2018 Anatomy of the thorax Sameh S. Akkila
THE THORACIC CAGE The thoracic cage consists of the sternum anteriorly, the twelve thoracic vertebrae and their intervertebral discs posteriorly and the twelve pairs of ribs on the sides [Figure 1]. Figure 1. Anterior view of the thoracic cage skeleton. The sternum The sternum is a flat bone on the anterior part of the thorax. It consists of three articulated parts: the upper part is the manubrium sterni, the middle large part is the body of the sternum and the lower small part is the xiphoid process (xiphisternum). The manubrium sterni is the roughly square upper part of the sternum. Its superior margin is notched in the middle by the jugular (suprasternal) notch which lies at the level of the disc between T2 and T3 vertebrae. The right and left upper angles articulate with the sternal (medial) ends of the clavicles at the sternoclavicular joints. The right and left margins articulate with the first costal cartilages. The inferior margin of the manubrium articulates with the superior margin of the body at the manubriosternal joint at a palpable angle called the sternal angle of Louis, which lies opposite of the disc between T4 and T5 vertebrae. This angle lies opposite the articulation of the second costal cartilage to the sternum and is used to identify the second rib by palpation for counting the ribs. The first rib cannot be palpated because it lies deep to the clavicle. The body of the sternum is originally composed of 4 pieces (sternebrae) that fuse in one bone between the time of puberty and the age of 25 years. The anterior surface of the body of the sternum is often marked by transverse ridges that represent lines of fusion between the sternebrae. The body 1
is about 4 fingerbreadths (10 cm) long and provides articulation for the 2 nd to 7 th costal cartilages. It is rich with red bone marrow. The xiphoid process is the smallest part of the sternum. Its shape is variable: it may be wide, thin, pointed, bifid, curved, or perforated. It begins as a cartilaginous structure, which becomes ossified in the adult. On each side of its upper lateral margin is a demifacet for articulation with the medial end of the seventh costal cartilage. The xiphoid process articulates with the body of the sternum at the xiphisternal joint which lies at the level of T10 vertebra. Figure 2. Anterior (A) & lateral (B) views of the sternum. Figure 3. Muscles attached to the anterior (A) & posterior (B) surfaces of the sternum. 2
The ribs A rib is an elongated flattened bone that curves anteroinferiorly from the thoracic vertebrae to join a cartilaginous part (costal cartilage) anteriorly at the costochondral junction. The costal cartilage then passes anterosuperiorly towards the sternum. The ribs are separated from each other by the intercostal spaces. The ribs, as well as the costal cartilages, increase in length from the first to the seventh, and their obliquity increases from the first to the ninth. The 8 th rib is the most laterally projecting rib (widest transverse diameter). Therefore, while counting the ribs by palpation, the examiner s hand should move inferolaterally. All the ribs articulate posteriorly with the vertebral column. Anteriorly, the arrangement is divided into three patterns: The upper 7 ribs articulate directly and individually with the sternum and are called the true or vertebrosternal ribs. The costal cartilages of the 8 th, 9 th and 10 th ribs join each other and the cartilage of the 7 th rib to form the costal margin. They are called the false or vertebrochondral ribs. The 11 th and 12 th ribs end in free anterior cartilages between the muscles of the anterior abdominal wall and are called the floating or vertebral ribs. Ribs may also be classified into typical and atypical according to certain anatomical characteristics: The typical ribs are the 3 rd to the 9 th ribs. They have the following characteristics: The posterior end consists of the head, neck and tubercle. The head contains 2 demifacets for articulation with the corresponding vertebra and the vertebra above. A crest lies between the 2 facets for articulation with the intervertebral disc. The neck is the posteriorly projecting narrow flat part between the head and the tubercle. The tubercle is a bony prominence that projects posteriorly from the junction of the neck with the shaft. It consists of articular and non-articular parts. The articular part is medial and has an oval facet for articulation with the transverse process of the corresponding vertebra. The non-articular part is roughened by ligamentous attachment. The angle is the part where the neck and tubercle meet the shaft. It is the most posterior part of the rib and represents the points of contact of the rib cage with a flat surface in the supine position. The shaft (body) is a long twisted, curved part that passes anterolaterally and then medially downward towards the costal cartilage. The shaft is flattened in the vertical plane with superior and inferior borders and inner and outer surfaces. The superior border of the shaft is thick, round and smooth. The inferior border is thin, sharp and grooved throughout its length by the costal groove which lodges the intercostal nerve and vessels. The outer and inner surfaces provide area for muscular attachments. 3
Figure 4. Features of the typical rib The atypical ribs are those which lack one or more of the typical characteristics: The first rib is atypical because: The head has one facet for articulation with T1 vertebra only. The body is short and not twisted. It is flattened in the horizontal plane so that it has superior and inferior surfaces and inner and outer borders. The superior surface is characterized by the scalene tubercle which separates two smooth grooves that cross the rib approximately midway along the shaft. The anterior groove is for the subclavian vein, and the posterior groove is for the subclavian artery. The bony and cartilaginous parts of the shaft both run in a downward slope. The second rib has a shaft which is not twisted and is also flattened horizontally. It is twice as long as the first rib but its costal groove extends only for a short distance posteriorly. 4
The 10 th rib has one articular facet on its head for articulation with T10 vertebra only. The 11 th and 12 th ribs each has one facet on the head for the corresponding vertebra. They have no necks or tubercles. Their angles are greatly diminished and the costal grooves are often not recognized. The thoracic vertebrae According to the shape of the different parts of the vertebra and the orientation of the articular processes, the vertebrae of each region of the vertebral column are divided into typical and atypical. The typical thoracic vertebrae are the 2 nd o the 8 th vertebrae which share the following characteristics: The body is medium-sized and heart-shaped. Each vertebral body has two articular demifacets for articulation with the head of the corresponding rib and the head of the rib below. The laminae are broad and overlap with those of the vertebra below. The superior articular processes are flat, with their articular surfaces facing almost directly posteriorly, while the inferior articular processes project from the laminae and their articular facets face anteriorly. The transverse processes are club shaped and project posterolaterally. Each process has a facet for articulation with the tubercle of the corresponding rib. The vertebral foramen is small and circular. The spine is long and inclined downwards. Figure 5. Features of a typical thoracic vertebra in superior (A) & lateral (B) views. 5
