Last's Anatomy: Regional and Applied

Part nine. Lungs

Each lung, consisting of two lobes on the left and three on the right, lies within its own side of the thoracic cavity and is surrounded by the visceral layer of the pleura. Each has a principal or main bronchus, one pulmonary artery and two pulmonary veins, all of which undergo subdivisions within the lung substance, together with bronchial vessels, nerves and lymphatics. The region of the mediastinum where the lung is connected to the trachea and heart is the root of the lung, and the region of the lung where the bronchi and pulmonary vessels enter or leave is the hilum.

The lung surface is mottled, and in colour is pink or grey according to the atmosphere in which its owner has lived. It is crepitant to the touch.

The lung conforms to the shape of the cavity which contains it. It has a convex costal surface and a concave diaphragmatic surface, separated from each other by a sharp inferior border. The posterior border of each lung is generously rounded to fit the paravertebral gutter, and is continued up to the convex apex. The anterior border is thin and sharp; on the left side the lower part of this border is deeply concave—the cardiac notch. The mediastinal surfaces differ somewhat. On the left side the cardiac notch is seen to be the anterior margin of a deep concavity produced by the pericardium in front of the hilum; the arch and ascending aorta make a deep groove on the lung surface around the hilum. Above the aortic arch are vertical impressions made by the subclavian artery and (behind the artery) the oesophagus. On the right the cardiac impression is much shallower; a groove for the azygos vein curves over the hilum to meet the impression made by the superior vena cava. Above the azygos arch and behind the vena cava is a shallow groove for the trachea and right vagus. The apices are grooved by the subclavian arteries (Fig. 4.10). Inferiorly, in front of the lower end of the pulmonary ligament, both lungs may have shallow impressions, on the left lung for the oesophagus and on the right lung for the inferior vena cava.

Lung roots

The left and right lung roots are similar but not identical.

In the left lung root (Fig. 4.13) the upper part is occupied by the left pulmonary artery lying within the concavity of the arch of the aorta. Below it is the left bronchus. There are two pulmonary veins, one in front of and the other below the bronchus. These structures are enclosed in a sleeve of pleura continuous below with the pulmonary ligament.

In the right lung root (Fig. 4.12) the general arrangement of structures is similar to that on the left, but the bronchus to the upper lobe and the branch of the pulmonary artery to the upper lobe originate outside the lung. Thus the upper lobe bronchus and its accompanying artery are found above the level of the main bronchus and pulmonary artery, the arteries lying in front of their respective bronchi; the right hilum is consequently larger than the left. The two pulmonary veins are disposed as on the left side, in front of and below the main bronchus. The root of the right lung lies within a sleeve of pleura with a dependent pulmonary ligament, as on the left.

In addition to the above large structures, each root contains bronchial vessels, autonomic nerves and lymph nodes and channels.

Pulmonary arteries

The left pulmonary artery attached to the undersurface of the aortic arch by the ligamentum arteriosum, quickly spirals over the top of the left bronchus. The right pulmonary artery, longer than the left, passes below the carina anterior to the oesophagus, and at the lung root is anterior to the right main bronchus (Fig. 4.29). It gives off its branch to the upper lobe and then enters the hilum.

Fissures

The oblique fissure (Figs 4.30 and 4.31) extends from the surface of the lung to the hilum and divides the organ into separate upper and lower lobes which are connected only by the lobar bronchi and vessels. In some lungs the fissure may not be complete. On the right lung a horizontal fissure (Fig. 4.30) passes from the anterior margin into the oblique fissure to separate a wedge-shaped middle lobe from the upper lobe. The visceral pleura, clothing the surface of the lung, extends inwards to line the depths of the fissures. The middle lobe of the right lung may not be completely separate from the upper lobe, the fissure separating it from the upper lobe being incomplete or even absent. In the left lung the lowest and most medial part of the upper lobe that overlaps the front of the pericardium is the lingula and forms the boundary of the cardiac notch.

Surface markings

The hilum of each lung lies approximately behind the third and fourth costal cartilages at the sternal margin, and level with T5–7 vertebrae.

On upper costal walls and the supraclavicular region, the surface markings of the lungs coincide with those of the pleura (Fig. 4.28). The anterior border of the right lung falls very little short of the pleura, lying within the lateral margin of the sternum; that of the left lung in contrast curves laterally to uncover the area of superficial cardiac dullness from the fourth costal cartilage out to the fifth intercostal space just medial to the midclavicular line. The lower border of the lung lies nearly horizontally around the chest wall, but two ribs higher than the pleural reflexion, i.e. in the midclavicular line at the sixth rib, midaxillary line at the eighth rib, and at the lateral border of erector spinae at the tenth rib.

