Human Anatomy and Physiology - N. I. Fedyukovich 2003

Internal Organs
Respiratory System
Lungs

The Lungs (pulmones) are the primary Organs of the Respiratory system, responsible for oxygenating the Blood and removing carbon dioxide. The right and left lungs are located in the thoracic cavity, each enclosed within its own pleural sac (see Fig. 80). Inferiorly, the lungs rest on the Diaphragm; anteriorly, laterally, and posteriorly, each lung is in contact with the thoracic wall. The right dome of the diaphragm lies higher than the left, making the right lung shorter and wider than the left. The left lung is narrower and longer because The Heart, with its apex pointed to the left, occupies the left side of the thoracic cavity.

The apices of the lungs project 2–3 cm above the clavicle. The inferior border of the lung crosses the VI rib along the midclavicular line, the VII rib along the anterior axillary line, the VIII rib along the midaxillary line, the IX rib along the posterior axillary line, and the X rib along the paravertebral line.

The inferior border of the left lung is located slightly lower. During deep inspiration, the inferior margin descends by an additional 5–7 cm.

The posterior border of the lungs runs along THE Vertebral Column starting from the II rib. The anterior border (PROJECTION OF THE anterior margin) originates from the apices of the lungs and runs almost parallel at a distance of 1.0–1.5 cm at the level of the IV costal Cartilage. At this point, the border of the left lung deviates to the left by 4–5 cm, forming the cardiac notch. At the level of the VI costal cartilage, the anterior borders of the lungs transition into the inferior borders.

Each lung has three surfaces: the convex costal surface, adjacent to the inner aspect of the thoracic wall; the diaphragmatic surface, resting on the diaphragm; and the medial (mediastinal) surface, facing the Mediastinum. The hilum of the lung is located on the medial surface, serving as the entry point for the main bronchus, pulmonary artery, and nerves, and the exit point for the two Pulmonary Veins and Lymphatic vessels. Together, these vessels and Bronchi constitute the ROOT of the lung.

Each lung is divided into lobes by fissures: the right lung into three (superior, middle, and inferior) and the left lung into two (superior and inferior).

The division of the lungs into bronchopulmonary segments is of great clinical significance; there are 10 segments in both the right and left lungs (Fig. 81). The segments are separated from one another by Connective Tissue septa (pauci-vascular zones) and are cone-shaped, with the apex pointing toward the hilum and the base facing The surface of the lung. A segmental bronchus and a segmental artery run through the center of each segment, while a segmental vein runs along the boundary with the adjacent segment.

Each lung consists of branching bronchi that form the bronchial tree and a system of pulmonary alveoli. Initially, the main bronchi divide into lobar bronchi, and then into segmental bronchi. The latter, in turn, branch into subsegmental (medium) bronchi. Subsegmental bronchi continue to divide into smaller Branches of the 9th–10th order. A bronchus with a diameter of about 1 mm is called a lobular bronchus, which further branches into 18–20 terminal bronchioles. There are approximately 20 000 terminal bronchioles in the right and left lungs of a human. Each terminal bronchiole divides into respiratory bronchioles, which subsequently branch dichotomously (into two) and transition into alveolar ducts.

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Fig. 81. Diagram of lung segments:

A — anterior view; B — posterior view; C — right lung (lateral view); D — left lung (lateral view)

Each alveolar duct terminates in two alveolar sacs. The walls of the alveolar sacs are composed of pulmonary alveoli. The diameter of an alveolar duct and an alveolar sac is 0.2–0.6 mm, and that of an alveolus is 0.25–0.30 mm.

The respiratory bronchioles, along with the alveolar ducts, alveolar sacs, and pulmonary alveoli, form the alveolar tree (pulmonary acinus), which is the Structural and functional unit of the lung. The number of pulmonary acini in a single lung reaches 15 000; the average number of alveoli is 300–350 million, and the respiratory surface area of all alveoli is approximately 80 m².

To supply the lung tissue and bronchial walls, blood enters the lungs via the bronchial Arteries from the Thoracic Aorta. Blood from the bronchial walls drains via the bronchial veins into the tributaries of the pulmonary veins, as well as into the azygos and hemiazygos veins. Deoxygenated (venous) blood enters the lungs through the left and right pulmonary arteries, becomes oxygenated through gas exchange, releases carbon dioxide, and, now oxygenated (arterial) blood, flows through the pulmonary veins into the left atrium.

The lymphatic Vessels of the lungs drain into the bronchopulmonary, as well as the inferior and superior tracheobronchial Lymph Nodes.