The 1 st, 9 th, 10 th, 11 th and 12 th vertebrae are atypical as shown in the diagram below. Figure 6. Features of the atypical thoracic vertebrae. THE THORACIC APERTURES The Superior thoracic Aperture and Suprapleural Membrane The superior aperture of the thorax (thoracic inlet) is directed anterosuperiorly. It is bound by the manubrium sterni anteriorly, the inner border of the first rib on the sides and by upper border of the 1 st thoracic vertebra posteriorly. It transmits structures between the neck and the thoracic cavity. The aperture is partially covered by the suprapleural membrane. This membrane is a thickening of the endothoracic fascia. It spreads like a tent from the transverse processes of C7 vertebra to the inner border of the 1 st rib and its costal cartilage. Medially, the membrane invests the deep fascia covering the structures that pass between the neck and thoracic 6
cavity. The membrane protects the upper surface of the pleural and pulmonary domes and prevents alterations of intrathoracic pressure. It may contain few muscle fibers (scalenus minimus muscle) that help to keep it tense. Figure 7. Plane of the thoracic inlet. Figure 8. Attachment (A) and relations (B) of the suprapleural membrane. The Inferior Thoracic Aperture and Diaphragm The inferior thoracic aperture is directed anteroinferiorly. The skeletal boundaries of the inferior thoracic aperture are: The body of T12 vertebra posteriorly. The 12 th rib and the distal end of the 11 th rib posterolaterally. The costal cartilages of the 7 th 10 th ribs (costal margin). The xiphoid process anteriorly. 7
The aperture is closed by the diaphragm which is traversed by structures that pass between the thoracic and abdominal cavities. The Diaphragm The diaphragm is a dome shaped skeletal muscle that separates the thoracic from the abdominal cavities. It is the most important muscle of respiration. The muscle fibers arise from a peripheral origin and converge to a trifoliate central tendon which represents the insertion. The central tendon lies inferior to the heart. On each side of the central tendon, the muscle fibers of the diaphragm are elevated as the right and left domes. The central tendon lies at the level of the xiphisternal junction. The right dome lies at the level of theupper border of the 5 th rib or the 4 th intercostal space. The left dome lies at the level of the lower border of the 5 th rib. The left dome is lower than the right because it is depressed by the heart. The origin of the diaphragm arises from the margins of the inferior thoracic aperture and the fascia of the muscles of the posterior abdominal wall. It is divided into three parts: The sternal part consists of right and left slips that arise from the posterior surface of the xiphisternum. On each side, a small gap known as the sternocostal triangle is present between the sternal and costal parts. It transmits the superior epigastric vessels and some lymphatics, and it may be the site of a diaphragmatic hernia. The costal part consists of several slips that originate from the inner surfaces of the lower 6 ribs. The portion of the costal part of the diaphragm that arises from 11 th and 12 th ribs is often separated from the vertebral part by an interval termed the vertebrocostal trigone. This area is occupied by connective tissue that separates the pleura above from the suprarenal gland and upper pole of the kidney below. The vertebral part consists of muscular fibers that arise from the crura and ligaments of the diaphragm. The diaphragm has two crura, right and left. The right crus originates from the sides of the bodies of the upper 3 lumbar vertebrae and their intervertebral discs. Fibers of the right crus pass to the left to surround the esophageal opening in the diaphragm forming a weak external sphincter that helps preventing gastric reflux. The left crus originates from the sides of the bodies of the upper 2 lumbar vertebrae and their intervertebral discs. Five ligaments provide attachment to the posterolateral fibers of the diaphragm. The single median arcuate ligament represents the medial fibrous borders of the two crura joined at the midline. The medial arcuate ligament (paired) is the thickened upper portion of the fascia covering the anterior surface of psoas major muscle. It extends from the side of the body of L1 or L2 vertebra to the tip of the transverse process of L1 vertebra. The lateral arcuate ligament (paired) is the thickened upper part of the fascia covering the anterior surface of quadratus lumborum muscle. It extends from the tip of the transverse process of L1 vertebra to the lower border of the 12 th rib. 8
Figure 9. Inferior aspect of the diapghram The diaphragm is supplied by the phrenic nerves (C3, C4, C5). Contraction of the diaphragm pulls down the central tendon and increases the vertical diameter of the thoracic cavity which reduces the intrathoracic pressure and results in inspiration. The reduction in intrathoracic pressure also increases venous and lymphatic return to the thorax. While diaphragmatic contraction reduces intrathoracic pressure, it increases the intra-abdominal pressure and allows evacuation of its viscera. Thus, the diaphragm is a muscle of inspiration and evacuation and a thoracic pump for venous and lymphatic return. Moreover, during weight-lifting, it supports the contracting anterior abdominal wall muscles by increasing the intra-abdominal pressure. During quiet breathing, the diaphragm moves a distance of 0.5-1 cm while in deep breathing it ascends and descends as far as a 10 cm distance. The arterial supply to the diaphragm is from vessels that arise superiorly and inferiorly to it. From above, pericardiophrenic and musculophrenic branches of the internal thoracic artery supply the diaphragm. Superior phrenic arteries, which arise directly from the lower part of the thoracic aorta, and small branches from intercostal arteries, contribute to the supply. The largest arteries supplying the diaphragm arise from below it. These arteries are the inferior phrenic arteries, which branch directly from the abdominal aorta. 9
