ANATOMY OF PLEURA,LUNGS AND

THE TRACHEOBRONCHIAL

TREE

The lungs are the essential organs of respiration. The functional design of the thorax facilitates this complex process.

The muscles of respiration and the diaphragm, acting togather, increase the intrathoracic volume, creating a negative pressure within the pleural space which surrounds the lung and causing expansion of the lung

PLEURAEach lung is covered by pleura, a serous membrane

arranged as a closed invaginated sac. The visceral or pulmonary pleura adheres closely to the pulmonary surface and its interlobar fissures. Its continuation, the parietal pleura, lines the corresponding half of the thoracic wall and covers much of the diaphragm and structures occupying the middle region of the thorax.

The visceral and parietal pleurae are continuous with each other around the hilar structures, and they remain in close, though sliding, contact at all phases of respiration.

The potential space between them is the pleural cavity, which is maintained at a negative pressure by the inward elastic recoil of the lung and the outward pull of the chest wall.

The right and left pleural sacs form separate compartments and touch only behind the upper half of the sternal body .

The region between them is the mediastinum (interpleural space).

The left pleural cavity is the smaller of the two, because the heart extends further to the left.

The upper and lower limits of the pleurae are about the same on the two sides, but the left sometimes descends lower in the midaxillary line.

PARIETAL PLEURA Different regions of parietal pleura are customarily

distinguished by name. The costovertebral pleura lines the internal surface

of the thoracic wall and the vertebral bodies .The diaphragmatic pleura lies on the thoracic

surface of the diaphragm. The cervical pleura lies over the pulmonary apices

(and is therefore also called the dome of the pleura). And the mediastinal pleura is applied to the

structures between the lungs.

Costovertebral pleura Costovertebral pleura lines the sternum, ribs, transversus thoracis

and intercostal muscles and the sides of the vertebral bodies;. External to the pleura is a thin layer of loose connective tissue, the endothoracic fascia.

Anteriorly, the costal pleura begins behind the sternum, where it is continuous with the mediastinal pleura. Posteriorly, it is again continuous with the mediastinal pleura.

The costovertebral pleura is continuous with the cervical pleura at the inner margin of the first rib and below it becomes continuous with the diaphragmatic pleura.

This line of costodiaphragmatic reflection begins behind the xiphoid process, passes behind the seventh costal cartilage to reach the eighth rib in the midclavicular line, the tenth rib in the midaxillary line, and then ascends slightly to cross the twelfth rib level with the upper border of the twelfth thoracic spine

Diaphragmatic pleura The diaphragmatic pleura is a thin, tightly

adherent layer which covers most of the upper surface of the diaphragm.

It is continuous with the costal pleura and on its medial aspect is continuous with the mediastinal pleura along the line of attachment of the pericardium to the diaphragm.

Cervical pleura The cervical pleura is a continuation of the

costovertebral pleura over the pulmonary apex .

The cervical pleura extends 3–4 cm above the first costal cartilage .

The cervical pleura is strengthened by a fascial suprapleural membrane.

The cervical pleura (like the pulmonary apex) reaches the level of the seventh cervical spine approximately 2.5 cm from the midline, its summit being 2.5 cm above the clavicle.

Mediastinal pleuraThe mediastinal pleura is the lateral

boundary of the mediastinum . At the hilum of the lung it turns laterally to

form a tube that encloses the hilar structures and is continuous with the pulmonary pleura

VISCERAL PLEURA

The pulmonary pleura is inseparably adherent to the lung over all its surfaces, including those in the fissures, except at the root or hilum of the lung and along a line descending from this, which marks the attachment of the pulmonary ligament .

INFERIOR PULMONARY LIGAMENTS

Below the hilum the mediastinal pleura extends as a double layer, the pulmonary ligament.

It is continuous above with the pleura around the hilar structures and below it ends in a free sickle-shaped border.

The interlobar fissures and posterior azygo-oesophageal and retrosternal pleural reflections are the only aspects of the normal pleura that can be visualized on a chest radiograph or CT scan.

Demonstration of significant pleural shadowing in any other regions usually implies pathological abnormalities of the pleura.

A, Axial section through the lungs below the carina demonstrates the left and right major fissures. The circular avascular area anterior to the right major fissure and surrounded by a halo of white (arrow) is due to the horizontal (minor) fissure. In this subject the fissure is

slightly domed and passing through the plane of the section. B, Axial section through the lungs at the mid left atrial level demonstrating the left and right major fissures as they pass more anteriorly and separate the lower lobes from the lingula and middle lobes

C, Sagittal section through the right lung demonstrating the major and minor fissures (arrows). D, Sagittal section through the left lung demonstrating the major fissure

PLEURAL RECESSES

The pleura extends considerably beyond the inferior border of the lung.

