ANATOMY AND EMBRYOLOGY

RESPIRATORY SYSTEM

The respiratory system is made up of the structures involved in the exchange of oxygen and carbon dioxide between the blood and the atmosphere, so-called external respiration . The exchange of gases between the blood in the capillaries of the systemic circulation and the tissues in which these capillaries are located is referred to as internal respiration .

The respiratory system consists of the external nose, internal nose, and paranasal sinuses; the pharynx, which is the common passage for air and food; the larynx, where the voice is produced; and the trachea, bronchi, and lungs. Accessory structures necessary for the operation of the respiratory system are the pleurae, diaphragm, thoracic wall, and muscles that raise and lower the ribs in inspiration and expiration. The muscles of the anterolateral abdominal wall are also accessory to forceful expiration (their contraction forces the diaphragm upward by pressing the contents of the abdominal cavity against it from below) and are used in “abdominal” respiration. Certain muscles of the neck can elevate the ribs, thus enlarging the anteroposterior diameter of the thorax, and under some circumstances, the muscles attaching the arms to the thoracic wall can also help change the capacity of the thorax.

In Plates 1-1 through 1-16 , the anatomy of the respiratory system and significant accessory structures is shown. It is important not only to visualize these structures in isolation but also to become familiar with their blood supply, nerve supply, and relationships with both adjacent structures and the surface of the body. One should keep in mind that these relationships are subject to the same degree of individual variation that affects all anatomic structures. The illustrations depict the most common situations encountered. No attempt is made to describe all of the many variations that occur.

An important and clinically valuable concept that is worth emphasizing at this point is the convention of subdividing each lung into lobes and segments on the basis of branching of the bronchial tree. From the standpoint of its embryologic development, as well as of its function as a fully established organ of respiration, the lung is indeed the ultimate branching of the main bronchus that leads into it. Knowledge of the subdivision of the lung on this basis is essential to anatomists, physiologists, pathologists, radiologists, surgeons, and chest physicians because without this three-dimensional key, there is no exact means of precisely localizing lesions within the respiratory system.

BONY THORAX

The skeletal framework of the thorax—the bony thorax—consists of 12 pairs of ribs and their cartilages, 12 thoracic vertebrae and intervertebral discs, and the sternum. The illustration also includes one clavicle and scapula because these bones serve as important attachments for some of the muscles involved in respiration.

The sternum is made up of three parts—the manubrium, body, and xiphoid process. The manubrium and body are not in quite the same plane and thus form the sternal angle at their junction, a significant landmark at which the costal cartilage of the second rib articulates with the sternum. The superior border of the manubrium is slightly concave, forming what is called the suprasternal notch .

The costal cartilages of the first through seventh ribs ordinarily articulate with the sternum and are called true ribs . The costal cartilages of the eighth through tenth ribs ( false ribs ) are usually attached to the cartilage of the rib above, and the ventral ends of the cartilages of the eleventh and twelfth ribs ( floating ribs ) have no direct skeletal attachment.

All of the ribs articulate dorsally with the vertebral column in such a way that their ventral end (together with the sternum) can be raised slightly, as occurs in inspiration. The articulations of the costal cartilages with the sternum, except those of the first rib, are true or synovial joints that allow more freedom of movement than there would be without this type of articulation.

The deep surface of the scapula (the subscapular fossa) fits against the posterolateral aspect of the thorax over the second to seventh ribs, where, to a great extent, it is held by the muscles that are attached to it. The acromion process of the scapula articulates with the lateral end of the clavicle; this acts as a strut to hold the lateral angle of the scapula away from the thorax. On the dorsal surface of the scapula, a spine protrudes and continues laterally into the acromion process. At its vertebral end, the spine flattens into a smooth triangular surface with the base of the triangle at the vertebral border. The spine separates the supraspinous fossa from the infraspinous fossa. Three borders of the scapula are described—superior, lateral, and medial or vertebral. On the superior border is a notch or incisura, and lateral to this, the coracoid process protrudes anteriorly.

The lateral angle of the scapula presents a slight concavity, the glenoid fossa, for articulation with the head of the humerus. At the superior end of the glenoid fossa is the supraglenoid tuberosity, and at its inferior margin is the infraglenoid tuberosity.