Structures that pass through the diaphragm Structures that pass through the diaphragm can be divided into three groups: Structures that pass through the diaphragmatic openings: the diaphragm contains three major openings; The caval opening lies in the right part of the central tendon at the level of T8 vertebra and transmits the inferior vena cava and right phrenic nerve. The esophageal opening is surrounded by the right crus at the level of T10 vertebra. It transmits the esophagus, the vagus nerves, the esophageal branches of the left gastric vessels and the lymph vessels of the lower third of the esophagus. The aortic opening is bound by the median arcuate ligament and lies in the posterior part of the diaphragm at the level of T12 vertebra. It transmits the aorta, the thoracic duct and the azygos veins. Structures that pass between the muscle slips: The thoracic splanchnic nerves pass posterior to the crura. The sympathetic trunk passes posterior to the crura or medial arcuate ligament. The intercostal nerves and vessels of the 7 th to the 11 th spaces and the subcostal neurovascular bundle pass between the slips of the costal origin. The superior epigastric vessels pass through the sternocostal triangle. Structures that pierce the diaphragm: The left phrenic nerve pierces the left dome of the diaphragm. THE INTERCOSTAL SPACES There are 11 intercostal spaces between the 12 ribs. Each intercostal space contains three layers of skeletal muscles: external, internal and innermost. The intercostal muscles of each space are supplied by an intercostal neurovascular bundle that runs in the plane between the internal and innermost intercostal muscles. In each space, the bundle consists of the intercostal vein, artery and nerve; arranged in that order from above downwards. The intercostal nerve and vessels run in the costal groove i.e. at the lower border of each rib. The vessels and nerve that run on the lower border of the 12 th rib are called the subcostal vessels and nerve. 10
Deep to the intercostal spaces and ribs, and separating them from the underlying pleura, is a layer of loose connective tissue, called endothoracic fascia, which contains variable amounts of fat. Superficial to the spaces and ribs are deep fascia, superficial fascia, and skin. Muscles associated with the upper limbs and back overlie the spaces. Figure 10. Arrangement of intercostal muscles on the internal aspects of the sternum (A) and ribs (B) and in relation to the intercostal neurovascular bundle (C). The Intercostal Muscles The Intercostals The external intercostal muscle is attached in each space to the lower border of one rib and passes anteroinferiorly to the upper border of the rib below. In the horizontal plane, the muscle fibers begin at the level of the rib tubercle posteriorly but end anteriorly at the level of the costochondral junction. From the level of the costochondral junction to the sternum, the muscle fibers are replaced by a connective tissue aponeurosis called the external intercostal membrane. The external intercostal is most active during inspiration and acts mainly to elevate the ribs. The internal intercostal muscle is attached in each space to the upper border of one rib and passes anterosuperiorly to the lower border of the rib above. Its fibers run in a direction opposite to that of the external intercostal. In the horizontal plane, the internal intercostal fibers begin at the parasternal level anteriorly and extend between the cartilaginous and bony 11
parts of the ribs to end at the level of the rib angle posteriorly. From the level of the rib angle to the vertebral column, the muscle fibers are replaced by a connective tissue aponeurosis called the internal intercostal membrane. The interosseous part of the internal intercostal is most active during expiration and acts mainly to depress the ribs. The interchondral part of the muscle is most active in inspiration. The innermost (intimal) intercostal muscle has a similar orientation and action to the internal intercostal. The innermost intercostal is attached to the upper margin of the costal groove while the internal intercostal is attached to the lower margin of the groove and the neurovascular bundle runs between the two muscles. The innermost intercostal muscles are not clearly recognized in all spaces and may even be absent. The Subcostales and subcostalis These two groups of muscles lie in the same plane of the internal intercostals and are believed to be part of an incomplete inner layer of intercostal muscles. The subcostales: are muscular slips that pass from the lower border of one rib and insert to the upper borders of the 2 or 3 ribs below, extending over more than one space. They are most evident in the lower intercostal spaces and act to raise the lower ribs during inspiration. Transversus thoracis (sternocostalis): takes origin from the posterior surface of the xiphisternum, the lower part of the body of the sternum and the adjacent parts of the costal cartilages of the lower true ribs. Its fibers radiate superolaterally to be inserted to the lower borders of the 2 nd (or 3 rd ) to the 6 th costal cartilages and help to depress them during expiration. 12
Movements of the Thoracic Wall during Respiration During breathing, the dimensions of the thorax change in the vertical, transverse and anteroposterior directions. Elevation and depression of the diaphragm significantly alter the vertical diameter of the thorax. Depression results when the muscle fibers of the diaphragm contract. Elevation occurs when the diaphragm relaxes. Changes in the anteroposterior and lateral dimensions result from elevation and depression of the ribs. Because the anterior ends of the ribs lie at a lower level than the posterior ends, movement of the ribs (primarily 2 nd -6 th ) at the costovertebral joints around an axis passing through the necks of the ribs causes the anterior ends of the ribs to rise. When the ribs are elevated, they move the sternum upward and forward. When the ribs are depressed, the sternum moves downward and backward. This 'pump handle' type of movement changes the anteroposterior diametr of the thorax. As well as the anterior ends of the ribs being lower than the posterior ends, the middles of the shafts tend to be lower than the two ends. When the ribs are elevated, the middles of the shafts move laterally. This 'bucket handle' movement changes the lateral diameter of the thorax. Inspiratory muscles: Inspiration is always an active process resulting from changes in thoracic diameters that increase the thoracic volume and decrease the intrathoracic pressure. Muscles responsible for such changes include: The diaphragm The external intercostals Serratus posterior superior and Levatores costarum The subcostales and the interchondral parts of the internal intercostal and innermost intercostal muscles - The lower ribs are fixed during inspiration by quadratus lumborum muscle. - Accessory muscles of inspiration (used in resisted inhalation) elevate the upper ribs and include the scalenes, sternocleidomastoid, pectoralis minor and serratus