The line of pleural reflection from the thoracic wall to the diaphragm forms a narrow slit, the costodiaphragmatic recess.

In quiet inspiration the lower limit of the lung is normally 5 cm above the lower pleural limit.

A similar costomediastinal recess exists behind the sternum and the costal cartilages, where the thin anterior margin of the lung falls short of the line of pleural reflection.

Diagrams of pleural reflections. A, Coronal section of the right lung and pleura. Lines B to E indicate the respective planes and levels of sections shown in diagrams B to E. B, Upper horizontal section. Note the costomediastinal and retro-esophageal recesses. C, Middle horizontal section. Note that the anterior border of the pleura forms

the edge of the costomediastinal recess and that the oblique fissure reaches almost to the hilus. D, Lower horizontal section, showing also relationships to the pericardium. Note that the pulmonary ligament is formed by the double reflection of the pleura below the hilus of the lung. The mediastinal pleura is adherent to the

fibrous pericardium, except where the phrenic nerve descends between them (not shown). E, Sagittal section.

VASCULAR SUPPLY AND LYMPHATIC DRAINAGE

The parietal and visceral pleurae are developed from somatopleural and splanchnopleural layers of the lateral plate mesoderm respectively.

The parietal pleura is supplied by arteries from somatic sources.

The costovertebral pleura is supplied by branches of intercostal and internal thoracic arteries.

The mediastinal pleura is supplied by branches from bronchial, upper diaphragmatic, internal thoracic and mediastinal arteries.

The cervical pleura is supplied by branches from the subclavian artery.

And the diaphragmatic pleura is supplied by the superficial part of the microcirculation of the diaphragmatic muscle

. The veins join systemic veins in the thoracic wall which drain into the superior vena cava.

Lymph from the costovertebral parietal pleura drains into the internal thoracic chain anteriorly and intercostal chains posteriorly.

That from the diaphragmatic pleura drains into the mediastinal, retrosternal and coeliac axis nodes.

The visceral pleura forms an integral part of the lung and accordingly its arterial supply and venous drainage are provided by the bronchial vessels.

The visceral pleura is drained by pulmonary veins, apart from an area around the hilum that drains into bronchial veins.

The lymphatic drainage of the visceral pleura is to the deep pulmonary plexus within the interlobar and peribronchial spaces

INNERVATION

The costal and peripheral diaphragmatic parietal pleurae are innervated by intercostal nerves, and the mediastinal and central diaphragmatic parietal pleurae are innervated by the phrenic nerve.

The visceral pleura is innervated by visceral afferents that reach it by travelling along the bronchial vessels with the autonomic fibres.

LUNGS The lungs are situated on either side of the

heart and other mediastinal contents . Each lung is free in its pleural cavity,

except for its attachment to the heart and trachea at the hilum and pulmonary ligament respectively.

At birth the lungs are pink, but in adults they are dark grey and patchily mottled. As age advances, this maculation becomes black, as granules of inhaled carbonaceous material are deposited in the loose connective tissue near the lung surface.

The adult right lung on average weighs about 625g and left lung weighs about 565g.

PULMONARY SURFACE FEATURES

Each lung has an apex, base, three borders and two surfaces. In shape, each lung approximates to half a cone.

Apex The apex, the rounded upper extremity,

protrudes above the thoracic inlet where it contacts the cervical pleura, and is covered in turn by the suprapleural membrane.

As a consequence of the obliquity of the inlet, the apex rises 3–4 cm above the level of the first costal cartilage.It is level posteriorly with the neck of the first rib.

Its summit is 2.5 cm above the medial third of the clavicle.

Base The basal surface is semilunar and concave,

and rests upon the superior surface of the diaphragm.

Since the diaphragm extends higher on the right than on the left, the concavity is deeper on the base of the right lung.

Posterolaterally, the base has a sharp margin that projects a little into the costodiaphragmatic recess.

Costal surface The costal surface of the lung is smooth and

convex, and its shape is adapted to that of the thoracic wall.

It is in contact with the costal pleura.In specimens that have been preserved in

situ, this surface exhibits grooves that correspond with the overlying ribs.

Medial surface The medial surface has a posterior vertebral

and anterior mediastinal part. The vertebral part lies in contact with the sides

of the thoracic vertebrae and intervertebral discs, the posterior intercostal vessels and the splanchnic nerves.

The mediastinal area is deeply concave, because it is adapted to the heart at the cardiac impression.

