by the thoracic duct.
Figure 24-6 Functional components of the phrenic nerve. For purposes of simplification, each component is shown as a single fiber. Afferent (blue), efferent (red), and sympathetic (black) fibers are distinguished.
Figure 24-7 Functional components of the vagus nerve. For purposes of simplification, each component is shown as a single fiber. Afferent (blue), efferent (red), and parasympathetic (interrupted) fibers are distinguished. The distinction between accessory and vagal components is not shown.
Figure 24-8 Functional components of the sympathetic trunk, showing the origin of preganglionic fibers and the sites of ganglion cells. Afferent (mostly pain) fibers from viscera are indicated in blue.