Lungs

Characteristic

Right Lung

Left Lung

Size

Shorter, broader

Longer, narrower

Weight

~700 grams

~600 grams

Lobes

3 (upper, middle, lower)

2 (upper, lower)

Fissures

2 (oblique, horizontal)

1 (oblique only)

Cardiac Impression

Shallow or absent

Deep with cardiac notch

Main Bronchus Length

~2.5 cm

~5 cm

</p><p>Arterial Supply</p><p>The bronchial arteries supply nutrition to the bronchial tree and to the pulmonary tissue. These are small arteries that vary in number, size and origin, but usually they are as follows:</p><p>The root is made up of the following structures.</p><p>1 Principal bronchus on the left side, and eparterial and hyparterial bronchi on the right side in posterior part.</p><p>2 One pulmonary artery in middle part.</p><p>3 Two pulmonary veins, superior and inferior, in</p><p>anterior part (Figs 16.4a and b).</p><p>4 Bronchial arteries—one on the right side and two on</p><p>the left side.</p><p>5 Bronchial veins</p><p>6 Anterior and posterior pulmonary plexuses of nerves</p><p>7 Lymphatics of the lung</p><p>8 Bronchopulmonary lymph nodes</p><p>9 Areolar tissue</p><p>LUNGS</p><p>267</p><p>Table 16.1: Structures related to the mediastinal surfaces of the right and left lungs</p><p>Right side (Fig.16.2) Left side (Fig.16.3)</p><p>1. Right atrium and auricle</p><p>2. A small part of the right ventricle</p><p>3. Superior vena cava</p><p>4. Lower part of the right brachiocephalic vein</p><p>5. Azygos vein</p><p>6. Oesophagus</p><p>7. Inferior vena cava</p><p>8. Trachea</p><p>9. Right vagus nerve</p><p>10.Right phrenic nerve</p><p>1. Left ventricle, left auricle, infundibulum and adjoining part of the right ventricle</p><p>2. Pulmonary trunk</p><p>3. Arch of aorta</p><p>4. Descending thoracic aorta</p><p>5. Left subclavian artery</p><p>6. Thoracic duct</p><p>7. Oesophagus</p><p>8. Left brachiocephalic vein 9. Left vagus nerve</p><p>10. Left phrenic nerve</p><p>11. Left recurrent laryngeal nerve</p><p>1 On the right side, there is one bronchial artery which arises from the third right posterior inter- costal artery.</p><p>2 On the left side, there are two bronchial arteries, both of which arise from the descending thoracic aorta, the upper opposite fifth thoracic vertebra and the lower just below the left bronchus.</p><p>Deoxygenated blood is brought to the lungs by the</p><p>two pulmonary arteries and oxygenated blood is returned to the heart by the four pulmonary veins.</p><p>There are precapillary anastomoses between bronchial and pulmonary arteries. These connections enlarge when any one of them is obstructed in disease. This ensures adequate blood supply to the lung tissue, allowing for proper gas exchange even in compromised conditions. </p><p>Right lung (Fig. 16.4a) Left lung</p><p>1. Shorter and broader</p><p>2. Larger and heavier, weighs about 700 g</p><p>3. Anterior border is straight</p><p>4. Cardiac impression shallow/absent</p><p>5. Absence of lingula</p><p>6. It has 2 fissures and 3 lobes</p><p>1. Longer and narrower</p><p>2. Smaller and lighter, weighs about 600 g</p><p>3. Anterior border is interrupted by the cardiac notch</p><p>4. Cardiac impression deep</p><p>5. Lingula present</p><p>6. It has only one fissure and 2 lobes</p><p>Section 2</p><p>Thorax</p><p>Venous Drainage of the Lungs</p><p>The venous blood from the first and second divisions of the bronchi is carried by bronchial veins. Usually there are two bronchial veins on each side. The right bronchial veins drain into the azygos vein. The left bronchial veins drain into the hemiazygos vein.</p><p>The greater part of the venous blood from the lungs is drained by the pulmonary veins.</p><p>Lymphatic Drainage</p><p>There are two sets of lymphatics, both of which drain into the bronchopulmonary nodes.</p><p>1 Superficial vessels drain the peripheral lung tissue lying beneath the pulmonary pleura. The vessels pass round the borders of the lung and margins of the fissures to reach the hilum.</p><p>2 Deep lymphatics drain the bronchial tree, the pulmonary vessels and the connective tissue septa. They run towards the hilum where they drain into the bronchopulmonary nodes (Fig. 16.4a).