The clavicle articulates at its medial end with the superolateral aspect of the manubrium of the sternum and at its lateral end with the medial edge of the acromion process of the scapula. Its medial two-thirds are curved slightly anteriorly, and its lateral third is curved posteriorly. Muscular attachments to the medial and lateral parts of the clavicle leave its middle portion less protected and thus readily subject to fracture.

The vertebral levels of the bony landmarks on the ventral aspect of the thorax are variable and differ somewhat with the phase of respiration. In general, the upper border of the manubrium is at the level of the second to third thoracic vertebrae, the sternal angle opposite the fourth to fifth thoracic vertebrae, and the xiphisternal junction at the level of the ninth thoracic vertebra.

RIB CHARACTERISTICS AND COSTOVERTEBRAL ARTICULATIONS

A typical rib has a head, a neck, and a body. The head articulates with one or two vertebral bodies (see below). A tubercle at the lateral end of the relatively short neck articulates with the transverse process of the lower of the two vertebrae with which the head of the rib articulates. As the body is followed anteriorly, the “angle” of the rib is formed. At the inferior border of the body is the costal or subcostal groove, partially housing the intercostal artery, vein, and nerve. Each rib is continued anteriorly by a costal cartilage by which it is attached either directly or indirectly to the sternum, except for the eleventh and twelfth ribs, which have no sternal attachment.

The first and second ribs differ from the typical rib and therefore need special description. The first rib—the shortest and most curved of all the ribs—is quite flat, and its almost horizontal surfaces face roughly superiorly and inferiorly. On its superior surface are grooves for the subclavian artery and subclavian vein, separated by a tubercle for the attachment of the scalenus anterior muscle.

The second rib is a good deal longer than the first, but its curvature is very similar to the curvature of the first rib. The angle of the second rib, which is close to the tubercle, is not at all marked. Its external surface faces to some extent superiorly but a bit more outward than that of the first rib.

The typical articulation of a rib with the vertebral column involves both the head and tubercle of the rib. The head has two articular facets—the superior facet making contact with the vertebral body above and the inferior one with the vertebral body below. Between these, the head of the rib is bound to the intervertebral disc by the intraarticular ligament. The articular facet on the tubercle of the rib contacts the transverse process of the lower of the two vertebrae. These are true or synovial joints, with articular cartilages, joint capsules, and synovial cavities. The articulations of the first, tenth, eleventh, and twelfth ribs are each with only one vertebra, the vertebra of the same number.

The ligaments related to the typical articulation of a rib with the vertebral column are as follows: for articulation of the head of the rib, the intraarticular ligament and the capsular ligament, with a thickening of its anterior part forming the radiate ligament; and for the costotransverse joint, the thin capsular ligament, the lateral costotransverse ligament between the lateral part of the tubercle of the rib and the tip of the transverse process, and the superior costotransverse ligament attached to the transverse process of the rib above.

The first and the last two (or three) ribs each has a single articular facet that makes contact with an impression on the side of the thoracic vertebra of the same number. No intraarticular ligament is present, so there is just a single synovial cavity, in contrast to the two synovial cavities present for the, typical rib. The lowest ribs do not have synovial joints between their tubercles and the transverse processes of the related vertebrae.

ANTERIOR THORACIC WALL

The anterior thoracic wall is covered by skin and the superficial fascia, which contains the mammary glands. Its framework is formed by the anterior part of the bony thorax, described and illustrated in Plate 1-2 .

The muscles here belong to three groups: muscles of the upper extremity, muscles of the anterolateral abdominal wall, and intrinsic muscles of the thorax (see Plates 1-4, 1-5, and 1-6 ).

DORSAL ASPECT OF THE THORAX

The dorsal aspect of the thorax is also covered by skin and superficial fascia, with the cutaneous nerves to the skin of the back ramifying in the latter. These cutaneous nerves are branches of the posterior primary divisions (dorsal rami) of the thoracic nerves—for the upper six thoracic levels the medial branch and for the lower six the lateral branch.

The more superficial muscles on the posterior aspect of the thorax belong to the group connecting the upper extremity to the vertebral column. They are the trapezius, latissimus dorsi, rhomboideus major, rhomboideus minor, and levator scapulae.