anterior. Expiratory muscles: Expiration during quiet breathing is a passive process that depends on the elastic recoil of the lungs. Muscular activity is required during rapid breathing or forced expiration. Muscles with expiratory activity include: Serratus posterior inferior Transversus thoracic (sternocostalis) Interosseous parts of the internal intercostals and innermost intercostals. Quadratus lumborum and muscles of the anterior abdominal wall contract forcibly to accelerate diaphragmatic elevation during coughing and sneezing. The Intercostal Nerves The intercostal nerves represent the ventral rami of the 1 st - 11 th thoracic spinal nerves. The subcostal nerve is the ventral ramus of the 12 th thoracic nerve. The intercostal nerves are divided into typical and atypical nerves. The typical intercostal nerves are the 3 rd -6 th intercostal nerves. They run along the costal grooves of their corresponding ribs between the internal and innermost intercostal muscles. Each typical intercostal nerve gives the following branches: White and grey rami communicantes pass to and from the corresponding sympathetic ganglion. Collateral branches arise near the angles of the ribs and descend to run along the superior margin of the rib below. They assist in supplying intercostal muscles and parietal pleura. 13
The lateral cutaneous branch arises near the midaxillary line, pierces the internal and external intercostal muscles and divides into anterior and posterior branches. These terminal branches supply the skin of the lateral thoracic and abdominal walls. The anterior cutaneous branch pierces the muscles and membranes of the intercostal space at the parasternal line and divides into medial and lateral branches. These terminal branches supply the skin on the anterior aspect of the thorax and abdomen. Muscular branches supply the intercostal, subcostal, transversus thoracis, levatores costarum, and serratus posterior muscles. Figure 11. Distribution of the typical intercostal nerve The atypical intercostal nerves have the following features: The 1 st and 2 nd intercostal nerves course on the internal surface of the 1 st and 2 nd ribs, instead of running along the inferior margin in costal grooves. The 1 st intercostal nerve has no anterior cutaneous branch and often no lateral cutaneous branch. It represents only a small part of the ventral ramus of the 1 st thoracic spinal nerve. Most of the 1 st thoracic ventral ramus joins the brachial plexus. The 2 nd intercostal nerve gives rise to a large lateral cutaneous 14
branch, the intercostobrachial nerve; it emerges from the 2 nd intercostal space at the midaxillary line, penetrates the serratus anterior, and enters the axilla and arm to supply the floor of the axilla and then communicates with the medial cutaneous nerve of the arm to supply the medial and posterior surfaces of the arm. The 7 th -11 th intercostal nerves, after giving rise to lateral cutaneous branches, cross the costal margin and continue on to supply abdominal skin and muscles. No longer being between ribs (intercostal), they now become thoraco-abdominal nerves of the anterior abdominal wall. Their anterior cutaneous branches pierce the rectus sheath, becoming cutaneous close to the median plane. The Intercostal Arteries The intercostal spaces receive their main arterial supply from the intercostal arteries. Additional supply is provided to the superficial layers of the intercostal spaces by the branches of the axillary artery. With the exception of the 10 th and 11 th intercostal spaces, each intercostal space is supplied by three arteries: two small anterior intercostal arteries (superior and inferior) and one large posterior intercostal artery. In the 10 th and 11 th intercostal spaces, there is only a posterior intercostal artery and no anterior intercostal arteries. Figure 12. The intercostal arteries in a horizontal plane. The arteries anastomose with each other at the midclavicular line. Although the distributions of the anterior and posterior intercostal arteries overlap, the anterior intercostal arteries supply each space from the sternum to the level of the midclavicular line. The posterior intercostal artery and its collateral branch supply the space from the level of the midclavicular line backwards. In addition to the intercostal muscles and overlying skin and fascia, the intercostal arteries supply the parietal pleura, the parietal peritoneum and the peripheral parts of the diaphragm. 15
The anterior intercostal arteries There are two anterior intercostal arteries in each of the upper 9 spaces only. In the 10 th and 11 th spaces, they are absent and replaced by continuation of the posterior intercostal artery. The anterior intercostal arteries of the upper 6 spaces are branches of the internal thoracic artery. Those of the 7 th, 8 th, and 9 th spaces arise from the musculophrenic artery, which is a branch of the internal thoracic artery. The internal thoracic (mammary) vessels The internal thoracic artery begins as a branch from the first part of the subclavian artery posterior to the sternoclavicular joint. It passes anteriorly over the cervical dome of pleura and descends vertically through the superior thoracic aperture. On each side, the internal thoracic artery lies posterior to the costal cartilages of the upper six ribs about 1 cm lateral to the sternum. At the level of the sixth intercostal space, it ends by dividing into two terminal branches; the musculophrenic and superior epigastric arteries. Along its course, the internal thoracic artery gives the following branches: Figure 13. The internal thoracic artery, anterior view. Two anterior intercostal arteries to each of the upper 6 intercostal spaces. The mediastinal artery mainly supplies the thymus gland in the anterior mediastinum. The pericardiophrenic artery runs with the phrenic nerve to supply the fibrous and parietal pericardium. Perforating branches pierce the intercostal muscles and pass anteriorly to supply the overlying skin and fascia. Long perforating branches pass to the breast as the medial mammary arteries. 16