</p><p>The superficial vessels have numerous valves and</p><p>the deep vessels have only a few valves or no valves at all. Though there is no free anastomosis between the superficial and deep vessels, some connections exist which can open up, so that lymph can flow from the deep to the superficial lymphatics when the deep vessels are obstructed in disease of the lungs or of the lymph nodes.</p><p>Nerve Supply</p><p>1 Parasympathetic nerves are derived from the vagus. These fibres are:</p><p>a. Motor to the bronchial muscles, and on stimul- ation cause bronchospasm.</p><p>b. Secretomotor to the mucous glands of the bronchial tree.</p><p>c. Sensoryfibresareresponsibleforthestretchreflex of the lungs, and for the cough reflex.</p><p>2 Sympathetic nerves are derived from second to fifth sympathetic ganglia. These are inhibitory to the smooth muscle and glands of the bronchial tree. That is how sympathomimetic drugs, like adrenaline, cause bronchodilatation and relieve symptoms of bronchial asthma.</p><p>Both parasympathetic and sympathetic nerves first</p><p>form anterior and posterior pulmonary plexuses situated in front of and behind the lung roots: From the plexuses, nerves are distributed to the lungs along the blood vessels and bronchi (Fig. 16.4).</p><p>BRONCHIAL TREE</p><p>Features</p><p>The trachea divides at the level of the lower border of the fourth thoracic vertebra into two primary principal bronchi, one for each lung. The right principal bronchus is 2.5 cm long. It is shorter, wider and more in line with the trachea than the left principal bronchus (Fig. 16.5a). Inhaled particles or foreign bodies, therefore, tend to pass more frequently to the right lung, with the result that infections are more common on the right side than on the left.</p><p>The left principal bronchus is 5 cm. It is longer, narrower and more oblique than the right bronchus. Right bronchus makes an angle of 25° with tracheal bifurcation, while left bronchus makes an angle of 45° with the trachea.</p><p>Each principal bronchus enters the lung through the hilum, and divides into secondary lobar bronchi, one for each lobe of the lungs. Thus there are three lobar bronchi on the right side, and only two on the left side. Each lobar bronchus divides into tertiary or segmental bronchi, one for each bronchopulmonary segment; which are 10 on the right side and 10 on the left side. The segmental bronchi divide repeatedly to form very small branches called terminal bronchioles. Still smaller branches are called respiratory bronchioles (Fig. 16.6).</p><p>Each respiratory bronchiole aerates a small part of the lung known as a pulmonary unit. The respiratory bronchiole ends in microscopic passages which are termed:</p><p>1 Alveolar ducts (Fig. 16.7)</p><p>2 Atria</p><p>3 Air saccules</p><p>4 Pulmonary alveoli (Latin small cavity). Gaseous</p><p>exchanges take place in the alveoli.</p><p>Bronchopulmonary Segments</p><p>The most widely accepted classification of segments is given in Table 16.3. There are 10 segments on the right side and 10 on the left side (Figs 16.5a–c and 16.8 a and b).</p><p>Definition</p><p>Bronchopulmonary segments are well-defined anatomical segments aerated by tertiary/segmental bronchus. These are pyramidal in shape with apex directed towards hilum and base directed towards periphery having their own arterial supply; but venous drainage is shared by adjacent bronchopulmonary segment.</p><p>Right lung</p><p>Lobes Segments</p><p>A. Upper</p><p>B. Middle C. Lower</p><p>1. Apical</p><p>2. Posterior</p><p>3. Anterior</p><p>4. Lateral</p><p>5. Medial</p><p>6. Superior</p><p>7. Medial basal 8. Anterior basal 9. Lateral basal</p><p>10. Posterior basal</p><p>Left lung</p><p>A. Upper</p><p>• Upper division</p><p>• Lower division B. Lower</p><p>1. Apical</p><p>2. Posterior 3. Anterior</p><p>4. Superior lingular 5. Inferior lingular 6. Superior</p><p>7. Medial basal</p><p>8. Anterior basal</p><p>9. Lateral basal </p><p>10. Posterior basal</p><p>Features</p><p>1 These are well-defined anatomic, functional and surgical sectors of the lung.</p><p>2 Each one is aerated by a tertiary or segmental bronchus.</p><p>3 Each segment is pyramidal in shape with its apex directed towards the root of the lung (Fig. 16.8).