The trapezius muscle arises from about the medial third of the superior nuchal line, the external occipital protuberance and the posterior margin of the ligamentum nuchae, and the spinous processes of the seventh cervical and all of the thoracic vertebrae and the related supraspinous ligaments. The lower fibers converge into an aponeurosis that slides over the triangular area at the medial end of the spine of the scapula and is attached at the apex of this triangle. The middle group of fibers is inserted on the medial margin of the acromion and the upper margin of the posterior border of the spine of the scapula. The upper group of fibers ends on the posterior border of the lateral third of the clavicle. The trapezius is supplied by the spinal part of the eleventh cranial nerve and branches from the anterior divisions (ventral rami) of the third and fourth cervical nerves. When contracting, the muscle tends to pull the scapula medially while at the same time rotating it, thus carrying the shoulder superiorly. If the shoulder is fixed, the upper fibers tilt the head so that the face goes upward toward the opposite side.

The latissimus dorsi muscle has a broad origin—by a small muscular slip from the outer lip of the iliac crest just lateral to the sacrospinalis muscle and by an extensive aponeurosis attached to the spinous processes of the lower six thoracic vertebrae, the lumbar and sacral vertebrae, and the related supraspinous ligaments. This muscle is inserted into the depth of the intertubercular groove of the humerus. Its nerve supply comes from the sixth, seventh, and eighth cervical nerves by way of the thoracodorsal branch of the brachial plexus. This muscle helps with extension, adduction, and medial rotation at the shoulder joint and helps to depress the raised arm against resistance.

The rhomboideus major and minor muscles are often difficult to separate. The rhomboideus major arises from the tips of the spinous processes and supraspinous ligaments of the second to fifth thoracic vertebrae. Its insertion is into the vertebral border of the scapula via a tendinous arch running from the lower angle of the smooth triangle at the root of the spine to the inferior angle. The rhomboideus minor muscle arises from the spinous processes of the first thoracic and last cervical vertebrae and the lower part of the ligamentum nuchae and is inserted into the vertebral border of the scapula at the base of the triangle, forming the root of the scapular spine. The rhomboideus muscles are supplied by fibers from the fifth and sixth cervical nerves by way of the dorsoscapular branch of the brachial plexus. The rhomboideus major and minor muscles tend to draw the scapula toward the vertebral column and slightly superiorly, with the lower fibers of the major muscles helping to rotate the scapula so that the shoulder is depressed.

The levator scapulae muscle originates in four tendinous slips attached to the transverse processes of the first four cervical vertebrae. Its insertion is the vertebral border of the scapula from its superior angle to the smooth triangle at the medial end of the spine scapula. Its nerve supply is primarily by cervical plexus branches from the ventral rami of the third and fourth cervical nerves. The levator scapulae, as the name indicates, elevates the scapula, drawing it medially and rotating it so that the tip of the shoulder is depressed.

Just deep to the group of muscles connecting the upper extremity to the vertebral column lie the serratus posterior superior and serratus posterior inferior muscles.

The serratus posterior superior muscle has an origin via a thin aponeurosis attached to the lower part of the ligamentum nuchae and to the spinous processes and related supraspinous ligaments of the seventh cervical and upper two or three thoracic vertebrae. It is inserted by fleshy digitations into the upper borders of the second to fifth ribs lateral to their angles. This muscle helps to increase the size of the thoracic cavity by elevating the ribs. The serratus posterior inferior muscle arises by means of a thin aponeurosis from the spinous processes and related supraspinous ligaments of the last two thoracic vertebrae and the first two or three lumbar vertebrae. This muscle inserts by fleshy digitations into the lower borders of the last four ribs, just beyond their angles. It tends to pull the last four ribs downward and outward. The serratus posterior muscles receive branches of the ventral rami of the thoracic nerves at the levels at which they are located.

Just deep to the serratus posterior superior muscle lie the thoracic portions of the splenius cervicis and capitis muscles.

