Anatomy, Thorax, Lungs

Excerpt

The purpose of the lung is to provide oxygen to the blood. The respiratory system divides into airways and lung parenchyma. The airways consist of the bronchus, which bifurcates off the trachea and divides into bronchioles and then further into alveoli. The parenchyma is responsible for gas exchange and includes the alveoli, alveolar ducts, and bronchioles. Lungs have a spongy texture and have a pinkish-gray hue. Also, they are anatomically described as having an apex, three borders, and three surfaces. Further, they subdivide into lobes and segments. The lung parenchyma also is covered by a pleura.

Anatomy

Anatomically, the lung has an apex, three borders, and three surfaces. The apex lies above the first rib.

The three borders include the anterior, posterior, and inferior borders. The anterior border of the lung corresponds to the pleural reflection, and it creates a cardiac notch in the left lung. The cardiac notch is a concavity in the lung that forms to accommodate the heart. The inferior border is thin and separates the base of the lung from the costal surface. The posterior border is thick and extends from the C7 to the T10 vertebra, which is also from the apex of the lung to the inferior border.

The three surfaces of the lung include the costal, medial, and diaphragmatic surfaces. The costal surface is covered by the costal pleura and is along the sternum and ribs. It also joins the medial surface at the anterior and posterior borders and diaphragmatic surfaces at the inferior border. The medial surface is divided anteriorly and posteriorly. Anteriorly, it is related to the sternum, and posteriorly, it is related to the vertebra. The diaphragmatic surface (base) is concave and rests on the dome of the diaphragm; the right dome is also higher than the left dome because of the liver.

The right and left lung anatomy are similar but asymmetrical. The right lung consists of three lobes: the right upper lobe (RUL), the right middle lobe (RML), and the right lower lobe (RLL). The left lung consists of two lobes: the left upper lobe (LUL) and the left lower lobe (LLL). The right lobe is divided by an oblique and horizontal fissure, where the horizontal fissure divides the upper and middle lobes, and the oblique fissure divides the middle and lower lobes. In the left lobe, there is only an oblique fissure that separates the upper and lower lobe.

The lobes further divide into segments that are associated with specific segmental bronchi. Segmental bronchi are the third-order branches off the second-order branches (lobar bronchi) that come off the main bronchus.

The right lung consists of ten segments. There are three segments in the RUL (apical, anterior, and posterior), two in the RML (medial and lateral), and five in the RLL (superior, medial, anterior, lateral, and posterior). The oblique fissure separates the RUL from the RML, and the horizontal fissure separates the RLL from the RML and RUL.

There are eight to nine segments on the left, depending on the division of the lobe. In general, there are four segments in the left upper lobe (anterior, apicoposterior, inferior, and superior lingula) and four or five in the left lower lobe (lateral, anteromedial, superior, and posterior).

The hilum (root) is a depressed surface at the center of the medial surface of the lung and lies anteriorly to the fifth through seventh thoracic vertebrae. It is the point at which various structures enter and exit the lung. The hilum is surrounded by the pleura, which extends inferiorly and forms a pulmonary ligament. The hilum contains mostly bronchi and pulmonary vasculature, along with the phrenic nerve, lymphatics, nodes, and bronchial vessels. Both the left and right hilum contain a pulmonary artery, pulmonary veins (superior and inferior), and bronchial arteries. Also, in the left hilum, there is one bronchus, the principal bronchus, and in the right hilum, there are two bronchi, the eparterial and hyparterial bronchi. From anterior to posterior, the order in the hilum is the vein, artery, and bronchus.