Sternal branches pass medially deep to the sternum. The superior epigastric artery passes through the sternocostal triangle to enter the rectus sheath and supply the rectus abdominis muscle as far downwards as the level of the umbilicus. The musculophrenic artery curves around the costal margin and supplies the diaphragm. It also gives two anterior intercostal arteries to the 7 th, 8 th and 9 th intercostal spaces. Occasionally, a lateral costal artery arises from the internal thoracic artery near its origin and runs inferolaterally on the inner surface of the thoracic cage. The internal thoracic vein begins at the 6 th intercostal space by the union of the musculophrenic and superior epigastric veins and ascends medial to the internal thoracic artery to drain into the subclavian vein. It receives tributaries corresponding to the branches of the internal thoracic artery. The posterior intercostal arteries The 1 st and 2 nd posterior intercostal arteries arise from the supreme (highest) intercostal artery which is a branch of the costocervical trunk of the subclavian artery. The 3 rd -11 th posterior intercostal arteries and the subcostal artery are all branches of the descending aorta. Because the aorta is on the left side of the vertebral column, the right posterior intercostal arteries cross the midline anterior to the bodies of the vertebrae and therefore are longer than the left posterior intercostal arteries. Each posterior intercostal artery gives a dorsal branch that accompanies the dorsal ramus of the spinal nerve to supply the spinal cord, vertebral column, back muscles, and skin. It also gives a small collateral branch that crosses the intercostal space and runs along the superior border of the rib below. A lateral cutaneous branch is also given and accompanies the lateral cutaneous branch of the intercostal nerve. The Intercostal Veins Like the arteries, each intercostal space contains three intercostal veins; two anterior and one posterior. There is only a posterior intercostal vein in the 10 th and 11 th spaces. The anterior intercostal veins of the upper 6 spaces drain to the internal thoracic vein. The anterior intercostal veins of the 7 th, 8 th and 9 th spaces drain to the musculophrenic vein. The posterior intercostal veins drain to the brachiocephalic veins and azygos venous system as follows: On the right side, the first posterior intercostal vein drains directly to the right brachiocephalic vein. The 2 nd, 3 rd and 4 th posterior intercostal veins unite to form the right superior intercostal vein which drains to the azygos vein. The remaining posterior intercostal veins and the subcostal vein drain segmentally to the azygos vein. On the left side, the first posterior intercostal vein drains directly to the left brachiocephalic vein. The 2 nd, 3 rd and usually the 4 th posterior intercostal veins unite to form the left superior intercostal vein which drains to the left brachiocephalic vein. The 5 th -8 th posterior intercostal veins drain to the superior (accessory) hemiazygos vein. The remaining posterior intercostal veins and subcostal vein drain to the inferior hemiazygos vein. 17
Figure 14. The posterior intercostal veins and the azygos venous system. Lymph Vessels/Nodes Lymphatic vessels of the thoracic wall drain into three groups of lymph nodes: The parasternal lymph nodes lies along the internal thoracic vessels and drain to the bronchomediastinal trunk. The intercostal lymph nodes lie in each space at the level of the heads and necks of the ribs. Diaphragmatic lymph nodes lie posterior to the xiphisternum and at sites where the phrenic nerves penetrate the diaphragm. 18
THE PULMONARY CAVITIES The right and left pulmonary cavities lie on each side of the mediastinum ( a septum between the lungs extending from the sternum to the vertebral column and from the upper thoracic aperture to the diaphragm). The pulmonary cavities contain the lungs surrounded by the pleura. Each pulmonary cavity extends for a short distance above the first rib into the root of the neck. The diaphragm and the thoracic wall form the inferior and lateral walls, respectively, while the mediastinum forms the medial wall of each cavity. THE PLEURA During embryological development, the thoracic cavity contains three serous sacs whose walls consist of a single layer of mesothelial tissue. These are the two pleural sacs and one pericardial sac. The pleural sacs become invaginated by the growing lung buds from the mediastinum while the pericardial sac is invaginated by the developing heart. Each sac is therefore reduced into a thin double-layered cavity. The fully developed pleura is a double-layered serous membrane that surrounds the lung and allows it to move freely within the pulmonary cavity. Like other serous membranes in the body, the pleura consists of an inner visceral layer which is closely adherent to the lungs and extends into their fissures; and an outer parietal layer which lines the walls of the pulmonary cavity. The film-thin space between the two layers is the pleura cavity which contains a small amount (30 ml) of pleural fluid. The fluid lubricates the pleural surfaces and allows the layers of pleura to slide smoothly over each other during respiration. The surface tension of the pleural fluid also provides the cohesion that keeps the lung surface in contact with the thoracic wall. Parts of the Pleura The visceral pleura is firmly attached to the surface of the lung, including both opposed surfaces of the fissures that divide the lungs into lobes. The visceral pleura is continuous with parietal pleura at the hilum of each lung where structures enter and leave the lung. The parietal pleural lines the walls of the pulmonary cavity and is divided into four regions, named according the part of the wall they line: The cervical pleura (pleural cupola) is the cup-like dome over the apex of each lung. It reaches its summit 2-3 cm superior to the level of the medial third of the clavicle at the level of the neck of the 1 st rib. It project as much as 3-4 cm above the first costal cartilage, but does not extend above the neck of the 1 st rib. The cervical pleura is reinforced by the suprapleural membrane. The costal pleura lines the internal surface of the thoracic wall (ribs and intercostal muscles) extending from the sides of the vertebrae posteriorly to the sternum anteriorly. It is separated from the internal surface of the thoracic wall by a thin, extrapleural layer of loose connective tissue called the endothoracic fascia. The diaphragmatic pleura covers the superior surface of the diaphragm on each side of the mediastinum. A thin, more elastic layer of endothoracic fascia called the phrenicopleural fascia connects the diaphragmatic pleura with the muscular fibers of the diaphragm. 19
The mediastinal pleura covers the mediastinal structures on each side of the mediastinum. It continues superiorly into the root of the neck as cervical pleura. It is continuous with costal pleura anteriorly and posteriorly and with the diaphragmatic pleura inferiorly. In the region of the 5 th -7 th thoracic vertebrae, the mediastinal pleura reflects off the mediastinum as a tubular, sleeve-like covering for structures that pass between the lung and mediastinum. This sleeve-like covering, and the structures it contains, form the root of the lung. Here, the mediastinal pleura is continuous with the visceral pleura. The pleural sleeve hangs below the lung root for a short distance as a double-layered membrane called the pulmonary ligament. This arrangement allows upward and downward movement of the root structures with the lung during respiration. Figure 15. Parts of the pleura in horizontal (left) and coronal (right) sections. Pleural Reflections and Recesses The abrupt lines along which the parietal pleura changes direction as it passes from one wall of the pulmonary cavity to another are the lines of pleural reflection. They represent areas where the different parts of the parietal pleura meet each other. The pleural recesses are potential empty parts of the pleural sac that lodge part of the lung only during deep inspiration. The lungs do not completely fill the anterior or posteroinferior regions of the pleural cavities. This results in recesses in which two layers of parietal pleura become opposed. The recesses also provide potential spaces in which fluids can collect and from which fluids can be aspirated. There are three lines of pleural reflection but only two pleural recesses: The sternal reflection occurs where the costal pleura meets the mediastinal pleura anteriorly. This area contains the costomediastinal recess as a narrow (1 cm) vertical gutter. The left recess is larger because the cardiac notch in the left lung is more concave than the corresponding notch in the pleural sac. 20
