HUMAN ANATOMY – VOLUME 1

Fig. 162. Segments of right and left lungs.

A, B — right lung: 1 — apical segment (upper lobe)(C1), 2 — posterior segment (CII), 3 — anterior segment (CIII), 4 — lateral segment (CIV); 5 — medial segment (CV); 6 — apical segment (inferior lobe) (CVI); 7 — medial (cardiac) basal segment (CVII); 8 — anterior basal segment (CVIII); 9 — lateral basal segment (CIX); 10 — posterior basal segment (CX);

C, D — left lung: 1 — apical segment (CI); 2 — posterior segment (CII); 3 — anterior segment (CIII); 4 — superior lingular segment (CIV); 5 — inferior lingular segment (CV); 6 — apical segment (inferior lobe) (CVI); 7 — medial cardiac segment (CIX); 8 — anterior basal segment (CVIII); 9 — lateral basal segment (CIX); 10 — posterior basal segment (CX).

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numerous mixed glands. These glands are absent only in small bronchi (less then 2 mm in diameter). The fibrocartilaginous layer in main bronchi contains closed cartilage rings. In lobar, segmental and subsegmental bronchi the walls contain cartilage plates. Lobular bronchi 1 mm in diameter contains separate segments of cartilage tissue, and bronchi of smaller size do not contain cartilage in their walls at all. The adventitia of the bronchi is made up of fibrous connective tissue, which is continuous with interlobular connective tissue.

After passing through the bronchial tree air enters the alveolar tree, which contains both conducting and respiratory components. The alveolar tree consists of structures called acini, which are the functional units of the lungs. Each lung contains approximately 150000 acini. An acinus is formed by one terminal bronchiole (branch of lobular bronchus), which branches into 14–16 primary respiratory bronchioles, which in turn branch into secondary and tertiary respiratory bronchioles. Tertiary respiratory bronchioles divide into alveolar ducts (100 mm in diameter). Each alveolar duct ends in two alveolar sacs. The walls of alveolar ducts and sacs have bubble-like protrusions, formed by vesicles called alveoli. One alveolar duct connects with approximately 20 alveoli. The diameter of an alveolus is approximately 280 mm. The total number of alveoli for both lungs reaches 600– 700 million. The total surface area of alveoli fluctuates during inhalation and exhalation between 40 and 120 m2. (Fig.163).

The a c i n u s has a complex structure. Respiratory bronchioles are lined with cuboid epithelium which contains primarily non-ciliated epitheliocytes and some ciliary and Clara cells. Beneath lies a thin discontinuous layer of smooth myocytes. Alveolar ducts are lined with squamous epithelium. The entrance into each alveolus is circled by thin fascicles of smooth myocytes. Alveoli are lined by two kinds of cells, including type I (respiratory, or squamous epithelial) and type II (large granular) alveolocytes. Among these cells there may also be macrophages. The respiratory alveolocytes form the majority of the alveolar lining. These cells are 0.1– 0.2 mm thick; they contain a convex nucleus, numerous micropinocytotic vesicles and ribosomes. Other organelles are weakly developed. The process of gas exchange takes place through these alveolocytes. The large (granular) alveolocytes are situated in groups of two or three cells. These cells contain a large rounded nucleus and well developed organelles. Their apical surface has microvilli. These cells participate in formation of surfactant and act as a source of regeneration of the alveolar lining.

Surfactant is a complex substance composed of proteoglycolipids. It supports surface tension inside alveoli, preventing their collapse during

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Fig. 163. Structure of acinus of lung.

1 — terminal bronchiole; 2 — alveolar bronchiole I; 3 — alveolar bronchiole II; 4 — alveolar bronchiole III; 5 — alveolar ducts; 6 — alveolar sacs; 7 — alveoli.

exhalation. Surfactant also has certain bactericidal qualities and penetration of liquid into the alveolar lumen.

The blood-air barrier, through which gas exchange takes place, is approximately 0.2–0.5 mm thick. It is formed by thin respiratory alveolocytes (90–95 nm), basement membrane of alveolocytes,

basement membrane of blood capillaries and a thin layer of endotheliocytes (20–30 nm) (Fig. 164). The thickness of the two merged basement membranes is 90–100 nm. Capillaries are situated around alveoli in thick networks. Each capillary may adjoin one or several alveoli. Along with gas exchange the lungs participate in regulation of the acid-base balance, production of immunoglobulins by plasma cells, excretion of immunoglobulins into the lumen of respiratory passage and other functions.

I n n e r v a t i o n of the lungs: Branches of the vagus nerve and sympathetic trunk form the pulmonary plexus in the root of the lung. Branches of the pulmonary plexus penetrate inside the lung and form the peribronchial plexus.