The splenius cervicis muscle has a tendinous origin from the spinous processes of the third to sixth thoracic vertebrae and wraps around the deeper muscles to insert by tendinous fasciculi onto the transverse processes of the upper two or three cervical vertebrae. The splenius capitis muscle arises from the inferior half of the ligamentum nuchae and the spinous processes of the seventh cervical and the first three or four thoracic vertebrae. It is inserted onto the occipital bone just inferior to the lateral third of the superior nuchal line. The splenius muscles tend to pull the head and neck backward and laterally and to turn the face toward the same side. They are supplied by branches of the posterior primary divisions of the middle and lower cervical nerves.

The groove lateral to the spinous processes of the thoracic vertebrae is filled by the sacrospinalis muscle , which is covered by the thoracic part of the lumbodorsal fascia. Deep to the sacrospinalis muscle lie the short vertebrocostal and intervertebral muscles; they are not described here.

INTERCOSTAL NERVES AND ARTERIES

The typical thoracic spinal nerve is formed by the junction of a dorsal root and a ventral root near the intervertebral foramen below the vertebra having the same number as the nerve. The dorsal root is made up of a series of rootlets that emerge from one segment of the spinal cord between its dorsal and lateral white columns; it contains the nerve cell bodies of the afferent neurons that enter the spinal cord through it. This collection of nerve cell bodies causes a swelling of the root, named the dorsal root ganglion . A series of rootlets composed of axons of ventral-born gray cells leaves the same segment of the cord between the lateral and ventral white columns to form the ventral root of the spinal nerve.

The dorsal and ventral roots join near the intervertebral foramen to make up the very short common trunk of the spinal nerve, which divides almost immediately into the dorsal ramus (posterior primary division) and the ventral ramus (anterior primary division). The white and gray rami communicantes, which connect the ganglia of the sympathetic trunk and the thoracic nerves of the same level, join the ventral ramus near its origin.

The dorsal ramus of the thoracic nerve, passing posteriorly, pierces the erector spinae muscle (which it supplies), the trapezius muscle, and the other superficial muscles of the back (depending on the level) to reach the superficial fascia. There it divides into a smaller medial branch and a longer lateral cutaneous branch, which supply the skin.

The ventral ramus of the thoracic nerve is the intercostal nerve of that particular level (for the twelfth thoracic nerve, the subcostal nerve). From the seventh to the eleventh thoracic levels, the ventral rami of the thoracic nerves continue from the intercostal spaces into the anterior abdominal wall. The intercostal nerve runs forward in the thoracic wall between the innermost intercostal muscle and the internal intercostal muscle. It lies inferior to the intercostal vein and intercostal artery and gives off a collateral branch to the lower part of the space, as do the vein and artery. The intercostal nerve has a lateral cutaneous branch at the lateral aspect of the thorax that pierces the overlying intercostal muscles to reach the subcutaneous tissue. There it divides into an anterior (mammary) and a posterior branch. At the anterior end of the intercostal space, the intercostal nerve ends by becoming the anterior cutaneous nerve, which divides into a lateral branch and a shorter and smaller medial branch.

The aorta, lying on the anterior aspect of the vertebral bodies, gives off pairs of posterior (aortic) intercostal arteries. The right posterior intercostal arteries lie on the anterior aspect and the right side of the vertebral bodies as they travel to reach the intercostal spaces of the right side. The right and left posterior intercostal arteries course forward in the upper part of the intercostal spaces between the intercostal vein above and the intercostal nerve below to anastomose with the anterior intercostal branches of the internal thoracic and musculophrenic arteries. Collateral branches run in the inferior parts of the intercostal space.

To reach the pleural cavity from the outside at the anterolateral aspect of the thorax, a needle would pass through the following layers: skin, superficial fascia, intercostal muscles and related deep fascial layers, subpleural fascia, and parietal layer of the pleura. If the needle is carefully inserted near the lower part of the intercostal space (i.e., above the rib margin), one is reasonably sure of avoiding the intercostal nerve and vessels.

DIAPHRAGM (VIEWED FROM ABOVE)

The diaphragm is a curved musculotendinous septum separating the thoracic from the abdominal cavity, forming the floor of the thoracic cavity with its convex upper surface facing the thorax. The dome of the diaphragm on the right side is as high as the fifth costal cartilage (varying with the phase of respiration) and on the left is only slightly lower, so that some of the abdominal viscera are covered by the thoracic cage.

The origin of the diaphragm is from the outlet of the thorax and has three parts: sternal, costal, and lumbar.

