The lung (pulmo) () is the organ in which oxygen from the atmosphere and carbon dioxide from the blood are exchanged. The lungs serve a passive function in the mechanical act of respiration. The diaphragm, when it contracts, enlarges the pleural cavity by moving caudally. When the intercostal muscles contract and draw the ribs cranially, the size of the thoracic cavity is also increased, and thus air is drawn into the lungs because of the negative pressure that is produced. Aiding in expulsion of the air from the lungs are the abdominal muscles, which contract and force the abdominal viscera against the caudal surface of the diaphragm. The efects of age on lung function and structure were reviewed by Mauderly and Hahn (1982). In general there is considerable fibrosis and loss of function in the lungs of old dogs. Robinson (1982) summarized some functional consequences of species diferences in lung anatomy. There is no explanation for the great variation seen in lung lobation of domestic and wild mammals.
The two lungs (pulmo sinister et dexter) possess many features in common. Each has a slightly concave base (basis pulmonis), which lies adjacent to the diaphragm, and an apex (apex pulmonis), which lies in the thoracic inlet. The apex of the left lung is more pointed and extends farther cranially than the apex of the right lung (). The curved lateral surface of each lung is called the costal surface (facies costalis), and the fattened surface, which faces the mediastinum, is called the medial surface (facies medialis). Because the vertebral bodies protrude ventrally from the dorsal wall of the thorax and intervene between the two lungs, this portion of the medial wall of each lung is known as the vertebral part (pars vertebralis). The remaining ventral portion of each medial wall faces the mediastinum and is known as the mediastinal part (pars mediastinalis). The medial surface of each lung is deeply indented by the heart over an area between the third and the sixth ribs. This is called the cardiac impression (impressio cardiaca). On each side, the mediastinal pleura covering the pericardial sac are displaced sufciently by the underlying heart so that its ventral portion lies in contact with the costal pleura. The cardiac notch of the right lung (incisura cardiaca pulmonis dextri) () is V-shaped, with the apex located dorsally. The right cardiac notch is formed by the ventrally diverging borders of the cranial and middle lobes. Its dorsal apex lies opposite the beginning of the distal fourth of the fourth rib. On the left side usually no obvious cardiac notch is formed.
Each lung has a diaphragmatic surface () (facies diaphragmatica) that is concave because it lies against the convex surface of the diaphragm. The diaphragmatic surface of the right lung is approximately one-third larger than that of the left lung, this larger area being caused by the accessory lobe of the right lung extending ventrally and to the left, ending in a process at the apex of the heart.
The margin along the vertebral part of the lung is the dorsal margin (margo dorsalis [obtusus]) () and extends from the apex to the base of the lung. The costal surface of each lung is continuous with the medial surface at an acute angle ventrally, lying in the costomediastinal recess. This margin, extending from the apex to the base of the lung, is called the ventral margin of the lung (margo ventralis). Caudally, the ventral margin of the lung is continuous with the peripheral margin of the base of the lung, or the basal margin (margo basalis). The basal margin is where the costal and diaphragmatic surfaces meet. This margin extends into the costodiaphragmatic recess. The combined ventral and basal margins constitute the acute margin (margo acutus).
The area of each lung that receives the principal bronchi and furnishes passages for the pulmonary and bronchial vessels and nerves is known as the hilus of the lung (hilus pulmonis).
The root of the lung (radix pulmonis) consists of the aggregate of those structures that enter the organ at the hilus. Lung lobes are determined by the pattern of branching of the principal bronchi. Interlobar fissures are the external indications of these divisions. The caudal interlobar fissure (fssura interlobalis caudalis) is between the middle and caudal lobes of the right lung and the cranial and caudal lobes of the left lung. The cranial interlobar fissure (fssura interlobaris cranialis) is between the cranial and middle lobes of the right lung. The surfaces of adjacent lobes that lie in contact with each other are called the interlobar surfaces (facies interlobares).
Shape of Lobes and Position of Interlobar Fissure
Lung lobes are named for the branching of principal bronchi into lobar bronchi. The left principal bronchus has two lobar bronchi, cranial and caudal. The cranial lobar bronchus immediately divides into two segmental bronchi that serve the cranial and caudal parts of the cranial lobe. The right principal bronchus has four lobar bronchi, one to each of its four lobes.