</p><p>4 Each segment has a segmental bronchus, segmental artery, autonomic nerves and lymph vessels.</p><p>5 The segmental venules lies in the connective tissue between adjacent pulmonary units of bronchopul- monary segments.</p><p>6 During segmental resection, the surgeon works along the segmental veins to isolate a particular segment.</p><p>Relation to Pulmonary Artery</p><p>The branches of the pulmonary artery accompany the bronchi. The artery lies dorsolateral to the bronchus. Thus each segment has its own separate artery (Fig. 16.9).</p><p>Relation to Pulmonary Vein</p><p>The pulmonary veins do not accompany the bronchi or pulmonary arteries. They run in the intersegmental planes. Thus each segment has more than one vein and each vein drains more than one segment. Near the hilum, the veins are ventromedial to the bronchus.</p><p>It should be noted that the bronchopulmonary segment is not a bronchovascular segment because it does not have its own vein.</p><p>DEVELOPMENT OF RESPIRATORY SYSTEM</p><p>The lower respiratory tract primordium appears in the third week of intrauterine life in the form of an outgrowth (respiratory diverticulum) from the ventral wall of the primitive pharynx, i.e. the part of the foregut caudal to the hypobranchial eminence. Hence epithelial lining of the respiratory system is endodermal in origin. It forms the lining of the larynx, the trachea, the bronchi and the pulmonary alveoli. The connective tissue, cartilage and smooth muscles of these structures develop from splanchnic mesenchyme surrounding the foregut. As develop- ment progresses, the diverticulum separates from the foregut by the tracheo-oesophageal septum (except at the entrance to the larynx).</p><p>The respiratory diverticulum below the larynx grows caudally and forms the trachea in the midline. This bifurcates into two lateral outpocketings; the lung buds. In the fifth week of intrauterine life, the proximal parts of each lung bud forms the principal bronchi. Each of these grows laterally and invaginates the pericardio- peritoneal canals (primitive pleural cavities). Following this, the primary bronchi divide into secondary bronchi (3 on the right side and 2 on the left side). These divide dichotomously into tertiary bronchi. Each tertiary bronchus with its surrounding mesenchyme forms a bronchopulmonary segment. By 24th week, about 17 orders of branches are formed and the lung parenchyma develops in four stages.</p><p>1 Pseudoglandular stage (between 5 and 17 weeks). In this stage, developing lung resembles a gland.</p><p>2 Canalicular stage (between 16 and 25 weeks), the</p><p>lumina of bronchi and bronchioles become larger and</p><p>tissue becomes more vascular.</p><p>3 Terminal sac stage (between 24 weeks to birth). Many</p><p>saccules appear at the ends of terminal bronchioles</p><p>(terminal sacs). Capillaries bulge into these sacs.</p><p>4 Alveolar stage (late fetal period to 8 years after birth). The epithelial lining of the sacs becomes an extremely thin squamous layer and the alveolocapillary</p><p>membrane allows exchange of gases.</p><p>The four stages overlap each other because the</p><p>cranial segments of the lungs mature faster than the caudal ones.</p><p>By 28–32 weeks, some of the alveolar epithelial cells secrete a substance which is capable of lowering the surface tension at the air–alveolar interface and thus helps maintaining the patency of the alveoli; this is known as pulmonary surfactant.</p><p>Molecular Regulation</p><p>1. Transcription factor (TBX4) expressed in the endoderm of gut tube at the site of respiratory diverticulum induces formation of lung bud and is responsible for growth and differentiation of lungs.</p><p>2. Fibroblast growth factor 10 (FGF10) and other signals from splanchnic mesenchyme probably induces the outgrowth of tracheal bud.</p><p>3. Sonic hedgehog (SHH-GLi) and other signaling molecules are involved in the epithelial mesenchyme interaction which governs the branching of tracheal bud and its proliferation. </p><p>Congenital Anomalies</p><p>1 Tracheo-oesophageal fistula: This abnormal communication between the trachea and the oesophagus is due to a deviation of the oesophago- tracheal septum or from mechanical factor pushing the dorsal wall of the foregut anteriorly.