posterosuperior to the right pulmonary artery, the right principal bron-
Anterior relations
chus crosses the posterior aspect of the artery, enters the pulmonary
The cervical trachea is covered anteriorly by skin and by the superficial hilum posteroinferiorly, and divides into middle and inferior lobar
and deep cervical fasciae. It is crossed by the jugular arch and over- bronchi.
lapped by sternohyoid and sternothyroid. The second to fourth tracheal
cartilages are crossed by the isthmus of the thyroid gland, above which Right superior lobar bronchus
an anastomosis connects the two superior thyroid arteries; anteroinfe-
riorly are the pretracheal fascia, inferior thyroid veins, thymic remnants
The right superior lobar bronchus arises from the lateral aspect of the
and the thyroid ima artery (when present). In children, the brachio-
parent bronchus and runs superolaterally to enter the hilum; 1 cm from
cephalic trunk crosses obliquely in front of the trachea at, or a little
its origin it divides into three segmental bronchi.
above, the upper border of the manubrium; the left brachiocephalic
vein may also rise a little above this level. An enlarged thyroid gland Segmental anatomy
may cover the cervical trachea and reach the superior mediastinum,
The apical segmental bronchus continues superolaterally towards the
usually anterior to the left brachiocephalic vein.
apex of the lung, which it supplies, and divides near its origin into
Obstruction of the upper airways, craniofacial trauma and neck
apical and anterior branches. The posterior segmental bronchus serves
cancers (especially laryngeal), in either the acute or the chronic set-
the posteroinferior part of the superior lobe, passes posterolaterally and
ting, require a tracheostomy: the creation of a hole (stoma) in the
slightly superiorly, and soon divides into a lateral and a posterior
anterior aspect of the trachea that can serve independently or as a site
branch. The anterior segmental bronchus runs anteroinferiorly to
for a dedicated tube allowing the spontaneous or mechanical ventila-
supply the rest of the superior lobe, and divides near its origin into a


tion of the patient. This manœuvre usually involves the second and
lateral and an anterior branch of equal size (see Fig. 54.7).
third cartilages; in particular cases, the lumen of the larynx can be
approached (laryngotomy or cricothyroidotomy). Tracheostomy may
Bronchus intermedius
be performed surgically or percutaneously (Ciaglia et al 1985, Griggs
et al 1990).
The bronchus intermedius is the continuation of the right principal
Posterior relations bronchus distal to the right upper lobe bifurcation, coursing in the same
The oesophagus is a posterior relation of the cervical trachea, separating direction as the former but with a slight rightward deviation. It is
it from the vertebral column and the prevertebral fascia. slightly flattened, 10 mm wide and 2.5 cm long. It begins at the base
of the right upper lobe bronchus and gives rise to the common middle
Lateral relations lobe and lower lobe bronchi, and sometimes to an additional variant
The paired lobes of the thyroid gland, which descend to the fifth or subsuperior segmental bronchus (see below).
sixth tracheal cartilage, and the common carotid and inferior thyroid
arteries are lateral relations. The recurrent laryngeal nerves ascend on Right middle lobar bronchus
each side, in or near the grooves between the sides of the trachea and
the oesophagus. The right middle lobar bronchus usually starts 2 cm below the superior
lobar bronchus and descends anterolaterally.
Relations of the thoracic part of the trachea
Segmental anatomy
Anterior relations The right middle lobar bronchus soon divides into a lateral and a
medial segmental bronchus distributed to the homonymous segments
As it descends through the superior mediastinum, the thoracic trachea
of the middle lobe (see Fig. 54.7).
lies behind the manubrium sterni, the origins of sternohyoid and ster-
nothyroid, the thymic remnants and the inferior thyroid veins. The
Right inferior lobar bronchus
brachiocephalic and left common carotid arteries come to lie on the
right and left of the trachea, respectively, as they diverge upwards into