The cranial part of the cranial lobe of the left lung (pulmo sinistra, lobus cranialis, pars cranialis), formerly the apical lobe (), is transversely compressed between the heart and the lateral thoracic wall. In a 22-pound dog it is 10 cm long, 3 cm wide. It extends from the dorsal part of the fifth rib to and through the thoracic inlet, where its apex lies not only cranial to a transverse plane through the first ribs but also largely to the right of the median plane. The parenchyma of the cranial and caudal parts of the left cranial lobe are fused over a transverse distance of 2.5 cm from the vertebral border to the fissure between the lobe parts.
The caudal part of the cranial lobe of the left lung () (pulmo sinister, lobus cranialis, pars caudalis) was formerly called the cardiac lobe. It presents a thin dorsocranially convex border that overlies the caudal thickened portion of the cranial part of the cranial lobe, or these features and positions of the adjacent lobes are reversed. The ventral margin of the caudal part of the cranial lobe of the left lung lies nearly in a dorsal plane 1 cm from the midventral line. The left lung does not possess a cardiac notch.
The caudal lobe of the left lung (pulmo sinister, lobus caudalis), formerly diaphragmatic lobe, is pyramidal in shape and is completely separated from the cranially lying caudal part of the cranial lobe by the caudal interlobar fissure, which extends from the costal surface to the root of the lung. When the lungs are moderately distended, the fissure begins at the vertebral end of the sixth rib and ends near the costochondral junction of the seventh rib.
The right lung () is divided into cranial, middle, accessory, and caudal lobes. The cranial interlobar fissure separates the cranial and middle lobes of the right lung. The caudal interlobar fissure separates the right middle and caudal lobes.
The cranial lobe of the right lung (pulmo dexter, lobus cranialis), formerly the apical lobe, extends from the dorsal part of the cranial interlobar fissure cranially and ventrally to the right of the median plane. It does not end in a definite apex, as does the left lung, but rather its most cranial and ventral part has a gentle curved convex border that goes from an acute, cranial, and dorsal margin to a slightly convex surface cranial to the heart. This portion of the cranial lobe extends across the median plane to the left side, whereas its most cranioventral portion lies adjacent to the caudal portion of the apex of the left lung that extends across the midline to the right side. These two lung lobes remain separated by cranial mediastinum here. The caudoventral margin of the cranial lobe of the right lung is separated from the craniodorsal portion of the middle lobe by the curved cranial interlobar fissure (fssura interlobalis cranialis).
The middle lobe of the right lung (pulmo dexter, lobus medius) begins at the cranial interlobar fissure, where its costal surface is broad and tapers to a narrow, pyramid-shaped ventral extremity that lies caudal or caudosinistral to the apex of the heart. Its medial surface is deeply excavated by the heart, resulting in the cardiac impression. The cranioventral one-fourth of its border diverges sharply from the rounded transversely located caudal portion of the cranial lobe, thus exposing a portion of the atrial surface of the heart to the thoracic wall. This notch in the right lung is the cardiac notch (incisura cardiaca pulmonis dextri).
The accessory lobe of the right lung (pulmo dexter, lobus accessorius) (), formerly intermediate lobe, is the most irregular of all of the lobes of the lungs. Caudally it is molded against the diaphragm. Cranially it lies in contact with the apex of the heart and the adjacent portion of the right caudal lobe. The caudal mediastinum separates it from the left caudal lobe. The accessory lobe possesses a thickened middle portion and three processes — a dorsal, a ventral, and a right lateral. The dorsal process is a sharp-pointed pyramid-shaped eminence that extends caudally in contact with the caudoventral face of the dorsomedial portion of the caudal lobe of the right lung. Its free caudal apex does not reach as far caudally as do the caudal lobes. The ventral process of the accessory lobe runs almost directly ventrally to the dorsal surface of the sixth sternebra. It is wedged in the mediastinal recess, the space between the diaphragm and the apex of the heart. Separating the dorsal and the right lateral processes is the notch through which the caudal vena cava and the right phrenic nerve pass.
