Human Anatomy - Kotsan I. Y. 2009

Respiratory System
Lungs

The Lungs (pulmones) are located in the right and left halves of the thoracic cavity (Fig. 167). Inferiorly, the lungs rest against the Diaphragm; laterally and posteriorly, against the thoracic wall; and medially, against the mediastinal Organs. Because the right dome of the diaphragm sits higher than the left, the right lung (pulmo dexter) is shorter and wider than the left. The left lung (pulmo sinister) is narrower and longer because a portion of the left thoracic cavity is occupied by The Heart, whose apex is directed to the left. In shape, the lungs resemble a cone with its apex pointing upward. Each lung features a base (basis pulmonis), which rests on the diaphragm, and a rounded apex (apex pulmonis), which points upward toward the supraclavicular fossa region and extends 3–4 cm above the first rib. At the lung apex, There is a small subclavian groove (sulcus subclavius) formed by the pressure of the adjacent Subclavian Artery.

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Fig. 167. The right and left lungs

1 — right lung; 2 — apex of the lung; 3 — Larynx; 4 — Trachea; 5 — left lung; 6 — superior lobe of the left lung; 7 — main bronchus of the left lung; 8 — inferior lobe of the left lung; 9 — inferior margin; 10 — cardiac notch of the left lung; 11 — medial margin of the right lung; 12 — inferior lobe of the right lung; 13 — oblique fissure; 14 — middle lobe of the right lung; 15 — horizontal fissure; 16 — superior lobe of the right lung

Each lung has three surfaces: the diaphragmatic surface (facies diaphragmatica), facing downward toward the diaphragm; the costal surface (facies costalis), facing the Ribs and convex to match their curvature; and the medial surface (facies medialis). The medial surface is subdivided into a posterior vertebral part (pars vertebralis), adjacent to THE Vertebral Column, and an anterior part—the mediastinal surface (facies mediastinalis), adjacent to the Mediastinum. The mediastinal surface of the lungs features the cardiac impression (impressio cardiaca), which accommodates the Pericardium, and the hilum of the lung (hilus pulmonis), through which the bronchus, pulmonary artery, and nerves enter the lung, while the Pulmonary Veins and Lymphatic vessels exit it. The topographical arrangement of these structures within the hila differs between the two lungs. In the hilum of the left lung, the anterosuperior position is occupied by the pulmonary artery, the intermediate position by the main bronchus, and the posteroinferior position by the pulmonary veins (the superior-to-inferior abbreviation being APV). In the hilum of the right lung, the anterosuperior position is occupied by the bronchus, the intermediate position by the artery, and the posteroinferior position by the veins (abbreviation PAV). The collective assembly of all these structures (vessels, Lymph Nodes, nerves, and Bronchi) constitutes the ROOT of the lung (radix pulmonis).

The surfaces of the lung are separated from one another by margins: the inferior and anterior margins. The inferior margin (margo inferior) separates the costal and medial surfaces from the diaphragmatic surface. The anterior margin (margo anterior) separates the costal surface from the mediastinal surface. In the lower half of the anterior margin of the left lung, there is a cardiac notch (incisura cardiaca), below which lies the so-called lingula of the left lung (lingula pulmonis sinistri).

Each lung is divided into lobes by deep grooves called fissures. The left lung has a single oblique fissure (fissura obliqua), which divides it into two lobes: the superior and inferior lobes (lobus superior et lobus inferior). The right lung has two fissures: the oblique fissure (fissura obliqua) and the horizontal fissure (fissura horizontalis), which divide it into three lobes: the superior lobe (lobus superior), the middle lobe (lobus medius), and the inferior lobe (lobus inferior). The course of the oblique fissure is quite similar in both lungs. It begins 6–7 cm below the apex of the lung posteriorly on the medial surface, roughly at the level of the spinous process of the 3rd thoracic vertebra, runs across the costal surface laterally forward and downward to the Base of the lung, and from there turns back onto the medial surface, ascending upward and backward toward the lung hilum. The horizontal fissure is somewhat shorter and shallower than the oblique fissure. It originates on the costal surface of the right lung at about the midpoint of the oblique fissure, runs forward almost horizontally at the level of the 4th rib to the anterior margin of the lung, crosses onto its medial surface, and reaches the hilum. The lung lobes are separated from one another by a layer of loose Connective Tissue. The apposed surfaces of adjacent lobes are termed interlobar surfaces (facies interlobares).

A lobe (lobus) is a Structural and functional unit of the lung that is ventilated by a secondary bronchus, possesses its own lobar neurovascular supply, and exhibits visible external boundaries. Lung lobes are further subdivided into segments.

A bronchopulmonary segment (segmentum) is a structural and functional unit of a lung lobe that is ventilated by a tertiary bronchus and supplied by a single artery (veins run through the intersegmental partitions and are generally shared between two adjacent segments). Segments are separated from one another by connective tissue septa and have the shape of irregular cones or pyramids, with their apices directed toward the lung hilum and their bases facing the lung surface. Each lung contains ten segments.