The oblique fissures of each lung are indicated by a line joining the spine of the T3 vertebra, which is opposite the posterior end of the fifth rib, to the sixth rib in the midclavicular line. More simply, this is approximately the line of the fifth rib, or level with the vertebral border of the scapula when the arm is fully abducted above the head. On the right the fourth costal cartilage overlies the horizontal fissure between the upper and middle lobes; continued horizontally this line meets the oblique fissure in the midaxillary line.

Lobar and segmental bronchi

Because the left lung grows into a smaller cavity than the right, the way bronchi divide to supply segments of lung is not identical on the two sides, although there are close similarities.

From the bifurcation of the trachea each main bronchus (Fig. 4.32) passes downwards and laterally to enter the hilum of the lung. The right main bronchus is 2.5cm long and shorter, wider and more vertical than the left, which is 5cm long. At the bifurcation the lowest tracheal cartilage has a hook-shaped process, the carina, which curves backwards between the bronchi and raises an anteroposterior internal ridge that lies to the left of the midline. Foreign bodies that fall down the trachea are more likely to enter the right bronchus.

Each main bronchus gives rise to lobar bronchi (Fig. 4.32) that supply the lobes of the lung. The right main bronchus gives off the upper lobe bronchus outside the hilum and ends within the hilum by dividing into middle and lower lobe bronchi. The left main bronchus divides within the hilum into upper and lower lobar bronchi. The tissues of the bronchi are supplied by the bronchial arteries. The veins of the right main bronchus drain to the azygos vein and those of the left to the accessory hemiazygos vein.

Each lobar bronchus gives rise to further branches, the segmental bronchi (Fig. 4.32), for each segment of the lung. There are typically 10 bronchopulmonary segments in each lung and therefore 10 segmental bronchi; some left segmental bronchi may share a common stem. Each lung segment is roughly pyramidal in shape, with its apex towards the hilum and base towards the surface of the lung. The bronchopulmonary segments are given the same names and numbers as the segmental bronchi, which are listed as follows:

From the above it can be seen that the bronchi and segments of the two lungs are very similar. The upper lobe of the right lung bronchus arises from the lateral aspect of the main bronchus and divides into three segmental bronchi. The middle lobe bronchus arises from the front of the main bronchus and divides into lateral and medial segmental bronchi. The upper lobe of the left lung has five segments, the first two (numerically) being supplied by bronchi that have a common apicoposterior stem. The lingular part of the left upper lobe has segments called superior and inferior; otherwise the names on the two sides are identical. Both lower lobes have five segmental bronchi and segments. On the left the medial and anterior basal segments arise from a common stem bronchus which subsequently subdivides.

On both sides the superior (apical) segment of the lower lobe is supplied by a bronchus (6) which is the highest to arise from the posterior surface of the bronchial tree. Material aspirated by a supine, comatose or anaesthetized patient would tend to gravitate into the superior segment of the right lower lobe, which is consequently a common site for aspiration pneumonia and abscess formation.

Blood supply

The bronchial tree receives its own arterial supply by the bronchial arteries. There are usually three: two on the left which are direct branches from the aorta, and one on the right coming from the third right posterior intercostal artery. They supply the bronchi from the carina to the respiratory bronchioles and the visceral pleura. The bronchial veins fall into a superficial system draining from the hilar region and visceral pleura into the azygos vein on the right and the accessory hemiazygos on the left, and a deep system from the deeper lung substance draining to a main pulmonary vein (thus mixing venous with oxygenated blood). The alveoli contain within their walls a rich capillary plexus which is fed with deoxygenated blood by the pulmonary artery. The pulmonary artery divides with the bronchi; every bronchus is accompanied by a branch of the artery. The artery supplies no bronchus but it does supply the alveoli, giving them all they need except oxygen, of which they have more than enough. There is some anastomosis between the bronchial and pulmonary arteries at precapillary level. The pulmonary veins are formed from tributaries which do not closely follow the bronchi but tend to run in the intersegmental septa. Two pulmonary veins leave each hilum, one from above and one from below the oblique fissure.