The caudal lobe of the right lung (pulmo dexter, lobus caudalis) is similar in shape and comparable in location to the left lobe, except that it lies to the right of the median plane. It is smaller than the left caudal lobe, and does not extend as far ventrally as does the left caudal lobe. Furthermore, its diaphragmatic surface is irregularly excavated in its central part by the accessory lobe. Around its periphery it is concave in all directions, in conformity with the convex surface of the diaphragm against which it lies.
Relationship of Lungs to Other Organs
The heart produces large impressions on the medial surfaces of each lung. The cardiac impression of the right lung (impressio cardiaca pulmonis dextri) is a deep excavation of the medial or mediastinal surfaces of the right cranial lobe cranially, the middle lobe laterally, and the accessory lobe caudally. Ventrally and on the right the cranial and middle lobes fail to cover the heart, so that the pericardial mediastinal pleura lies in contact with the costal pleura over an area that is V-shaped, with the apex located dorsally. This notch in the right lung is called the cardiac notch of the right lung (incisura cardiaca pulmonis dextri). The cardiac notch exposes approximately 5 square cm of the atrial surface of the heart to the thoracic wall. The ventral margin of the left cranial lobe is arciform. It is located, approximately in a sagittal plane, approximately 1 cm from the median plane, ventrally. The notch for the caudal vena cava (sulcus venae cavae caudalis) () is located between the dorsal and the right lateral processes of the accessory lobe. Passing through this notch in close association with the caudal vena cava is the right phrenic nerve as it runs to the diaphragm. Both structures are surrounded by the plica venae cavae, which attaches to the diaphragm. The medial surfaces of the cranial lobes of the lungs lie in contact with the mediastinum covering the thoracic portion of the thymus gland when the thymus is present. When it is fully developed, the thymus gland ends caudally opposite the left fifth costal cartilage. It therefore enters into the formation of the cardiac impression of the caudal part of the left cranial lobe.
The pulmonary arteries carry nonaerated blood from the right ventricle of the heart to the lungs for gaseous exchange. The pulmonary veins return aerated blood from the lungs to the left atrium of the heart. McLaughlin et al. (1961) have shown that the spatial and functional arrangements of the pulmonary vessels differ greatly among various species. In the dog, the pulmonary artery, in addition to supplying the distal portion of the respiratory bronchiole, alveolar duct, and alveoli, continues on to supply the thin pleura.
The pulmonary trunk (truncus pulmonalis) is the stem artery arising from the fibrous pulmonary ring, which extends into the media of the pulmonary trunk. It serves for the attachment of muscle fibers from the conus arteriosus. The pulmonary trunk bifurcates into the left and right pulmonary arteries, which ramify in the left and right lungs. The left pulmonary artery (a. pulmonalis sinistra) curves dorsally cranial to the vein from the cranial part of the cranial lobe that crosses the lobar bronchus to that lobe. Just prior to this crossing, a large branch of the left pulmonary artery arises and bifurcates. The larger terminal branch runs cranially as the main vessel to the cranial part of the left cranial lobe. The branch to the caudal part of the left cranial lobe lies cranial to the bronchus and caudal to the large vein. The veins from all of the lobes compose the most ventral part of the root of the lung ().
The right pulmonary artery (a. pulmonalis dextra) is shorter than the left. It runs caudolaterally across the heart base from left to right. It passes ventral to the left lobar bronchi and dorsal to the large left lobar veins. The artery divides unequally into a small branch that runs to the right cranial lobe and a large branch that courses caudally into the right caudal lobe. Near the origin of the large artery to the caudal lobe the relatively small right middle lobar artery runs laterally and enters the dorsal third of the lobe. It is related to the dorsal surface of its satellite vein and lies dorsocranial to the right middle lobar bronchus. It may arise from the right cranial lobar artery. The pulmonary lobar artery to the accessory lobe of the right lung enters the thickened middle portion of the lobe and trifurcates into a branch supplying each of the three processes of the lobe. This lobar artery lies ventral to the bronchus to this lobe and dorsal to its satellite vein.