the neck. At a lower level, the aortic arch, brachiocephalic trunk, left The right inferior lobar bronchus is the continuation of the bronchus
common carotid artery, left brachiocephalic vein, deep cardiac plexus intermedius beyond the origin of the common middle lobar
and lymph nodes are all anterior to the trachea. bronchus.
Posterior relations Segmental anatomy
The oesophagus is posterior to the trachea and separates it from the At or a little below its origin from the principal bronchus, the right
vertebral column. inferior lobar bronchus gives off a large superior (apical) segmental
bronchus posteriorly. This runs posteriorly to the upper part of the
Lateral relations inferior lobe, and then divides into medial, superior and lateral
Laterally and on the right are the right lung and pleura, right vagus branches; the first two usually arise from a common stem. After giving
nerve, right brachiocephalic vein, superior vena cava and azygos vein. off the superior segmental branch, the right inferior lobar bronchus
On the left are the arch of the aorta, left common carotid and subclavian descends posterolaterally. The medial basal segmental bronchus
arteries. The left recurrent laryngeal nerve is at first situated between the branches from its anteromedial aspect, and runs inferomedially to serve
trachea and aortic arch, and then lies within or just anterior to the a small region below the hilum. The inferior lobar bronchus continues
tracheo-oesophageal groove. downwards and divides into an anterior basal segmental bronchus,
The trachea and oesophagus can be compressed (or even eroded) by which descends anteriorly, and a trunk that soon divides into a lateral
vascular rings resulting from abnormal formation of the aorta and its basal segmental bronchus, which descends laterally, and a posterior


Pleura, lungs, trachea and bronchi
966.e1
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Normal variants in the bronchial anatomy are occasionally seen and
consist of either displaced or supernumerary airways (Ghaye et al
2001). Abnormalities include a common origin of the right upper and
middle lobe bronchi; an accessory cardiac bronchus; a right lower lobe
bronchus that may arise from the left main stem bronchus; and an
oesophageal bronchus. These anatomical variants are largely asympto-
matic, occasionally causing haemoptysis, recurrent infection and bron-
chiectasis. Congenital bronchial atresia is associated with a bronchocele
and emphysematous changes in the peripheral lung fields (Wang et al
2012). Left bronchial isomerism, characterized by two hyparterial
bronchi and two bilobed lungs, and right bronchial isomerism, charac-
terized by two eparterial bronchi and two trilobed lungs, may be fea-
tures of the heterotaxy syndrome, which may include congenital cardiac,
liver, stomach, intestinal and splenic abnormalities.


trachea and bronchi
967
45
retPahc
Fig. 54.14 A, A bronchogram showing the
branching pattern of the trachea and bronchi of
the right lung, in a slightly oblique, anteroposterior
view. In this procedure, a radiopaque contrast
medium has been introduced into the respiratory
tract to coat the walls of the respiratory passages.
For identification of the major branches, compare
with Figures 54.5B–E and 54.6. B–E, CT images
demonstrating branching of lobar and segmental
bronchi. These axial images correspond to level
T4 (carina) to T6 (bifurcation right middle lobe
C bronchus), respectively.
A D
B E
basal segmental bronchus, which descends posteriorly. In more than Left inferior lobar bronchus
half of all right lungs, a subsuperior (subapical) segmental bronchus
arises posteriorly from the bronchus intermedius or right inferior lobar
The left inferior bronchus descends posterolaterally and divides to
bronchus 1–3 cm below the superior segmental bronchus, and is dis-
supply territories of the lung that are distributed in essentially the same
tributed to the region of lung between the superior and posterior basal
manner as they are in the right lung.
segments (see Fig. 54.7).
Segmental anatomy
LEFT PRINCIPAL BRONCHUS 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 anteromedial and posterolateral
The left principal bronchus is narrower, less vertical and twice as long
stems. The latter divides into lateral and posterior basal segmental
(5 cm) as the right principal bronchus. Passing to the left, inferior to
bronchi. The anterior basal segmental bronchus is an occasional inde-
the aortic arch, it crosses anterior to the oesophagus, thoracic duct and
pendent branch of the inferior lobar bronchus, and a subsuperior (sub- descending aorta; the left pulmonary artery is at first anterior and then
apical) segmental bronchus occasionally arises posteriorly from the left