The sternal origin is by two fleshy slips from the back of the xiphoid process. The costal origin is by fleshy slips that interdigitate with the slips of origin of the transversus abdominis muscle and arise from the inner surfaces of the costal cartilages and adjacent parts of the last six ribs on each side. The lumbar portion of the origin is by a right and a left crus and right and left medial and lateral lumbocostal arches (sometimes termed arcuate ligaments). The tendinous crura blend with the anterior longitudinal ligament of the vertebral column and are attached to the anterior surfaces of the lumbar vertebral bodies and related intervertebral discs—to the first three on the right and the first two on the left. The medial lumbocostal arch, a thickening of the fascia covering the psoas major muscle, extends from the side of the body of the first or second lumbar vertebra to the front of the transverse process of the first (sometimes also the second) lumbar vertebra. The lateral lumbocostal arch, passing across the quadratus lumborum muscle, extends from the transverse process of the first lumbar vertebra to the tip and lower border of the twelfth rib.

From the extensive origin just described, the fibers converge to insert in a three-leafed central tendon. Contraction of the muscular portion of the diaphragm pulls the central tendon downward, thus increasing the volume of the thoracic cavity and bringing about inspiration.

The diaphragmatic nerve supply is by way of the right and left phrenic nerves, which are branches of the right and left cervical plexuses and receive their fibers primarily from the fourth cervical nerves, with some contribution from the third and fifth cervical nerves.

Several structures pass between the thoracic and abdominal cavities, mainly through apertures in the diaphragm.

The aortic aperture is at the level of the twelfth thoracic vertebra situated between the diaphragm and the vertebra. It transmits the aorta, azygos vein, and thoracic duct.

The esophageal aperture is located at the level of the tenth thoracic vertebra in the fleshy part of the diaphragm. It transmits the esophagus, the right and left vagus nerves, and small esophageal arteries and veins.

The inferior vena caval aperture is situated at the level of the disc between the eighth and ninth thoracic vertebrae at the junction of the right and middle leaflets of the central tendon. It is traversed by the inferior vena cava and some branches of the right phrenic nerve.

The right crus is pierced by the right greater and lesser splanchnic nerves, and the left crus is pierced by the left greater and lesser splanchnic nerves and the hemiazygos vein. The sympathetic trunks usually do not pierce the diaphragm but pass behind the medial lumbocostal arches.

The base of the fibrous pericardial sac is partially blended with the middle leaflet of the central tendon of the diaphragm. The diaphragmatic portions of the parietal pleura are closely blended with the upper surfaces of the right and left portions of the diaphragm. Where the diaphragmatic pleura reflects at a sharp angle to become the costal pleura, the costodiaphragmatic recess or costophrenic sulcus is formed. Where the costal pleura reflects to become pericardial pleura, the costomediastinal recess is formed.

TOPOGRAPHY OF THE LUNGS (ANTERIOR VIEW)

Because the apex of each lung reaches as far superiorly as the vertebral end of the first rib, the lung usually extends about 1 inch above the medial third of the clavicle when viewed from the front. Thus, the lung projects into the base of the neck.

The anterior border of the right lung descends behind the sternoclavicular joint and almost reaches the midline at the level of the sternal angle. It continues inferiorly posterior to the sternum to the level of the sixth chondrosternal junction. There the inferior border curves laterally and slightly inferiorly, crossing the sixth rib in the midclavicular line and the eighth rib in the midaxillary line. It then runs posteriorly and medially at the level of the spinous process of the tenth thoracic vertebra. These levels are, of course, variable and apply to the lung in expiration. In inspiration, the levels for the inferior border are roughly two ribs lower.

The anterior border of the left lung is similar in position to that of the right lung. However, at the level of the fourth costal cartilage, it deviates laterally because of the heart, causing a cardiac notch in this border of the lung. The inferior border of the left lung is similar in position to that of the right lung except that it extends farther inferiorly because the right lung is pushed up by the liver below the diaphragm on the right side.

The oblique fissure of the right lung, separating the lower lobe from the upper and middle lobes, ends at the lower border of the lung near the midclavicular line. The horizontal fissure separating the middle from the upper lobe begins at the oblique fissure and runs horizontally forward to the lung's anterior border, which it reaches at about the level of the fourth costal cartilage.