The vertebral reflection lies where the costal pleura meets the mediastinal pleura posteriorly. There is no recess here because the rounded posterior borders of the lungs completely fill the area on the sides of the vertebrae. The costodiaphragmatic (costophrenic) reflection and recess lie where the costal pleura meet the diaphragmatic pleura. In the erect position, the recess is the most redundant part of the pleural cavity and is the first area for accumulation of fluids. It is deepest (lowest) at the level of the midaxillary line. This area is therefore used for fluid aspiration from the pleural cavity. Arterial Supply The visceral pleura is supplied by branches from the bronchial arteries. The parietal pleura receives branches from the descending aorta, intercostal, musculophrenic and internal thoracic arteries. Nerve Supply The visceral pleura receives autonomic fibers from the pulmonary autonomic plexus and is sensitive to stretch and chemical irritation. The parietal pleura receives somatic nerve supply from the nerves that supply the walls of the pulmonary cavity. The intercostal nerves supply the costal pleura and the peripheral parts of the diaphragmatic pleura. The phrenic nerves supply the mediastinal and cervical pleurae and the central part of the diaphragmatic pleura. The parietal pleura is sensitive to general sensory modalities (pain, touch, temperature and vibration). Lymphatic Drainage Lymph from the visceral pleura drains to the subpleural pulmonary lymphatic plexus and then to the bronchopulmonary lymph nodes. The parietal pleura lymph drains to the intercostal, parasternal, diaphragmatic and mediastinal lymph nodes. Surface Anatomy of the Pleura and Lungs The outlines of the right and left pulmonary cavities are asymmetrical because the heart extends toward the left side, imposing on the left cavity more markedly than on the right. The outlines of the lung and pleura can be traced on the surface of the thorax by a continuous line that has three parts: anterior, inferior and posterior. The anterior border of the pleura begins at the summit of the pleural cupola 2-3 cm above the medial third of the clavicle. It descends behind the sternoclavicular joint to the level of the sternal angle (2 nd costal cartilage) where the right and left sides become closest to each other. Between the levels of the 2 nd and 4 th costal cartilages, the right and left lines descend in contact. The pleural sacs may even slightly overlap each other. At the level of the 4 th costal cartilage, the left border is deviated laterally from the sternum by the heart over the cardiac notch of the left lung. 21
Figure 16. Surface outline of the parietal pleura The right side continues a more vertical descent. Both sides then reach the level of the 6 th costal cartilage where the anterior border ends and the inferior border begins. The anterior border of the lung follows a similar course but it falls about 1cm behind the pleural border creating the costomediastinal recess. The inferior border of the pleura runs inferolaterally from the 6 th costal cartilage so that it crosses the midclavicular line anteriorly at the level of the 8 th rib and the midaxillary line laterally at the level of the 10 th rib. It then curves posteriorly to reach the paravertebral line at the level of the 12 th rib and ends at the sides of T12 vertebra. The inferior border of the lung runs a similar but higher course. It reaches the midclavicular line at the level of the 6 th rib, the midaxillary line at the 8 th rib and the paravertebral line at the 10 th rib. The difference between the inferior borders of the pleura and lung results creates the costophrenic recess. The posterior borders of the pleura and lung are superimposed. They ascend vertically from the level of T12 vertebra to the level of the neck of the 1 st rib to reach the pleural cupola and lung apex. 22
It is important to mention that the pleural outline extends outside the margins of the thoracic cage in three areas: at the right xiphi-costal angle overlying the stomach, and at the right and left costovertebral angles overlying the kidneys. Figure 17. Surface outline of the parietal pleura and lungs THE LUNGS The lung is a light, spongy soft organ responsible chiefly for gaseous exchange. The lungs lie on either side of the mediastinum surrounded by the pleural sacs. Air enters and leaves the lungs via the tracheobronchial tree. The pulmonary arteries bring deoxygenated blood to the lungs from the right ventricle of the heart. Oxygenated blood returns to the left atrium via the pulmonary veins. The right lung is heavier, broader, and shorter and has more fissures and lobes than the left lung. The difference in weight and width is due to the fact that the middle mediastinum, containing the heart, bulges more to the left than to the right. The difference in height is due to the higher right dome of the diaphragm. 23