superior to it (the hyparterial bronchus). It enters the hilum of the left
inferior lobar bronchus (see Fig. 54.7).
lung at the level of the sixth thoracic vertebra and divides into superior
and inferior lobar bronchi.
ASPIRATION PNEUMONIA
Left superior lobar bronchus
Penetration of foreign substances into the bronchial tree, usually oral
The left superior lobar bronchus arises from the anterolateral aspect of
or gastric contents, produces aspiration pneumonia. Chemical (gastric
its parent stem, curves laterally, and soon divides into two bronchi that
acid) injury or bacterial infection complicates the clinical picture. The
correspond to the branches of the right principal bronchus as it supplies
location depends on the position of the patient, since it is gravity-
the right superior and middle lobes, except that, on the left side, both
dependent; right middle and inferior lobes are the most common sites
are distributed to the left superior lobe because there is no separate
because of the larger and more vertical corresponding bronchi. Aspira-
middle lobe.
tion while standing leads to bilateral inferior lobar infiltrates. Patients
Segmental anatomy lying down will aspirate in the right superior lobe (e.g. alcoholics).
The superior division of the left superior lobar bronchus ascends 1 cm,
gives off an anterior segmental bronchus, continues a further 1 cm as
VASCULAR SUPPLY AND LYMPHATIC DRAINAGE
the apicoposterior segmental bronchus, and then divides into apical
and posterior branches. The apical, posterior and anterior segmental
Trachea
bronchi are largely distributed as they are in the right superior lobe. The
inferior division descends anterolaterally to the anteroinferior part of
the left superior lobe (the lingula) and forms the common lingular The trachea is supplied mainly by branches of the inferior thyroid arter-
bronchus, which divides into superior and inferior lingular segmental ies, which anastomose with ascending branches of the bronchial arter-
bronchi. This is different from the pattern in the right middle lobe, ies. Veins draining the trachea end in the inferior thyroid venous plexus.


where the corresponding distribution is lateral and medial (see The lymph vessels pass to the pretracheal and paratracheal lymph
Fig. 54.7). nodes.


Pleura, lungs, trachea and bronchi
968
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Bronchi the visceral pleura. Efferent vagal preganglionic axons synapse on small
ganglia within the walls of the tracheobronchial tree, likely acting as
sites of integration and/or modulation of the input from extrinsic
Bronchial arteries
nerves, or permitting local control of aspects of airway function by local
The bronchial arteries supply oxygenated blood and nutrition to
reflex mechanisms.
the bronchi and connective tissue of the lungs. Each bronchial artery
also has a branch that supplies the oesophagus. They are derived
from the descending thoracic aorta either directly or indirectly, travel MICROSTRUCTURE OF THE CONDUCTING AIRWAYS
and branch with the bronchi, and terminate around the level of the
respiratory bronchioles. They anastomose with the branches of the
The conducting airways are lined internally by a mucosa (see Fig. 2.16),
pulmonary arteries, and together supply the visceral pleura of
and the epithelium lies on a thin connective tissue lamina propria.
the lung. Much of the blood supplied by the bronchial arteries is
External to this is a submucosa, also composed of connective tissue, in
returned via the pulmonary veins rather than the bronchial veins
which are embedded airway smooth muscle, glands, cartilage plates
(see below).
(depending on the level in the respiratory tree), vessels, lymphoid tissue
The single right bronchial artery has a variable origin, usually arising
and nerves. Cartilage is present from the trachea to the smallest bronchi,
from the thoracic aorta as a common trunk with the right third posterior
but is absent (by definition) from bronchioles.
intercostal artery, sometimes from the superior left bronchial artery or
any number of the right intercostal arteries, mostly the third right pos-
Cartilaginous support
terior. There are normally two left bronchial arteries (superior and
inferior) that branch separately from the thoracic aorta.
The bronchial arteries accompany the bronchial tree and supply The trachea and extrapulmonary bronchi contain a framework of
bronchial glands, the walls of the bronchi and larger pulmonary vessels. incomplete rings of hyaline cartilage, which are united by fibrous tissue