The oblique fissure of the left lung is similar in its location to the corresponding fissure of the right side. The left lung ordinarily has only two lobes, and there is usually no horizontal fissure in this lung. Extra fissures may occur in either lung, usually between bronchopulmonary segments and, in the left lung, between the superior and inferior divisions of the upper lobe, giving rise to a three-lobed left lung.

The lungs seldom extend as far inferiorly as the parietal pleura, so some of the diaphragmatic parietal pleura is usually in contact with costal parietal pleura. This area—which, of course, varies in size with the phase of respiration—is called the costodiaphragmatic recess of the pleura or the costophrenic sulcus . A similar but much less extensive area is present where the anterior border of the lung does not extend to its limits medially—especially in expiration—and the costal and mediastinal parietal pleurae are in contact. This area is called the costomediastinal recess .

The diaphragm separates the liver from the right lung and, depending on the size of the liver, from the left lung. The left lung is also separated by the diaphragm from the stomach and the spleen.

The nipple in males usually overlies the fourth intercostal space in approximately the midclavicular line. In females, its position varies, depending on the size and functional state of the breast.

TOPOGRAPHY OF THE LUNGS (POSTERIOR VIEW)

The apex of the lung extends as far superiorly as the vertebral end of the first rib and therefore as high as the first thoracic vertebra. From there, the lung extends inferiorly as far as the diaphragm, with the base of the lung resting on the diaphragm and fitted to its superior surface. Because of the diaphragm's domed shape, the level of the highest point on the base of the right lung is about at the eighth to ninth thoracic vertebrae. The highest point on the base of the left lung is a fraction of an inch lower. From these high points, the bases of the two lungs follow the curves of the diaphragm to reach the levels described earlier for the inferior borders of the lungs.

The highest point on the oblique fissure of the two lungs is on their posterior aspects, at about the level of the third to fourth thoracic vertebrae, a little over 1 inch from the midline.

If the arm is raised over the head, the vertebral border of the scapula approximates the position of the oblique fissure of the lung. If the shoulder is brought forward as far as possible, the scapula is carried laterally, so that the area in which auscultation can be satisfactorily carried out on the posterior aspect of the chest is significantly widened.

The parietal pleura is separated from the visceral pleura by a potential space (the pleural cavity), which under normal circumstances contains only a minimal amount of serous fluid. Caudal to the inferior margin of the lung, the costal parietal pleura is in contact with the diaphragmatic parietal pleura, forming the costodiaphragmatic recess (costophrenic sulcus). This allows for the caudal movement of the inferior margin of the lung on inspiration.

Under abnormal circumstances, the pleural cavity may contain air, increased amounts of serous fluid, blood, or pus. The accumulation of a significant amount of any of these in the pleural cavity compresses the lung and causes respiratory difficulties.

The diaphragm separates the base of the left lung from the fundus of the stomach and the spleen. Because of this relationship, if the stomach is distended by food or gas, it may push the diaphragm upward and embarrass respiratory activity.

The diaphragm similarly separates the base of the right lung from the liver, which, if enlarged, elevates the diaphragm and pushes against the lung, possibly limiting its expansion. A hepatic abscess may rupture through the diaphragm to involve the related pleural cavity and lung.

In this illustration, the lungs are shown in relation to the bony thorax, scapula, and diaphragm, but overlying the structures shown are the deep and superficial muscles of the back in addition to the superficial fascia and skin.

MEDIAL SURFACE OF THE LUNGS

The medial (mediastinal) surfaces of the right and left lungs present concave mirror images of the right and left sides of the mediastinum so that in addition to the structures forming the root of the lung, the medial lung surface presents distinct impressions made by the structures constituting the mediastinum (see Plates 1-18 and 1-19 ).

BRONCHOPULMONARY SEGMENTS

A bronchopulmonary segment is that portion of the lung supplied by the primary branch of a lobar bronchus. Each segment is surrounded by connective tissue that is continuous with visceral pleura and forms a separate, functionally independent respiratory unit. The artery supplying a segment follows the segmental bronchus but the segmental veins are at the periphery of the segment and thus can be helpful in delineating it.