The superior lobe of the right lung contains three segments: the apical segment (segmentum apicale (S_I)), which occupies the upper medial region of the superior lobe; the posterior segment (segmentum posterius (S_II)), which borders the apical segment and lies posterior and lateral to it; and the anterior segment (segmentum anterius (S_III)), whose base faces forward, forming part of the ventral surface of the superior lobe, positioned anterior and inferior to the apical segment.

The middle lobe of the right lung contains two segments: the lateral segment (segmentum laterale (S_IV)), which forms the posterolateral part of the middle lobe, and the medial segment (segmentum mediale (S_V)), which forms the anteromedial part of the middle lobe, constituting its mediastinal and diaphragmatic surfaces.

The inferior lobe of the right lung contains five segments: the superior segment (segmentum superius (S_VI)), which occupies the wedge-shaped apex of the inferior lobe located in its posterior region; the medial basal segment (segmentum basale mediale (S_VII)), which lies in the inferomedial part of the inferior lobe, forming parts of its dorsal and medial surfaces; the anterior basal segment (segmentum basale anterius (S_VIII)), which makes up the anterolateral part of the inferior lobe, forming parts of its inferior and lateral surfaces; the lateral basal segment (segmentum basale laterale (S_IX)), which forms the mid-lateral part of the inferior lobe, also contributing to its inferior and lateral surfaces; and the posterior basal segment (segmentum basale posterius (S_X)), which is situated posterior to all other basal segments, forming the posteromedial part of the inferior lobe as well as its posterior and medial surfaces.

The superior lobe of the left lung contains five segments: the apicoposterior segment (segmentum apicoposterius (S_I+II)), composed of the apical and posterior segments, which occupy the upper medial area of the superior lobe and form parts of its posterior and anterior surfaces; the anterior segment (segmentum anterius (S_III)), the largest segment of the superior lobe, occupying portions of its costal and mediastinal surfaces between the 1st and 4th ribs; the superior lingular segment (segmentum lingulare superius (S_IV)), which forms the middle part of the superior lobe and contributes to all of its surfaces; and the inferior lingular segment (segmentum lingulare inferius (S_V)), which occupies the lower part of the superior lobe.

The inferior lobe of the left lung contains five segments that mirror those of the right inferior lobe: the superior segment (segmentum superius (S_VI)), which occupies the wedge-shaped apex of the lobe, located in its paravertebral region; the medial basal segment (segmentum basale mediale (S_VII)), which occupies a central position and helps form the inner surface of the lobe; the anterior basal segment (segmentum basale anterius (S_VIII)), which occupies the anterolateral part of the inferior lobe, forming parts of its inferior and lateral surfaces; the lateral basal segment (segmentum basale laterale (S_IX)), which occupies the mid-lateral part of the inferior lobe and contributes to its inferior and lateral surfaces; and the posterior basal segment (segmentum basale posterius (S_X)), which occupies the posteromedial part of the inferior lobe, forming its posterior and medial surfaces.

Lung segments are composed of lobules. A pulmonary lobule (lobula) is the morphological structural unit of a lung segment, ventilated by a 9th- to 10th-order bronchus. A single segment contains approximately 80 lobules. Lobules are separated from one another by interlobular connective tissue septa. In shape, lobules resemble irregular pyramids, with a base diameter of 0.5–1 cm. A lobular bronchus enters the apex of each lobule and branches into terminal bronchioles.

Lobules are made up of acini. A single lobule contains 18–20 acini. A pulmonary acinus (acinus) is a structural unit originating from a single terminal bronchiole, comprising two respiratory bronchioles along with their corresponding system of alveolar ducts and alveolar sacs containing alveoli (Fig. 168 A, B).

Pulmonary alveoli (alveoli pulmonis) resemble irregular air sacs (about 0.3 mm in diameter) separated by interalveolar septa. Each septum typically serves as the wall for two adjacent alveoli; it contains a dense network of Blood capillaries, elastic and Collagen fibers, and connective tissue Cells. Pores that allow neighboring alveoli to communicate with one another are frequently found within the interalveolar septa. The alveoli are the terminal compartments of the Respiratory Portion of the lung where gas exchange takes place. They are enveloped by a dense capillary meshwork, allowing oxygen from inhaled air to cross the ultra-thin alveolocapillary membrane into the bloodstream, while carbon dioxide passes from the blood into the alveolar lumen to be expelled from the lungs during expiration. In addition to gas exchange, alveoli perform excretory (eliminating volatile chemical substances from the blood) and thermoregulatory Functions. The total number of alveoli in both lungs is approximately 600–700 million, and the total surface area available for gas exchange averages 100 m².