B l o o d s u p p l y: The lungs receive arterial blood through bronchial branches (from thoracic part of the aorta)

Ve n o u s o u t f l o w: bronchial branches into pulmonary, azygos and hemiazygos veins. The lungs also receive venous blood through pulmonary arteries. In the lungs this blood becomes oxygenated and looses carbon dioxide. Arterial blood flows through pulmonary veins into the left atrium.

Ly m p h o u t f l o w: bronchopulmonary, inferior and superior tracheobronchial lymph nodes.

Questions for revision and examination

1.Describe the structure of the walls of the trachea and main bronchi.

2.Which organs adjoin the trachea and main bronchi in the thoracic cavity?

3.How long are the trachea and the main bronchi?

4.Name the segments of the right and left lungs.

5.What structures form a pulmonary acinus?

6.Describe the projections of the right and left lungs onto the skeleton.

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Fig. 164. Aerohaematic barrier of a lung.

1 — space of alveoles; 2 — surfactunt; 3 — alveocyte; 4 — endoteliocyte; 5 — space of capillary; 6 — erythrocytes within capillary space. Arrows show the way, how oxygen and dioxide pass across aerohaematic barrier.

PLEURA

The pleura (pléura) is a thin serous membrane, which consists of a parietal and visceral sheets (parietal and visceral pleurae). It is formed by a thin connective tissue base, which is covered by squamous epithelium (mesothelium), situated on a basement membrane. Cells of the mesothelium have numerous microvilli on their apical surfaces and weakly developed organelles. The connective tissue base is formed by a network of collagen and elastic fibers, separate fascicles of smooth myocytes and an insignificant amount of cell elements. The v i s c e r a l p l e u r a covers the

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lung from all sides, accretes with its surface and lines the surfaces of its fissures. It continues from the hilus of the lung onto the diaphragm as a fold, which forms the p u l m o n a r y l i g a m e n t. The p a r i e t a l p l e u - r a is continuous with the visceral pleural sheet. It lines the internal surface of the thoracic cavity and the mediastinum. The parietal pleura is divided into the costal, diaphragmatic and visceral parts. The c o s t a l p l e u r a lines the inside surfaces of the ribs and intercostal spaces. In the front near the sternum, and in the back near the spine the costal pleura continues into the mediastinal. The m e d i a s t i n a l p l e u r a covers the organs of the mediastinum. It extends from the inside surface of the sternum to the lateral surfaces of the spine. It is continuous with the pericardium. In the region of the lung root the mediastinal pleura continues into the visceral pleura. At the level of the head of rib 1 the mediastinal and costal pleurae connect, forming the p l e u r a l c u p u l a. From the front and medially this cupola is adjoined by the subclavicular artery and vein. At the bottom the costal and mediastinal pleurae continue into the d i a p h r a g m a t i c p l e u r a, which covers the diaphragm, excluding its central regions, which are accreted with the pericardium. Between the parietal and visceral pleurae there is a narrow space called the p l e u r a l c a v i t y (c áv u m p l é u r a e).

This cavity contains a small amount of serous fluid, which moistens the pleural membranes, allowing them to glide against each other more easily during breathing. In places of transition between the mediastinal and diaphragmatic pleurae the pleural cavity has recessions called pleural recesses, or sinuses, which serve as reserve spaces. The c o s t o d i a - p h r a g m a t i c r e c e s s (recéssus costodiaphragmáticus) is situated at the transition between the costal and diaphragmatic pleurae. Its deepest part (9 cm) corresponds to the middle axillary line. The p h r e n i c o m e - d i a s t i n a l r e c e s s (recéssus phrenicomediastinális) is situated at the transition between diaphragmatic and mediastinal pleurae in the form of a small sagittal fissure. The c o s t o m e d i a s t i n a l r e c e s s (recéssus costomediastinális) is a small fissure situated in the transition region between the anterior costal and mediastinal pleurae.

The right and left cupolas of the pleura extend 1.5–2 cm above the clavicle. Its anterior and posterior borders correspond to the borders of the lungs (Fig. 165, 166, 167). The inferior border of the pleural cavity lies one rib (2–3 cm) lower than the inferior margin of the lung. On the midclavicular line it corresponds to rib 7; on the anterior axillary line — to rib 8; on the middle axillary line — to rib 9; posterior axillary line — to rib 10; scapular line — to rib 11; and at the level of rib 12 it continues into the posterior border. The anterior borders of the pleura are parallel to each other between ribs 2 and 4. Above and below they diverge, forming inter-

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pleural fields. The superior interpleural field is situated behind the manubrium sterni and contains the thymus gland. The inferior interpleural field is triangular; it lies behind the inferior half of the sternum and adjacent ribs. It contains the part of the pericardium, which adjoins directly to the anterior thoracic wall.

