Normal Lung I

INTRODUCTION
Inhalation of approximately 10,000 L of air is necessary to meet the daily gas exchange requirements of the adult human lung. The normal lung has an extraordinary respiratory reserve. Arterial oxygenation commonly improves with exercise, and even under heavy work conditions, pulmonary gas exchange in a normal adult is rarely a cause of limitation to aerobic performance. To accomplish the efficient extraction of oxygen and exchange of carbon dioxide, the lung has an internal surface area approximately equal to that of a tennis court. The upper and lower respiratory tracts act to condition the inhaled air, and the lung has developed unique defense pathways to allow it to maintain its fine, delicate gas exchange surface while being continuously exposed to potentially injurious reactive or infectious agents in inhaled air. This chapter reviews the normal anatomy of the human lung and focuses on the unique structural characteristics that allow the lung to maintain normal function while being continuously exposed to inhaled reactive gases and particles.
THE NORMAL RESPIRATORY TRACT
Two normal adult lungs at maximal capacity contain 5 to 6 L of air and weigh an average of 850 g in men and 750 g in women. Blood makes up a substantial fraction of the lung weight, and in vivo the lungs have been estimated to contain as much as 360 mL of blood. Lung weight is approximately 1% of total body weight in a normal adult. Ninety percent of the volume of the lungs is made up of gas exchange regions or lung parenchyma, whereas lung weight is approximately equally divided between the parenchyma and structures other than parenchyma (airways and large vessels).
The right lung is commonly slightly larger than the left, comprising about 53% of the volume of both lungs on average. Each lung is completely covered by a visceral pleura. The visceral pleura subdivides each lung, although incompletely, into lobes. The right lung has three lobes, and the left is divided into two lobes. Incomplete fissures between the lobes commonly allow for some collateral ventilation between lobes. The bronchopulmonary segments are defined by the primary segmental bronchi that branch off the lobar bronchi. Lobar segments are not commonly subdivided by pleura. There are 10 segments in the right lung (Fig. 1) and eight in the left. Common terminology identifies 10 segments in each lung, with the first and second (apical posterior) segments of the left upper lobe being a combined segment and the anterior basal and medial basal segments being combined in the left lower lobe. The left lingula is anatomically part of the left upper lobe and is not commonly separated by a pleura-containing fissure. The fissure separating the right middle lobe from the right upper lobe is termed the horizontal fissure and can occasionally be recognized as a horizontal line on an anterior-posterior chest radiograph. The oblique or major fissures separating the upper and lower lobes of both the left and right lungs can be identified on lateral chest radiographs. The left major fissure commonly lies slightly apically and anteriorly to the right major fissure (Fig. 2). However, this apparent position can be easily altered by small variations in the orientation of a left lateral chest radiograph.

FIG. 1. Location of bronchopulmonary segments from anterior, lateral, and medial views. See color plate 1.

FIG. 2. Left lateral view of the lungs. Partially translucent image of the left lung allows the right lung to be seen. The location of the major fissures and the horizontal fissure of the right lung are illustrated in the positions in which they would appear on a left lateral chest radiograph. Note that the major fissure on the right side lies slightly anterior and apical to the major fissure on the left side.

A common variant in the lobation of the lung is the presence of a horizontal fissure partially demarcating the superior segment of either the right or left lower lobes. Another variant occurs when during development the azygos vein moves into the apical portion of the right pleural cavity. This displaces parietal pleura into the lung, producing a fissure in the apex of the upper lobe of the right lung. This partially separated lung lobe, known as an azygos lobe, occurs in slightly less than 1% of the population. The lingula of the left lung may also be demarcated by an anomalous fissure.
Contours along the lung surfaces for the heart, mediastinal structures, and major vessels are illustrated in Fig. 1. A fold of tissue containing connecting tissue and vessels that extends inferiorly from the hilum on both sides is termed the pulmonary ligament.