The bronchial branches form a capillary plexus in the muscular tunic and smooth muscle (see Fig. 54.16). Intrapulmonary bronchi contain
of the air passages that supports a second mucosal plexus, which com- discontinuous plates or islands of hyaline cartilage in their walls.
municates with branches of the pulmonary artery and drains into the
pulmonary veins. Other arterial branches ramify in interlobular loose Tracheal cartilages
connective tissue and most end in either deep or superficial bronchial Each of the 16–20 tracheal cartilages measures about 4 mm in height
veins; some also ramify on the surface of the lung, forming subpleural and 1 mm in thickness, appearing thicker in the middle part; their
capillary plexuses. The bronchopulmonary arterial anastomoses in the external surface is flat while the internal aspect is slightly convex. The
walls of the smaller bronchi and in the visceral pleura may be more cartilaginous ‘rings’ encompass the anterior two-thirds of the trachea,
numerous in the neonate and are subsequently obliterated to a marked the ring being completed in the posterior third by fibroelastic and
degree. muscular tissue. Cartilage loses elasticity and calcifies in older age, and
In addition to the three main bronchial arteries, smaller bronchial adjacent cartilages may variously fuse. The first and the last two carti-
branches arise from the descending thoracic aorta; some may course lages are more conspicuous, the first being the thickest (see Fig. 54.7).
through the pulmonary ligament and bleed during inferior lobectomy. The last tracheal ring merges into the incomplete rings at the origin of
The bronchial arteries are typically enlarged and tortuous in chronic each principal bronchus; the bifurcation is marked by a cartilaginous
pulmonary thromboembolic hypertension. They are not usually recon- spur, the carina (see Fig. 54.11).
nected following lung transplantation. Aneurysms of a bronchial artery
may mimic aortic aneurysms. Bronchial cartilages
Some degree of mixing of blood between bronchial and pulmonary Cartilaginous plates mainly surround the dorsal aspect of the lobar


circulations and through Thebesian veins (venae cordis minimae) bronchi. They become more irregular and form progressively less of the
draining into the left ventricle normally occurs. A small degree of shunt, bronchial walls in intrapulmonary bronchi, and disappear in the
i.e. perfusion without ventilation, appears as normal and is usually 2– bronchioles.
4%. Cardiac malformations with right-to-left flow, pulmonary arterio-
venous malformations, congenital diaphragmatic herniae or pulmonary
embolism cause more important shunting, which is refractory to PULMONARY DEFENSIVE MECHANISMS
administration of pure oxygen.
The respiratory tract presents a huge surface area that is vulnerable to
Bronchial veins
desiccation, freezing, microbial invasion and the mechanical and
Usually two on each side, the bronchial veins drain blood from larger chemical effects of inhaled particles. Inhaled air is humidified chiefly
bronchi and hilar structures. They form two distinct systems. Deep in the upper respiratory tract, where it passes, with some turbulence,
bronchial veins commence as intrapulmonary bronchiolar plexuses over the nasal and buccopharyngeal mucosae. The secretions of
that communicate freely with the pulmonary veins and eventually the various glands of the bronchial tree also help to prevent desicca-
join a single trunk that ends in a main pulmonary vein or in the left tion. Goblet cells secrete sulphated acid mucosubstances; cells in
atrium. Superficial bronchial veins drain extrapulmonary bronchi, vis- mucous glands beneath the epithelial surface contain mainly carboxy-
ceral pleura and the hilar lymph nodes. They also communicate with lated mucosubstances, particularly those associated with sialic acid,
the pulmonary veins and end in the azygos vein on the right, and in although sulphated groups also occur; cells of serous glands contain
the left superior intercostal or the accessory hemiazygos vein on the neutral mucosubstances. Goblet cells respond mainly to local irrita-
left. Bronchial veins do not receive all the blood conveyed by bron- tion, while tubular glands, both mucous and serous, are mainly under
chial arteries; some enters the pulmonary veins. The main bronchial neural and hormonal control. Excessive or altered secretions may