MEDIASTINUM

The mediastinum (mediastinum) is a complex of organs, which are situated between the sternum, vertebral column and right and left mediastinal pleurae. At the top it is limited by the superior aperture of the thorax, and at the bottom by the diaphragm. The mediastinum is divided into superior and inferior sections. The border between these sections is the plane drawn through the sternal angle and intervertebral disc between T4 and T5 vertebrae. The superior mediastinum contains the thymus gland, left and right brachiocephalic veins, beginning of the superior vena cava, arch of the aorta, beginning of the brachiocephalic trunk, left common

Fig. 165. Projections of borders of lungs and parietal pleura.

Part of pleural cavity between inferior margins of lungs and inferior border of parietal pleura is of green color. Rome digits ribs are enumerated.

A — anterior aspect: 1 — apex of lung; 2 — superior interpleural area; 3 — anterior margin of lung; 4 — inferior interpleural area; 5 — cardiac notch; 6 — inferior margin of lung; 7 — inferior border of parietal pleura; 8 — oblique fissure; 9 — horisontal fissure (of right lung). B — lateral aspect: 1 — apex of lung; 2 — horisontal fissure; 3 — oblique fissure; 4 — inferior margin of lung; 5 — inferior border of parietal pleura.

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Fig. 166. Projections of borders of lungs and parietal pleura. Posterior aspect.

A part of pleural cavity between inferior margins of lungs and inferior border of parietal pleura have blue color. Rome digits enumerate ribs.

1 — apex of lung; 2 — oblique fissure; 3 — inferior margin of lung; 4 — inferior margin of parietal pleura.

Fig. 167. Pleural and pericardial cavities; shematic section in frontal flat.

1 — left lung; 2 — pleural cavity; 3 — pericardial cavity; 4 — diaphragm; 5 — costodiaphragmatic sinus; 6 — mediastinal pleura; 7 — right lung.

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carotid and left subclavicular arteries. It also contains the trachea, part of the esophagus, thoracic duct, sympathetic trunk, vagus and phrenic nerves. The inferior mediastinum consists of an anterior, middle and posterior parts. The anterior mediastinum is situated between the body of the sternum and the anterior surface of the pericardium. It contains the internal thoracic arteries and veins, parasternal and prepericardial lymph nodes. Situated inside the middle mediastinum are the heart and pericardium, beginning parts of the aorta and pulmonary trunk, end parts of the superior and inferior venae cavae, main bronchi, pulmonary arteries and veins, phrenic nerves, pericardiophrenic vessels, inferior tracheobronchial and pericardial lymph nodes. The posterior mediastinum contains organs, situated behind the pericardium. This includes the thoracic part of the aorta, azygos and hemiazygos veins, sections of the right and left sympathetic trunks, greater and lesser splanchnic nerves, vagus nerves, esophagus, thoracic duct and prevertebral lymph nodes.

In clinical practice the mediastinum is divided into the anterior and posterior sections, which are divided by a frontal plane, drawn through the roots of the lungs. The anterior mediastinum contains the heart, pericardium, arch of the aorta, thymus gland, phrenic nerves, pericardiophrenic and internal thoracic arteries and veins, and parasternal, mediastinal and superior phrenic lymph nodes. The posterior mediastinum contains the esophagus, thoracic part of the aorta, thoracic duct, azygos and hemiazygos veins, vagus and splanchnic nerves, sympathetic trunks and posterior, mediastinal and prevertebral lymph nodes.

DEVELOPMENT OF THE RESPIRATORY SYSTEM

Development of the external nose and nasal cavity is associated with formation of the fascial skull, oral cavity and olfactory organs of the embryo. A sac-like protrusion forms in the front, at the border between the pharyngeal and truncal guts. It continues to grow in the ventrocaudal direction, forming the laryngotracheal tube. Its upper end is connected to the future pharynx. On the fourth week of embryogenesis the lower end of this tube divides into the left and right bronchial buds. The proximal part of the laryngotracheal tube develops into epithelium and glands of the larynx. Its distal part forms epithelium and glands of the trachea. The right and left bronchial buds form the epithelium of the bronchi and lungs. During formation of the larynx its endoderm derivatives (primary gut) become integrated with the surrounding mesenchyme. The mesenchyme gradually develops into connective tissue, cartilage, musculature, blood and lymph vessels. Germs of future laryngeal cartilages and muscles appear during the fourth week of development. The cartilages of the larynx develop from

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the second and third branchial arches. The muscles develop from a common muscle sphincter, situated to the outside of the pharyngeal gut. Secondary (lobar) bronchi begin to form during the fifth week. The lobar bronchial buds continue to divide into future segmental bronchial buds, which, in turn, form the rest of the bronchial tree. Bronchioles form between months 4 and 6, and alveolar ducts and sacs — between months 6 and 9. At the moment of birth the bronchial and alveolar tree consists of 18 orders of branches. After birth it continues to grow (up to 23 orders of branches) and differentiate.