</p><p>2 Tracheal stenosis</p><p>3 Azygos lobe of lung around vena azygos: This may be</p><p>due to an additional respiratory bud which develops</p><p>independently of the main respiratory system.</p><p>4 Hyaline membrane disease or distress syndrome: This is due to a deficiency of pulmonary surfactant.</p><p>5 Agenesis of lung.</p><p>CLINICAL ANATOMY</p><p>• Usually, the infection of a bronchopulmonary segment remains restricted to it, although tuberculosis and bronchogenic carcinoma may spread from one segment to another.</p><p>• Knowledge of the detailed anatomy of the bronchial tree helps considerably in:</p><p>a. Segmental resection (Fig. 16.12).</p><p>b. Visualising the interior of the bronchi through</p><p>a bronchoscope passed through the mouth and</p><p>trachea. The procedure is called bronchoscopy.</p><p>• Carina is a hook-shaped process projecting backwards from the lower margin of lowest tracheal ring. It helps to divide trachea into two primary bronchi. Right bronchus makes an angle of 25°, while left one makes an angle of 45°. Foreign bodies mostly descend into right bronchus (Fig. 16.13) as it is wider and more vertical than the left bronchus. Enlarged lymph nodes present</p><p>in this area may distort the carina.</p><p>• Carina(Latinkeel)ofthetracheaisasensitivearea.</p><p>When patient is made to lie on her/his left side, secretions from right bronchial tree flow towards the carina due to effect of gravity. This stimulates the cough reflex, and sputum is brought out. This is called postural drainage (Fig. 16.14).</p><p>Respiratory bronchiole is lined by cuboidal epithelium. The walls consist of collagenous connective tissue containing bundles of interlacing smooth muscle fibres and elastic fibres. At number of places, the alveolar sacs and alveoli arise from the respiratory bronchiole and its cuboidal epithelium is continuous with the squamous epithelium of alveolar sacs and alveoli.</p><p>Alveoli</p><p>Alveoli are thin-walled polyhedral sacs. The alveoli are lined by two types of cells, which rest on a basement</p><p>• Bronchial asthma is a common disease of res- piratory system. It occurs due to bronchospasm of smooth muscles in the wall of bronchioles. Patient has difficulty especially during expiration. It is accompanied by wheezing. Epinephrine, a sympathomimetic drug, relieves the symptoms.</p><p>• Auscultation of lung: Upper lobe is auscultated above 4th rib on both sides; lower lobes are best heard on the back. Middle lobe is auscultated between 4th and 6th ribs on right side.</p><p>• Superiorsegmentoflowerlobeisthemostdepen- dent bronchopulmonary segment in supine position. Foreign bodies are likely to be lodged here.</p><p>• Paradoxical respiration: During inspiration, the flail (abnormally mobile) segments of ribs are pulled inside the chest wall while during expira- tion the ribs are pushed out (Fig. 16.15).</p><p>• Tuberculosis of lung is one of the commonest diseases. A complete course of treatment must be taken under the guidance of a physician.</p><p>Bronchopulmonary segments of right lung “A PALM Seed Makes Another Little Palm”.</p><p>In order from superior to inferior: Apical</p><p>Posterior Anterior Lateral</p><p>Medial Superior Medial basal Anterior basal Lateral basal Posterior basal</p><p>Lung lobes: One having lingula, lobe numbers</p><p>Lingula is on Left</p><p>The lingula is like an atrophied lobe, so the left lung must have two "other" lobes, and, therefore, right lung has three lobes.</p><p>FACTS TO REMEMBER</p><p>• Large spongy lungs occupy almost whole of thoracic cage leaving little space for the heart and accompanying blood vessels, etc.</p><p>• Bronchopulmonary segments are independent functional units of lung.</p><p>• Lungs are subjected to lot of insult by the smoke of cigarette/bidis/pollution.</p><p>• Tuberculosisoflungisoneofthecommonestkiller in an underdeveloped or a developing country. Complete treatment of TB is a must, otherwise the</p><p>bacteria become resistant to antitubercular treatment. People harbouring resistant bacteria spread the disease to people around through their sputum.</p>