Lymph drainage

Lymphatic vessels in the lung originate in a superficial subpleural plexus and a deep submucosal plexus. They drain along the lung surface and with the pulmonary vessels to bronchopulmonary, or hilar, nodes in the hilum of each lung. Efferents from bronchopulmonary nodes run to tracheobronchial nodes. Lymphatics from the upper and middle lobes tend to drain to superior, and those from the lower lobes to inferior, tracheobronchial nodes, but these connections are not exclusive.

Nerve supply

Autonomic nerve fibres from the cardiac plexuses, and directly from the thoracic vagus and sympathetic chain, pass to the pulmonary plexuses, which are situated anterior and posterior to other hilar structures, the anterior plexus being much smaller than the posterior. From here nerve fibres pass into the lung with the bronchi and vessels. The parasympathetic (vagal) fibres are afferent (cell bodies in the inferior ganglion) and efferent (cell bodies in dorsal nucleus, with relay in the bronchial mucosa). They provide the afferent fibres for the cough reflex (see p. 395), important for clearing excess secretions and inhaled substances from the tracheobronchial tree; the receptors are unmyelinated endings in the epithelium. Included among the afferent fibres are those subserving pain. The vagal efferents are bronchoconstrictor, vasodilator and secretomotor to mucous glands. The sympathetic preganglionic fibres originate from cells in the lateral horn of the T2-4 segments of the spinal cord; they relay in the upper thoracic ganglia. The sympathetic efferents are bronchodilator and vasoconstrictor.

Structure

Bronchi have smooth muscle and hyaline cartilage in their walls and are lined by the typical respiratory type of epithelium, pseudostratified columnar ciliated, with mucous glands. By successive divisions they become smaller and smaller; when cartilage disappears (at a diameter of about 1mm) bronchi become bronchioles. After repeated branching, a lobular bronchiole enters each lung lobule and divides into terminal bronchioles which are the most distal air passages lined by typical respiratory epithelium. They subdivide into respiratory bronchioles, so called because some alveoli (air sacs) open directly off their walls. Beyond the respiratory bronchioles are alveolar ducts which have a lining of cubical epithelium (but no cilia) and many alveoli. Finally there are the alveolar sacs which have walls studded with alveoli.

The walls of the alveoli are lined by two types of epithelial cells covering a layer of connective tissue which contains capillaries. Over 90% of the lining consists of squamous type I alveolar cells through which gaseous exchange can occur. The rest of the alveolar wall is lined by smaller, rounded type II alveolar cells, containing lamellar bodies with a high phospholipid content; when discharged from the cell they produce the surfactant effect (reducing surface tension). Migratory macrophages are also present on the epithelial surface and within the alveolar lumen.

Development

Each lung develops from a bud at the lower end of the laryngotracheal tube that grows down from the floor of the primitive pharynx (see Fig. 1.19, p. 24). These endodermal buds form the epithelial part of the lung; the connective tissue, cartilage and muscle of the bronchial tree are derived from the surrounding mesoderm. By the fifth month the lung has a glandular appearance, with clumps of epithelial cells and hardly any recognizable lumina. The cell groups proliferate, become canalized and lie adjacent to capillaries so that by the seventh month there are sufficient alveoli to sustain a viable infant following premature birth at this time. Surfactant begins to be secreted about the sixth month.

In the fetus the lungs are not just a mass of collapsed air spaces but are full of fluid, largely secreted by the lungs but with a contribution from swallowed amniotic fluid. At the time of birth some is squeezed out by thoracic pressure and the rest escapes into blood capillaries and lymphatics, assisted by surfactant action. After birth there is no new development of any kind of bronchioles, but more than 80% of the adult number of alveoli are budded off during about the first 8 years of life.

Surgical approach

For pneumonectomy the approach on each side is usually a posterolateral thoracotomy through the bed of the sixth rib. It is necessary to divide the main pulmonary artery, both pulmonary veins and the main bronchus, but the sequence in which these principal structures are divided varies in accordance with their ease of isolation and the pathological indication for pneumonectomy. On the left the vagus is divided distal to the recurrent laryngeal branch, and the pulmonary artery is divided distal to the ligamentum arteriosum. The nearness of the aortic arch may make closure of the bronchus difficult. On the right the arch of the azygos vein is preserved if possible, as it affords collateral circulation if the superior vena cava is obstructed. The latter may have to be displaced forwards to allow satisfactory ligation of the pulmonary artery.