The visceral and parietal pleura develop from the splanchnopleura and somatopleura, respectively. Between them forms the pleural cavity.

AGE CHARACTERISTICS OF THE RESPIRATORY SYSTEM

Nasal cavity. In newborns the nasal cavity is narrow, with a height of 17–18 mm. The nasal conchae are thick, and the nasal meatuses are almost undeveloped. The paranasal sinuses are absent, with the exception of the maxillary sinus. The middle nasal meatus appears at the age of 6 months. The inferior meatus forms at age 2, and the superior — after the age 2 or 3. The frontal sinus develops during the second year of life. The sphenoid sinus appears by age 3; cells of the ethmoid bone develop between ages 3 and 6. By 8–9 years the maxillary sinus takes up almost the entire body of maxilla. The paranasal sinuses are almost completely developed by ages 10–14. The nasal cavity reaches its final size by age 18–20. During senescence the cartilages of the nasal cavity undergo partial calcification.

Larynx. A newborn child has a short, wide larynx, which has the shape of a funnel. It is situated higher than in an adult (at the level of C2– C4 vertebrae), and its superior aperture is relatively wider. The vestibule is short; the vocal fissure is situated at a high level and has the length of 6.5 mm. The cartilages of the larynx in newborns are thin, the Adam’s apple is absent, and the epiglottis extends above the root of the tongue. This position characteristic allows infants to breath and swallow (drink) simultaneously, because during swallowing the milk flows along the sides of the epiglottis. The muscles of the larynx in newborns are weakly developed. The larynx grows most intensively during the first four years of life. After age 6–7 it begins to develop sexual characteristics, such as the larger size, the Adam’s apple, and the longer vocal chords in boys. The larynx undergoes active growth after ages 10–12. As it grows its upper and lower borders descend. It reaches its adult position after ages 17–20. The cartilages of the larynx retain their flexibility until age 35–50. During senes-

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cence almost all cartilages (except for the epiglottis) begin to calcify, and the number of glands in the larynx decreases.

Trachea and main bronchi. In newborns the trachea is 3.2–4.5 cm long and less than 0.8 cm wide. The length of the right main bronchus is 0.6 cm, and of the left — 0.8 cm. Their mucosa is delicate. It contains a small amount of glands. The cartilages are soft and flexible, and the membranous wall is relatively wide. The trachea and main bronchi grow intensively during the first year, after which their growth slows down. By age 3–4 the width of their lumen increases by more than two times. Their growth intensifies during puberty. During the same period the upper and lower borders of the trachea descend. After ages 60–70 the cartilages of the trachea calcify and become brittle. The number of glands of the trachea and bronchi and their sizes decrease.

Lungs. The lungs of a newborn have a conoid shape. The superior lobes are relatively small, while the inferior lobes are comparatively large. The apexes of the lungs are situated at the level of rib 1, and the inferior margin is situated one rib higher than in an adult. The bronchial tree is formed. The number of alveoli inside the acini is small. During the first year of life there is significant growth of the bronchial tree and formation of new bronchioles, alveolar ducts and alveoli. The lungs undergo most intensive growth during puberty. The formation of alveolar ducts ends by age 9, and of alveoli — by age 12–15. The lung parenchyma is completely formed by age 20–25. The bronchial tree reaches maximal size between ages 35–40. After age 50 the length and diameter of the bronchi decreases. Sometimes their walls form diverticula. During senescence the alveoli increase in size and part of the interalveolar septa disappears. The connective tissue of the lungs proliferates, while the content of parenchyma and the volume of the lungs decrease. The inferior margin of the lungs in elderly people is situated 1–2 cm lower than at ages 30–40.

Pleura. During senescence the pleural cavity may contain adhesions between the parietal and visceral pleurae. The inferior margin of the pleura in elderly people projects lower than during ages 30–40.

VARIANTS AND ANOMALIES OF THE RESPIRATORY SYSTEM

External nose. The number of cartilages of the external nose varies. Often some of them are absent. In 20 percent of cases, near the posterior part of the nasal septum, there are a right and left vomeronasal cartilages. The shape and size of the nose and nostrils are highly variable.

Nasal cavity. Situated in the mucosa, which covers the anterior nasal spine, there is often a small blind canal called the vomeronasal (Jacobson’s) organ. This canal is directed upward and to the back, and is a rudi-

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