Posterosuperior to this concavity is the somewhat triangular hilum, where various structures enter or leave the lung.

Pulmonary borders The inferior border is thin and sharp where

it separates the base from the costal surface .

It corresponds, in quiet respiration, to a line drawn from the lowest point of the anterior border which passes to the sixth rib at about the midclavicular line, then to the eighth rib in the midaxillary line (usually 10 cm above the costal margin), and then continues posteriorly, medially and slightly upwards to a point 2 cm lateral to the tenth thoracic spine.

The right lung is vertically shorter (by approximately 2.5 cm) than the left but is broader, and has weight greater than the left.

The posterior border separates the costal surface from the mediastinal surface, and corresponds to the heads of the ribs.

The thin, sharp, anterior border overlaps the pericardium.

On the right it is almost vertical. On the left it shows a variable cardiac notch.

PULMONARY LOBES AND FISSURES

PULMONARY FISSURES AND LOBES

The right lung is divided into superior, middle and inferior lobes by an oblique and a horizontal fissure .

The upper, oblique fissure separates the inferior from the middle and upper lobes, and corresponds closely to the left oblique fissure, although it is less vertical, and crosses the inferior border of the lung approximately 7.5 cm behind its anterior end.

On the posterior border it is either level with the spine of the fourth thoracic vertebra or slightly lower. It descends across the fifth intercostal space and follows the sixth rib to the sixth costochondral junction.

RIGHT LUNG

The short horizontal fissure separates the superior and middle lobes.

It passes from the oblique fissure, near the midaxillary line, horizontally forwards to the anterior border of the lung, level with the sternal end of the fourth costal cartilage, then passes backwards to the hilum on the mediastinal surface.

The horizontal fissure is usually visible on a frontal chest radiograph

. The oblique fissure is usually visible on a lateral radiograph and on a high resolution CT scan as a curvilinear band from the lateral aspect to the hilum.

The small middle lobe is cuneiform and includes some of the costal surface, the lower part of the anterior border and the anterior part of the base of the lung.

Sometimes the medial part of the upper lobe is partially separated by a fissure of variable depth which contains the terminal part of the azygos vein, forming the ‘lobe of the azygos vein.

Radiographically, a pleural effusion may be limited to the azygos fissure.

Less common variations are the presence of an inferior accessory fissure and a superior accessory fissure.

Left lung The left lung is divided into a superior and an

inferior lobe by an oblique fissure which extends from the costal to the medial surfaces of the lung both above and below the hilum.

Traced around the chest, the fissure reaches the fifth intercostal space (at or near the midaxillary line) and follows this to intersect the inferior border of the lung close to, or just below, the sixth costochondral junction (7.5 cm from the midline).

The left oblique fissure is usually more vertical than the right.

A left horizontal fissure is a normal variant found occasionally.

The superior lobe, which lies anterosuperior to the oblique fissure, includes the apex, anterior border, much of the costal and most of the medial surfaces of the lung.

At the lower end of the cardiac notch a small process, the lingula, is usually present.

The larger inferior lobe lies behind and below the fissure, and contributes almost the whole of the base, much of the costal surface and most of the posterior border of the lung.

Bronchopulmonary segments Each of the principal bronchi divides into

lobar bronchi. Primary branches of the right and left lobar

bronchi are termed segmental bronchi because each ramifies in a structurally separate, functionally independent, unit of lung tissue called a bronchopulmonary segment

The main segments are named and numbered as follows:

PULMONARY HILA The pulmonary root connects the medial surface of the

lung to the heart and trachea and is formed by a group of structures which enter or leave the hilum.

These are the principal bronchus, pulmonary artery, two pulmonary veins, bronchial vessels, a pulmonary autonomic plexus, lymph vessels, bronchopulmonary lymph nodes and loose connective tissue, all of which are enveloped by a sleeve of pleura.

The pulmonary roots, or pedicles, lie opposite the bodies of the fifth to seventh thoracic vertebrae.

The major structures in both roots are similarly arranged, so that the upper of the two pulmonary veins is anterior, the pulmonary artery and principal bronchus are more posterior, and the bronchial vessels are most posterior.

Right hilum The right root is situated behind the

superior vena cava and right atrium and below the terminal part of the azygos vein.

The usual sequence of hilar structures from above downwards is: superior lobar bronchus, pulmonary artery, principal bronchus, and lower pulmonary vein

Left hilum The left root lies below the aortic arch and in

front of the descending thoracic aorta. The usual vertical sequence of structures at

the left hilum is pulmonary artery, principal bronchus, and lower pulmonary vein.