The pulmonary veins (vv. pulmonales) () are variable in number. All of the blood distributed to the bronchial tree is returned by the pulmonary veins except for a limited area around the hilus drained by bronchial veins. There is one pulmonary vein from each lobe, although there may be two veins that drain the right cranial lobe. The latter veins anastomose to form a larger vein that immediately receives the vein from the right middle lobe so that blood from all of these areas is returned to the right lateral part of the left atrium by a single large vein 1 cm in diameter. The blood from the right caudal and accessory lobes is drained by a single trunk that lies to the right of a similar trunk from the left caudal lobe. The pulmonary lobar veins from the left lung usually open individually into the dorsum of the left atrium. At the hilus of the lung the pulmonary veins lie most ventrally. The pulmonary arteries are dorsal to the veins, and the lobar bronchi are insinuated between the arteries and the veins. Holt et al. (2005) provided a revised anatomic description of the pulmonary veins of the right caudal and accessory lobes of the dog’s lung. They described similar conditions in the nine dogs they dissected. The pulmonary vein from the right caudal lung lobe initially paralleled the right caudal lung lobe bronchus, running cranially, medially, and ventrally. It diverged from the bronchus at the level of the pulmonary artery and bronchus of the accessory lung lobe. At this point the pulmonary vein from the right caudal lobe coursed dorsal to the pulmonary artery and bronchus of the accessory lobe. Medial to the bronchus of the accessory lobe, it received the pulmonary vein from the accessory lobe on its ventral surface. Within the pericardium this common venous trunk merged with the caudal aspect of the left atrium either with, or immediately adjacent to, the left caudal pulmonary vein.
Barone (1957), Ishaq (1980), Phalen and Oldham (1983), Amis and McKiernan (1986), and Shishkin et al. (1989) consider various aspects of the bronchial tree and lung in the dog. Holt et al. (2005) revised the anatomic description and embryologic implications of the right caudal and accessory lobe pulmonary veins.
The small bronchial arteries are variable in origin, although in the majority of dogs the parent trunk is the bronchoesophageal artery (), which arises from the right fifth intercostal artery close to its origin from the aorta. The bronchoesophageal artery crosses the left face of the esophagus and contributes an esophageal branch before entering the root of the lung as the bronchial artery. In its course, the bronchial artery supplies the tracheobronchial lymph nodes, peribronchial connective tissue, and the bronchial mucous membrane. At the level of the respiratory bronchiole the bronchial artery terminates in a capillary bed that is continuous with that of the pulmonary artery. Miller (1937), McLaughlin et al. (1961) and more recently Laitinen et al. (1989) were unable to demonstrate normally occurring bronchial arterypulmonary artery anastomoses in the dog. Laitinen et al. (1989) have described the structure of tracheal and bronchial blood vessels in the Greyhound, using vascular corrosion casts as well as electron micrographs. They found two networks of vessels of different types and sizes in the mucosa. One, a rich capillary network close to the epithelial basement membrane, converged to form venules that then extended to a deeper mucosal plexus formed by larger venules and arterioles.
Notkovitch (1957) found single bronchial arteries on each side in 75% of his specimens, and double bronchial arteries on each side in 10%. In all cases they arose from the first to the fourth right intercostal artery. Berry et al. (1931) found that the right and left bronchial arteries arose from the right sixth intercostal artery by a common trunk, which also supplied branches to the esophagus. They also described small bronchial vessels that supplied the hilus of the lung and that arose from the pericardiacophrenic or internal thoracic arteries. Michel (1982) demonstrated qualitative and quantitative morphologic diferences between small pulmonary arteries and veins. Pulmonary arteries of the dog gradually taper and change from elastic to transitional, to muscular, partially muscular, and fnally nonmuscular arteries.
True bronchial veins are found only at the hilus of the lung. They empty into the azygos vein or intercostal vein at the level of the seventh thoracic vertebra.
The afferent lymph vessels from the lobes of each lung run to the tracheobronchial lymph nodes of the respective side and to the middle tracheobronchial lymph node. From these locations lymph is drained via a chain of cranial mediastinal lymph nodes ().