Surface Features Each lung has a half-cone shape, with an apex, base, two surfaces and three borders: The apex is the blunt rounded upper part that projects 2.5 cm above the medial 1 /3 of the clavicle at the level of the neck of the 1 st rib, under cover of the cervical pleura and the suprapleural membrane. The base is the concave inferior surface that rests on the diaphragmatic domes. The right dome of the diaphragm separates the right base from the right lobe of the liver. The left dome separates the left base from the left lobe of the liver, the gastric fundus and the spleen. The right base is broader and more concave than the left. Each base has a sharp posterolateral border that projects into the costodiaphragmatic recess during deep inspiration. The two surfaces are the costal surface which lies immediately adjacent to the ribs and intercostal spaces of the thoracic wall and the mediastinal surface which lies against the mediastinum. The costal surface is large, smooth, and convex while the mediastinal surface is concave. The base may be considered as a third (inferior) surface. The three borders are anterior, inferior and posterior. The inferior border of the lung lies between the base and the costal surface. The anterior and posterior borders lie between the costal and mediastinal surfaces. The anterior and inferior borders are sharp while the posterior border is smooth and rounded. The anterior border of the left lung is notched by the heart at the cardiac notch, below which the lung extends in a tongue-like process called the lingula. The lungs lie directly adjacent to, and are indented by, structures contained in the mediastinum and the thoracic cage. The costal surface is characterized by horizontal ridges that correspond to the overlying ribs and intercostal spaces. The mediastinal surface contains the lung hilum and root and shows various impressions caused by different mediastinal structures. The lung hilum is the depression in the center of the mediastinal surface where structures enter and leave the lung. The lung root consists of a sleeve-like pleural fold that surrounds the collection of structures that pass through the hilum. These structures are the main bronchus, the pulmonary artery, two pulmonary veins, bronchial vessels, lymph vessels and nerves. Around the lung hilum, the mediastinal surface shows the following impressions: 24
25 FIGURE 18. THE MEDIASTINAL SURFACES OF THE LUNGS. THE SQUARE GRIDS ON THE UPPER CORNERS INDICATE THE ARRANGEMENT OF THE ROOT STRUCTURES (A: PULMONARY ARTERY, B: BRONCHUS, V: PULMONARY VEIN)
In the right lung: The cardiac impression lies anteroinferior to the hilum and is caused by the right atrium of the heart. Passing upwards from this area is the faint impression of the superior vena cava. The right subclavian vessels form an arched impression near the pulmonary apex behind the superior vena caval impression. Inferior to the cardiac impression is the small impression of the inferior vena cava. The azygos impression curves above the hilum and is caused by the azygos arch. The esophageal impression runs vertically behind the hilum. In the left lung: The cardiac impression lies anteroinferior to the hilum and is caused by the left ventricle. The aortic impression curves above the hilum and is caused by the aortic arch. The impression of the left subclavian artery is also seen passing upwards from here. The descending thoracic aorta leaves a vertical impression posterior to the hilum. The phrenic and vagus nerves, the sympathetic trunks and the thoracic duct are mediastinal structures that run near the medial surfaces of the lungs but do not leave clear impressions. Fissures, Lobes and Segments A pulmonary fissure is a deep furrow that extends from the lung surface to the hilum dividing the lung into lobes. The pulmonary fissures are covered with visceral pleura. The pulmonary lobes are subdivided by connective tissue septa into bronchopulmonary segments. The segments are further subdivided into functional respiratory lobules. The right lung has oblique and horizontal fissures that divide it into three lobes: superior, middle, and inferior. The left lung has only an oblique fissure that divides it into superior and inferior lobes. Chest auscultation for lung sounds The orientations of the oblique and horizontal fissures determine where clinicians can listen for lung sounds from each lobe. The pulmonary apex is auscultated at the root of the neck. The largest surface of the superior lobe is in contact with the upper part of the anterolateral wall. It is therefore best heard at the upper part of the anterior thoracic wall. The surface of the middle lobe lies mainly adjacent to the lower anterior and lateral wall. It is heard best at the anterolateral part of the thoracic wall, usually at the 5 th intercostal space. The largest surface of the inferior lobe is in contact with the posterior and inferior walls. It is therefore auscultated posteroinferiorly below the scapula, usually at the level of T10 vertebra. The approximate position of the oblique fissure in both lungs on a patient, in quiet respiration, can be marked by a curved line on the thoracic wall that begins roughly at the spine of T3 vertebra 26
posteriorly, crosses the 5 th intercostal space laterally, and then follows the contour of 6 th rib anteriorly. The horizontal fissure follows the right 4 th rib or 4 th intercostal space from the sternum until it meets the oblique fissure as it crosses the right 5 th rib at the midaxillary line. Figure 19. Surface projections of the lung fissures and lobes. The bronchopulmonary segment is the smallest, functionally independent region of a lung that can be isolated and removed without affecting adjacent regions. Each segment is separated from the adjacent segments by connective tissue septa. The segment is supplied by its own segmental bronchus and a branch of the pulmonary artery and is drained by its own pulmonary vein and lymphatic vessels. A bronchopulmonary segment is pyramidal in shape with its apex directed towards the lung hilum and its base towards the lung surface. In each segment, the pulmonary and bronchial arteries run and divide with the segmental bronchus while the pulmonary veins and lymphatic vessels run in the intersegmental connective tissue septa. The common arrangement of bronchopulmonary segments in each lung is as follows: The right superior lobe is composed of 3 segments: apical, anterior and posterior. The right middle lobe is composed of 2 segments: medial and lateral. 27
The left superior lobe is composed of 5 segments: apical, anterior, posterior, superior lingular and inferior lingular. The lingular segments on the left correspond to the segments of the middle lobe on the right. The inferior lobe in each lung is composed of 5 segments: apical, anterior basal, posterior basal, medial basal and lateral basal. Figure 20. The lung lobes, fissures and bronchopulmonary segments. (Ant.: anterior,inf.: inferior, Lat.: lateral, Med.: medial, MB: medial basal, Post.: posterior, Sup.:superior) The Tracheobronchial Tree This is a branching system of respiratory passages. The trachea is a patent tube composed of C- shaped cartilaginous rings and smooth muscles. It begins with the trachea at the level of C6 vertebral body and ends at the level of the sternal angle by bifurcating into right and left primary or main bronchi. Each main bronchus enters the root of a lung and passes through the hilum into the lung itself. The right main bronchus is wider and takes a more vertical course than the left main bronchus. Therefore, inhaled foreign bodies tend to lodge more frequently in the right lung than in the left. The main bronchus divides within the lung into lobar or secondary bronchi, each of which supplies a lobe. The lobar bronchi further divide into segmental or tertiary bronchi, which supply the bronchopulmonary segments. Beyond the segmental bronchi, there are 20 to 25 generations of branching conducting bronchioles that eventually end as terminal bronchioles, the smallest conducting bronchioles. The walls of the bronchi are held open by discontinuous elongated plates of cartilage, but these are not present in bronchioles. Each terminal bronchiole gives rise to several generations of respiratory bronchioles, characterized by scattered, thin-walled outpocketings (alveoli) that extend from their lumens. 28