Targeted and magnified view from PA chest radiograph clearly showsthe hilar vessels. The right and left hilar points (where the upper lober veins

apparently “cross” the lower lobe artery) are indicated.

VASCULAR SUPPLY AND LYMPHATIC DRAINAGE

The lungs have two functionally distinct circulatory pathways.

The pulmonary vessels convey deoxygenated blood to the alveolar walls and drain oxygenated blood back to the left side of the heart.

The much smaller bronchial vessels, which are derived from the systemic circulation, provide oxygenated blood to the bronchi and larger bronchioles.

At the hilum, the pulmonary vessels accompany the main bronchial divisions.

This is not the case in the bronchopulmonary segments, where a segmental bronchus, its branches and associated arteries occupy a central position in each segment, but the many tributaries of the pulmonary veins run between segments.

Some veins also lie beneath the visceral pleura, including the pleura in the interlobar fissures.

It follows from this that a bronchopulmonary segment is not a complete vascular unit with an individual bronchus, artery and vein.

The relationship between the central airways and the pulmonary vessels. The numbers in the lower figure denote the numbers of the bronchopulmonary segments.

Pulmonary arteries

The pulmonary artery bifurcates into right and left pulmonary arteries which pass to the hila of the lungs. On entering the lung tissue, both arteries divide into branches that accompany segmental and subsegmental bronchi.

The right pulmonary artery divides into two large branches as it emerges behind the superior vena cava .

The superior branch, which is the smaller of the two, goes to the superior lobe.

The inferior branch gives off the branches to the middle lobe, the superior segment of the inferior lobe and the rest of the inferior lobe segments.

The branches of the left pulmonary artery are extremely variable.

Pulmonary veins

. Pulmonary veins, two from each lung, drain the pulmonary capillaries. Their radicles coalesce to form large vessels that ultimately accompany the arteries and bronchial tubes to the pulmonary hilum.

The pulmonary veins open into the left atrium and convey oxygenated blood for systemic distribution by the left ventricle

The pulmonary veins are devoid of valves.

Pulmonary veins, segmental veins. 1, Apical vein. 2, Posterior vein. 3, Anterior vein. 4, Lingular vein. 5, Apical basal vein. 6, Superior basal vein. 7, Inferior basal vein. 8, Anterior basal vein. 9, Lateral basal vein. 10, Posterior basal vein.

11, Superficial intersegmental vein between segment 7 and segment 10. 12, Left inferior pulmonary vein. 13, Left superior pulmonary vein. 14, Right superior pulmonary vein. 15, Right inferior pulmonary vein. 16, Middle lobe vein.

Lymphatics Pulmonary lymphatic vessels originate in a superficial

subpleural plexus. A deep plexus accompanies the branches of the pulmonary

vessels and bronchi. Superficial efferents turn round lung borders and the

margins of fissures to converge in the bronchopulmonary nodes.

There is little anastomosis between the superficial and deep lymphatics, except in the hilar regions.

There is a tendency for vessels from the upper lobes to pass to the superior tracheobronchial nodes, and those from lower lobes to the inferior tracheobronchial group.

Lymphoid aggregations, non-follicular in appearance, occur in peribronchial sites and in ‘placoid' formations adjoining pulmonary pleura.

INNERVATION

The autonomic nervous system controls many aspects of airway function, including regulation of airway smooth muscle tone, mucus secretion from submucosal glands and surface epithelial goblet cells, vascular permeability and blood flow .

Pulmonary plexuses

The pulmonary plexuses lie anterior and posterior to the other hilar structures of the lungs .

The anterior plexus is small and is formed by rami from vagal and sympathetic cervical cardiac nerves.

The posterior pulmonary plexus is formed by the rami of vagal and sympathetic cardiac branches from the second to fifth or sixth thoracic sympathetic ganglia.

The left plexus also receives branches from the left recurrent laryngeal nerve.

TRACHEA AND BRONCHI The trachea is a tube formed of cartilage and

fibromuscular membrane, lined internally by mucosa. . It is 10–11 cm long.It descends from the larynx from the level of the sixth

cervical vertebra to the upper border of the fifth thoracic vertebra, where it divides into right and left principal (pulmonary) bronchi.

It lies approximately in the sagittal plane, but its point of bifurcation is usually a little to the right. .

Its external transverse diameter is typically 2 cm in adult males, and 1.5 cm in adult females.The lumen in live adults has an average transverse diameter of 12 mm.

In the first postnatal year, the tracheal diameter does not exceed 4 mm, while during later childhood its diameter in millimetres is approximately equal to age in years.