Fig. 168. Diagram of The Structure of a pulmonary acinus

(A — cast of the lumen of a human pulmonary acinus; B — cross-sectional diagram of an acinus)

1 — terminal bronchiole; 2 — respiratory bronchiole; 3 — alveolar ducts; 4 — alveoli; 5 — alveolar sacs.

Despite their substantial volume, the mass of each lung is only 0.5–0.6 kg (hence the organ's name in Ukrainian, derived from the word for light/weightless). The vital capacity of an adult's lungs averages 4–5 liters. Through targeted athletic training (such as swimming or rowing), this capacity can reach 7 liters or more. At rest, with each respiratory cycle, a person inhales and exhales about 0.5 L (500 cm³) of air. Under heavy exertion, the volume of exchanged air can rise to 3.5 L. Even collapsed lungs still contain residual air and therefore float in Water. The lungs of stillborn infants contain no air and thus sink in water—a crucial factor taken into account during forensic autopsies.

The color of the lungs depends on the accumulation of atmospheric dust particles within the lung tissue that are not entirely cleared via the respiratory tract. In newborns, the lungs are pale pink, but with age, they transition to a dark gray with a bluish tinge.

The lung parenchyma consists of branching air passages (bronchi, their branches, bronchioles, and alveoli), branching Blood and Lymphatic vessels, and nerves. All these structures are interconnected by connective tissue. Under normal conditions, lung tissue is elastic and exhibits a finely porous appearance on cross-section.

Blood supply to the lungs and bronchi. Arterial blood to nourish the lung tissue and bronchial walls is delivered via bronchial branches originating from the Thoracic Aorta. Venous blood from the bronchial walls drains through bronchial veins into tributaries of the pulmonary veins, as well as into the azygos and hemiazygos veins. To facilitate gas exchange, the lungs receive not only arterial blood but also venous blood, which is delivered via the right and left pulmonary Arteries. Within the lungs, this venous blood gives off carbon dioxide and is enriched with oxygen through gas exchange, effectively transforming from venous into arterial blood. Oxygenated blood then flows from the lungs through the pulmonary veins (right and left) into the left atrium of the heart. Lymph from the lungs drains via efferent lymphatic vessels into bronchopulmonary and lower and upper tracheobronchial lymph nodes. Some lymphatic vessels pierce the diaphragm and drain into abdominal lymph nodes.

The Innervation of the lungs is provided by the Branches of the pulmonary plexus, which is formed by branches of the Vagus nerve and the Sympathetic trunk.

PROJECTION OF THE lungs onto the thoracic wall. The apex of the lung anteriorly rises 3 cm above the margin of the first rib, and posteriorly lies at the level of the neck of the first rib due to the fact that the plane of the superior thoracic aperture is not horizontal, but inclined downwards. The anterior margins of the lungs, extending from their apices downwards and medially, approach each other and converge at the level of the junction of the manubrium of the Sternum with its body. They bound the superior triangle, free of lungs, which contains the Thymus gland. From here, the anterior margin of the right lung descends to the Cartilage of the sixth rib and, turning laterally, continues into the inferior margin. Anteriorly, this margin crosses the sixth rib as it extends posteriorly, laterally it crosses the eighth, and posteriorly, at the level of the inferior angle of the scapula, it crosses the tenth, reaching the eleventh thoracic vertebra. The anterior margin of the left lung descends only to the fourth rib, where the cardiac notch begins. Its margin runs horizontally at first, then turns downwards, reaches the sixth rib anteriorly, and thereafter follows approximately the same course as the margin of the right lung.

Age-related Features of the lungs. The mass of both lungs in a newborn is 39–70 g, and their volume is 67 cm³. The apices of the lungs in a newborn do not project above the clavicles, but are located at the level of the first ribs. The inferior border of the lungs in a newborn lies one rib higher than in an adult. As the child grows older, this border gradually descends. In old age (after 60 years), the inferior BOUNDARIES OF THE lungs, due to the descent of the abdominal viscera and diaphragm, are located 1–2 cm lower than in individuals aged 30–40 years.

The bronchial tree is largely formed by the time of birth. It grows most intensively During the first year of life and during Puberty. In individuals aged 40–45 years, the bronchial tree reaches its maximum dimensions. Age-related involution of the bronchi begins after 50 years. The length and lumen diameter of many segmental bronchi gradually decrease, and protrusions on their walls and tortuosity of their course develop.

The pulmonary acini in a newborn contain a small number of small pulmonary alveoli. Throughout the first year of life and later, the acinus grows through The Development of new alveolar ducts and The formation of new pulmonary alveoli. The Formation of the pulmonary parenchyma is completed by the age of 15–25 years. From 25 to 40 years of age, the STRUCTURE OF THE pulmonary acinus remains practically unchanged. After 40 years, the Aging of the pulmonary tissue begins: interalveolar septa become smoothed out, pulmonary alveoli become smaller, alveolar ducts merge with one another, and the sizes of the acini increase.