The pulmonary alveolus is the basic structural unit of gas exchange in the lung. Each respiratory bronchiole gives rise to 2-11 alveolar ducts, each of which gives rise to 5-6 alveolar sacs. Alveolar ducts are elongated airways densely lined with alveoli, leading to common spaces, the alveolar sacs, into which clusters of alveoli open. Blood Supply of the Lungs The lungs have a dual blood supply, the pulmonary circulation for gas exchange and the bronchial circulation to supply the parenchymal tissue of the lung itself. The bronchial arteries supply the lung tissue with oxygenated blood. The bronchial veins drain the lung venous blood to the azygos venous system. The pulmonary circulation begins from the right ventricle of the heart with pumping of deoxygenated blood to the pulmonary trunk. This trunk divides into two pulmonary arteries, one for each lung. The pulmonary arteries enter the lungs and divide with the bronchi to reach the alveoli where gaseous exchange occurs and the blood becomes oxygenated. This oxygenated blood returns to the left atrium of the heart via four pulmonary veins, two from each lung. The bronchial arteries The bronchial arteries provide oxygen and nutrients to the lung tissue and visceral pleura. They interconnect within the lung with branches of the pulmonary arteries and veins. The bronchial arteries originate from the thoracic aorta or one of its branches: The two left bronchial arteries (upper and lower) arise directly from the anterior surface of the descending aorta. The single right bronchial artery usually arises from the third posterior intercostal artery, but occasionally, it originates from the upper left bronchial artery. The bronchial veins The bronchial veins drain only part of the blood supplied to the lungs by the bronchial arteries, the rest of the venous blood is drained by the pulmonary veins. The bronchial veins drain into the azygos vein on the right and into the superior intercostal vein or hemiazygos vein on the left. The pulmonary arteries The pulmonary trunk and arteries carry deoxygenated blood, but they are arteries in the sense that they transmit blood in a pulsatile manner away from the heart at a relatively high pressure (20 to 30 mmhg). They also have elastic walls like the aorta. Pulmonary arteries are commonly colored blue, like veins, in anatomical illustrations. The pulmonary trunk extends from the right ventricle to the concavity of the aortic arch, to the left of the ascending aorta, where it divides into the right and left pulmonary arteries. The bifurcation of the pulmonary trunk occurs to the left of the midline just inferior to the level of the sternal angle, and anteroinferiorly to the left of the bifurcation of the trachea. In the concavity of the aortic arch, the pulmonary trunk is attached to the aortic arch by the ligamentum arteriosum. This is a fibrous band that represents a remnant of the ductus arteriosus, an embryological arterial channel between the aorta and the pulmonary trunk. The right pulmonary artery is longer than the left and passes horizontally across the mediastinum. It runs anteroinferior to the tracheal bifurcation, anterior to the right main bronchus and posterior to the ascending aorta, superior vena cava and right superior pulmonary vein. 29
The left pulmonary artery is shorter than the right and lies anterior to the descending aorta and posterior to the left superior pulmonary vein. Each pulmonary artery passes through the hilum and branches within the lung into lobar and segmental branches. The pulmonary veins Two pulmonary veins, a superior and an inferior pulmonary vein on each side, carry oxygenated blood from the corresponding lobes of each lung to the left atrium of the heart. The right middle lobe vein is a tributary of the right superior pulmonary vein. Pulmonary veins are commonly colored red, like arteries, in anatomical illustrations. The pulmonary veins run independently of the arteries and bronchi in the lung, coursing between and receiving blood from adjacent bronchopulmonary segments as they run toward the hilum. Lymphatic Drainage of the Lungs Two lymphatic plexuses drain the lung. The superficial (subpleural) lymphatic plexus lies just deep to the visceral pleura and drains the lung parenchyma and visceral pleura. Lymphatic vessels from this plexus drain into the bronchopulmonary lymph (hilar) nodes at the hilum. The deep lymphatic plexus is located in the submucosa of the bronchi and in the peribronchial connective tissue. Lymphatic vessels from this plexus drain initially into the pulmonary lymph nodes, located along the lobar bronchi. Lymph vessels then pass from these nodes to follow the bronchi and pulmonary vessels to the hilum of the lung, where they also drain into the bronchopulmonary lymph nodes. From the bronchopulmonary lymph nodes, lymph passes to the superior and inferior tracheobronchial lymph nodes, superior and inferior to the bifurcation of the trachea and main bronchi, respectively. Lymph from the tracheobronchial lymph nodes passes to the right and left bronchomediastinal lymph trunks, the major lymph pathways draining the thoracic viscera. These trunks usually terminate on each side at the venous angles formed by the junction of the subclavian and internal jugular veins. However, the right bronchomediastinal trunk may first merge with other lymphatic trunks forming a short right lymphatic duct. The left bronchomediastinal trunk may terminate in the thoracic duct. 30
Figure 21. The lymphatic drainage of the lungs and pleura. Nerve Supply of the Lungs The lung innervation is provided by two interconnected nerve plexuses. The small anterior and larger posterior pulmonary nerve plexuses are formed at the hila anterior and posterior to the tracheal bifurcation and main bronchi, respectively. The plexuses consist of efferent sympathetic and parasympathetic fibers with afferent visceral fibers. The sympathetic fibers of the pulmonary plexuses are postganglionic fibers that emerge from the ganglia of the thoracic part of the sympathetic trunks (T2-T4 mainly). The sympathetic fibers are inhibitory to the bronchial muscles (bronchodilator), motor to the pulmonary vessels (vasoconstrictor), and inhibitory to the glands of the bronchial tree. The parasympathetic fibers conveyed to the pulmonary plexus are preganglionic fibers from the vagus nerve. They synapse at parasympathetic ganglia in the pulmonary plexuses. The postganglionic efferent parasympathetic fibers are motor to the smooth muscles of the 31