Targeted andmagnified view of the

tracheal carina (asterisk).The right main bronchus(thin arrows) is shorther

and more verticallyorientated than the left

RIGHT MAIN BRONCHUS The right principal bronchus is

approximately 2.5 cm long. It is wider, shorter and more vertical than

the left . The right main bronchus gives rise to its

first branch, the superior lobar bronchus, then enters the right lung opposite the fifth thoracic vertebra.

After giving off the superior lobar bronchus, it enters the pulmonary hilum and divides into a middle and an inferior lobar bronchus.

Right superior lobar bronchus

The right superior lobar bronchus arises from the lateral aspect of the parent bronchus and runs superolaterally to enter the hilum.

1 cm from its origin it divides into three segmental bronchi.

Segmental anatomy

The apical segmental bronchus continues superolaterally towards the apex of the lung, which it supplies, and divides near its origin into apical and anterior branches.

The posterior segmental bronchus serves the posteroinferior part of the superior lobe.

The anterior segmental bronchus runs anteroinferiorly to supply the rest of the superior lobe.

LEFT MAIN BRONCHUS

The left principal bronchus, which is narrower and less vertical than the right, is 5 cm long.

It enters the hilum of the left lung at the level of the sixth thoracic vertebra and divides into a superior and an inferior lobar bronchus.

Left superior lobar bronchus

The left superior lobar bronchus arises from its parent stem divides into two bronchi which correspond to the branches of the right principal bronchus as it supplies the right superior and middle lobes, except that on the left side both are distributed to the left superior lobe because there is no separate middle lobe.

Segmental anatomy

The superior division of the left superior lobar branches into the apical, posterior and anterior segmental bronchi.

They are largely distributed as they are in the right superior lobe.

The inferior division descends to the lingula and forms the lingular bronchus, which divides into superior and inferior lingular segmental bronchi. This is different from the pattern in the right middle lobe, where the corresponding distribution is lateral and medial.

Left inferior lobar bronchus The left inferior bronchus descends

posterolaterally and divides to supply territories of the lung that are distributed in essentially the same manner as they are in the right lung.

Segmental anatomy The superior (apical) segmental bronchus

arises from the inferior lobar bronchus posteriorly 1 cm from its origin.

After a further 1–2 cm, the inferior lobar bronchus divides into an anteromedial and a posterolateral stem.

The latter divides into lateral and posterior basal segmental bronchi.

SECONDARY PULMONARY LOBULES

Each segmental bronchus supplies a bronchopulmonary segment.

Progressive subdivisions of the bronchus occur within each segment and the bronchi become increasingly narrow.

The terminal bronchiole is the most peripheral bronchiole not to have alveoli in its wall. Distal to each terminal bronchiole is an acinus, which consists of three to four orders of respiratory bronchioles

. The primary lobule is the lung distal to the respiratory bronchiole.

The secondary lobule is the smallest subsection of the peripheral lung bounded by connective tissue septa and consists of approximately six terminal bronchioles.

VASCULAR SUPPLY AND LYMPHATIC DRAINAGE TRACHEA The trachea is supplied with blood mainly

by branches of the inferior thyroid arteries. The thoracic portion of the trachea is also

supplied by branches of the bronchial arteries .

Veins draining the trachea end in the inferior thyroid venous plexus.

The lymph vessels pass to the pretracheal and paratracheal lymph nodes.

Bronchi

BRONCHIAL ARTERIES

The bronchial arteries supply oxygenated blood to maintain the pulmonary tissues.

They are derived from the descending thoracic aorta either directly or indirectly.

The right bronchial artery is usually a branch of the third posterior intercostal artery.

There are normally two left bronchial arteries (upper and lower) that branch separately from the thoracic aorta.

The bronchial arteries accompany the bronchial tree and supply bronchial glands, the walls of the bronchial tubes and larger pulmonary vessels.

Bronchial arteries supply the bronchial wall as far as the respiratory bronchioles, and anastomose with branches of the pulmonary arteries in the walls of the smaller bronchi and in the visceral pleura.

Usually two on each side, the bronchial veins drain blood from larger bronchi and from hilar structure.

They also communicate with the pulmonary veins and end in the azygos vein on the right and in the left superior intercostal or the accessory hemiazygos veins on the left.

Bronchial veins do not receive all the blood conveyed by bronchial arteries, some enters the pulmonary veins.

Lymphatic drainage The deep lymphatic plexus reaches the

hilum by travelling along the pulmonary vessels and bronchi.

The walls of the alveoli have no lymphatic vessels.

INNERVATION The anterior and posterior pulmonary

plexuses innervate the trachea and the bronchi.

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