Normal Anatomy of the Lungs

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Introduction

The lungs incorporate the parenchyma, vasculature, bronchial tree (trachea, bronchi, and bronchioles), and a network of investing connective tissue that supports and connects the structures to one another. The human lungs are a pair of large spongy organs for gas exchange between blood and the air. The lungs are covered by a serous membrane called the visceral pleura, and the surrounding cavity formed between the lungs and the surrounding chest wall and mediastinum, covered by parietal pleura, is called the pleural cavity . Understanding diseases of the chest requires the radiologist to have a sound knowledge of the normal lung anatomy. This chapter describes the gross and radiologic anatomy of the lungs. Although imaging with computed tomography (CT) is commonplace, most chest imaging is still done with plain chest radiography.

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Surfaces

There are three surfaces of each lung—the costal, mediastinal, and diaphragmatic surfaces. The costal surface is the outer smooth and convex surface, which faces the ribs and the vertebrae. The mediastinal surface abuts the mediastinum. The mediastinal surface of the right lung carries the impression of the right subclavian artery, superior vena cava (SVC), and paratracheal soft tissues, including the esophagus, the azygous vein, the right atrium, and the inferior vena cava (IVC). The left lung carries the impression of the left subclavian artery, thoracic aorta, and the left atrium and ventricle. The diaphragmatic surface, also called the base of the lung, rests on the thoracic diaphragm ( Fig. 1.1 ).

Hilum

The hilum of the lung (also called the root of the lung ) is formed by the principal bronchi, the central pulmonary arteries and veins, the bronchial nerves and vessels, and the lymphatics, which enter and leave the lung from the mediastinum ( Fig. 1.2 ). The root of the right lung lies behind the SVC, the superior part of the right atrium, and below the azygos vein. The root of left lung lies beneath the aortic arch and in front of the descending aorta.

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Bronchopulmonary Segments

The bronchopulmonary segment is a smaller division of each lobe, supplied by a tertiary bronchus and its own segmental artery. The significance of such compartmentalization is that each segment is functionally and anatomically discrete, and thus can be removed surgically without affecting the neighboring segments. The veins and lymphatics run along the edges of the segments. There are typically 10 bronchopulmonary segments in the right lung (three, upper lobe; two, middle lobe; five, lower lobe) and eight segments in the left lung (four, upper lobe; four, lower lobe). Table 1.1 lists the typical segmental anatomy ( Fig. 1.3 ). The branching pattern at the segmental level is relatively constant, with the largest variations occurring in the lower lobes. The most common variation occurs in the left lower lobe (LLL), with 4% to 10% of the lobes demonstrating separate origins of the anterior and medial basal segments, similar to the right lower lobe (RLL). On CT, one can define these segments based on the fissures and bronchovascular anatomy. Knowledge of segmental anatomy for the bronchoscopist is important for localization, for the surgeons, because the segments are considered surgical units and can be resected with conservation of adjacent lung, and for the clinician, because many disease processes are segmental in distribution, such as bronchopneumonia, mycobacterial infections, tumor, aspiration, pulmonary infarction, and sequestration.

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Pulmonary Interstitium: Skeleton of the Lung

The pulmonary interstitial fibers are a network of connective tissues that provide support to the airways. The pulmonary interstitium is divided into two communicating compartments consisting of the axial or bronchovascular interstitium and the peripheral or subpleural interstitium. The axial interstitium extends from the hilum toward the periphery of the lung and surrounds the bronchovascular bundles. The axial interstitium is contiguous with the centrilobular interstitium, which surrounds the centrilobular arteriole and bronchiole within the secondary pulmonary lobule (SPL). The subpleural or peripheral interstitium lies beneath the visceral pleura and forms the borders of the SPL (interlobular septa). Extending between the centrilobular interstitium of the lobular core and the interlobular septa is a fine network of connective fibers known as the intralobular, parenchymal, or alveolar interstitium ( Fig. 1.4 ). The pulmonary gas exchange units, including the respiratory ducts, alveolar ducts, and alveoli, lie in close relation to the interlobular septa.

It is important to understand these different networks of interstitial fibers so that one can localize the interstitial disease processes and differentiate and diagnose interstitial lung diseases accurately. On radiographs, the interstitium is not well seen, and the diagnosis of the interstitial lung disease is frequently delayed. On high-resolution CT (HRCT), the interstitium is well seen, both centrally and peripherally. Therefore it is the modality of choice for interstitial lung diseases. The bronchovascular margins are smooth, and one should look for marginal blurring (edema, inflammation, and lymphangiectasia), irregularity (fibrosing conditions), and nodularity (granulomatous conditions and malignancy).

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Blood Supply and Bronchovascular Anatomy

The connective tissues, bronchi, and pleura are supplied in a dual fashion, in small part by the pulmonary arteries and veins and to a larger degree by the bronchial arteries and veins. The bronchial arteries arise from the aorta, are variable in number (more on the right side), and accompany the airways. Their branching pattern is similar to the bronchial tree. There is extensive anastomosis between the bronchial and pulmonary arteries under the pleura and adjacent to the small bronchi. They can be enlarged, and many chronic conditions are visible on CT, such as cystic fibrosis, bronchopulmonary aspergillosis, and tuberculosis. The bronchial veins drain via the pulmonary veins to the left atrium and the mediastinal veins, which later join the vena cava. The lymphatics drain the loose connective tissue below the pleura, arising from the interlobular septa and periarterial-peribronchial connective tissue. The lymphatics channel the lymph to the tracheal-bronchial, tracheal, and mediastinal lymph nodes.