Lungs: Pulmonary Borders, fissures and Lobes
Pulmonary borders
The inferior border is thin and sharp where it separates the base from the costal surface and extends into the costo diaphragmatic recess, and is more rounded medially where it divides the base from the mediastinal surface. It corresponds, in quiet respiration, to a line drawn from the lowest point of the anterior border which passes to the sixth rib at about the mid clavicular line, then to the eighth rib in the mid axillary line (usually 10 cm above the costal margin), and then continues posteriorly, medially and slightly upwards to a point 2 cm lateral to the tenth thoracic spine.
The diaphragm rises higher on the right to accommodate the liver, and so the right lung is vertically shorter (by approximately 2.5 cm) than the left. However, cardiac asymmetry means that the right lung is broader, and has a greater capacity and weight than the left.
The posterior border separates the
costal surface from the mediastinal surface, and corresponds to the heads of the ribs. The thin, sharp, anterior border overlaps the pericardium. On the right it corresponds closely to the costo mediastinal line of pleural reflection, and is almost vertical. On the left it approaches the same line above the fourth costal cartilage; below this point it shows a variable cardiac notch, the edge of which passes laterally for 3.5 cm before curving down and medially to the sixth costal cartilage 4 cm from the midline. It therefore does not reach the line of pleural reflection here (see Fig.( and so the pericardium is covered only by a double layer of pleura (area of superficial cardiac dullness). However, surgical experience suggests that the line of pleural reflection, the anterior pulmonary margin and the costo mediastinal pleural recess are all variable.
PULMONARY FISSURES AND LOBES
Right lung
The right lung is divided into superior,
middle and inferior lobes by an oblique and
a horizontal fissure (Fig.). The upper,
oblique fissure separates the inferior from
the middle and upper lobes, and corresponds closely to the left oblique fissure, although it is less vertical, and crosses the inferior border of the lung approximately 7.5 cm behind its anterior end.
On the posterior border it is either level with the spine of the fourth thoracic vertebra or slightly lower. It descends across the fifth intercostal space and follows the sixth rib to the sixth costochondral junction.
The short horizontal fissure separates the superior and middle lobes. It passes from the oblique fissure, near the mid axillary line, horizontally forwards to the anterior border of the lung, level with the sternal end of the fourth costal cartilage, then passes backwards to the hilum on the mediastinal surface. The horizontal fissure is usually visible on a frontal chest radiograph.
The oblique fissure is usually visible on a lateral radiograph and on a high resolution CT scan. The small middle lobe is cuneiform and includes some of the costal surface, the lower part of the anterior border and the anterior part of the base of the lung
Left lung
The left lung is divided into a superior and an inferior lobe by an oblique fissure which extends from the costal to the medial surfaces of the lung both above and below the hilum.
Superficially this fissure begins on the medial surface at the postero superior part of the hilum. It ascends obliquely backwards to cross the posterior border of the lung 6 cm below the apex, then descends forwards across the costal surface, to reach the lower border almost at its anterior end. It finally ascends on the medial surface to the lower part of the hilum.
At the posterior border of the lung the fissure usually lies opposite a surface point 2 cm lateral to the midline between the spines of the third and fourth thoracic vertebrae, but it may be above or below this level. Traced around the chest, the fissure reaches the fifth intercostal space (at or near the mid axillary line) and follows this to intersect the inferior border of the lung close to, or just below, the sixth costochondral junction (7.5 cm from the midline).
The superior lobe, which lies antero superior to the oblique fissure, includes the apex, anterior border, much of the costal and most of the medial surfaces of the lung. At the lower end of the cardiac notch a small process, the lingula, is usually present. The larger inferior lobe lies behind and below the fissure, and contributes almost the whole of the base, much of the costal surface and most of the posterior border of the lung.
Each of the principal bronchi divides into lobar bronchi. Primary branches of the right and left lobar bronchi are termed segmental bronchi.
PULMONARY HILA
The pulmonary root connects the medial surface of the lung to the heart and trachea and is formed by a group of structures which enter or leave the hilum . These are the principal bronchus, pulmonary artery, two
pulmonary veins, bronchial vessels that all of which are enveloped by a sleeve of pleura.
The pulmonary roots, or pedicles, lie opposite the bodies of the fifth to seventh thoracic vertebrae. The pulmonary ligament is a common inferior relation. The majo structures in both roots are similarly arranged, so that the upper of the two pulmonary veins is anterior, the pulmonary artery and principal bronchus are more posterior, and the bronchial vessels are most posterior.
Right hilum
The right root is situated behind the superior vena cava and right atrium and below the terminal part of the azygos vein. The usual sequence of hilar structures from above downwards is: superior lobar bronchus,
pulmonary artery, principal bronchus, and lower pulmonary vein.
Left hilum
The left root lies below the aortic arch and in front of the descending thoracic aorta. The usual vertical sequence of structures at the left hilum is pulmonary artery, principal bronchus, and lower pulmonary vein. The pulmonary artery is longer on the left side.