arteries and veins run on the dorsal aspect of the extrapulmonary obstruct the flow of air. Antibacterial and antiviral substances, e.g. lys-
bronchi. ozyme, IgA antibodies and possibly interferon, also appear in the
secreted fluid.
Lymphatic drainage
Inhaled particles may be removed via the mucociliary rejection
The deep lymphatic plexus reaches the hilum by travelling along the current. Cilia sweep the fluid overlying the surfaces of bronchioles,
pulmonary vessels and bronchi. In larger bronchi, the deep plexus has bronchi and trachea upwards at 1 cm/min, and much of the inhaled
submucosal and peribronchial parts, but in smaller bronchi there is matter trapped in the viscous fluid may be removed in this way. Particles
only a single plexus that extends to the bronchioles. The walls of the small enough to reach the alveoli may be removed by alveolar phago-
alveoli have no lymphatic vessels. cytes. Alveolar epithelium has limited powers of regeneration but nor-
mally is continually replaced. The lifespan of alveolar squamous cells
is approximately 3 weeks, and that of alveolar phagocytes is 4 days.
INNERVATION Inhaled particles may also be cleared by the cough reflex.
Numerous lymphoid nodules occur in the bronchial lining, provid-
The anterior and posterior pulmonary plexuses innervate the trachea ing foci for the production of lymphocytes and giving local immuno-
and bronchi. The two plexuses are interconnected. The trachea is inner- logical protection against infection both by cell-mediated (T-cell)
vated by branches of the vagi, recurrent laryngeal nerves and sympa- activities, and by the production of immunoglobulins (mainly IgA)
thetic trunks; the nerves enter the lung as networks that travel along from B cells, which are passed on to gland cells for secretion to the
branches of the bronchi and pulmonary and bronchial vessels as far as epithelial surface.


Pleura, lungs, trachea and bronchi
968.e1
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The predominant contractile innervation of airway smooth muscle Brush cells
is parasympathetic and cholinergic in nature, while the primary relaxant
Brush cells are slender, non-ciliated cells with characteristically long,
innervation of the airways is non-cholinergic (parasympathetic nerves
stiff apical microvilli, from which they derive their name. Although
containing nitric oxide synthase and vasoactive intestinal peptide).
infrequent, they are present throughout all parts of the conducting
These parasympathetic nerves are anatomically and physiologically
airway passages, including the respiratory epithelium of the nasophar-
distinct from one another and differentially regulated by reflexes.
ynx. They are in contact with afferent nerve fibres basally and are pre-
Sympathetic-adrenergic nerves play little, if any, role in directly regulat-
sumed to have a sensory receptor function.
ing smooth muscle tone in the human airways. Activation of airway
afferent nerves (rapidly adapting receptors, C fibres) can evoke increases Neuroendocrine cells
in airway smooth muscle parasympathetic nerve activity, or decreases Neuroendocrine cells, also termed dense-core or small granule (Kulchit-
in parasympathetic nerve activity (via activation of slowly adapting sky) cells, are found mainly in the basal part of the epithelium. They
receptors). Extrapulmonary afferents may modulate neurally mediated have a rounded shape and the cytoplasm basal to their nuclei contains
regulation of airway smooth muscle tone (Canning 2001a). numerous small, dense-cored vesicles, approximately 150 nm in diam-
Stimulation of cholinergic nerves causes not only bronchoconstric- eter. They form part of the dispersed neuroendocrine system of amine
tion but also mucus secretion and bronchial vasodilation. Sympathetic precursor uptake and decarboxylation (APUD) cells. Neuroendocrine
nerves may control tracheobronchial blood vessels. cells are most numerous in fetal lungs and their number decreases
dramatically after birth. There appears to be little further change in their
Epithelium frequency during adult life, although they may proliferate in certain
pulmonary diseases.


The epithelia of the trachea, bronchi and bronchioles are, in general, Lymphocytes
similar to each other, with graded variations in the numbers of different
T lymphocytes, derived from mucosa-associated lymphoid tissue
cell types. The extrapulmonary and larger intrapulmonary passages are
(MALT) in the walls of the passages, occur within the epithelium
lined with respiratory epithelium (see Fig. 2.2D), which is pseudostrati-
throughout the conducting airway tissues, particularly in the extrapul-
fied and predominantly ciliated, and contains interspersed mucus-
monary portion. They are concerned with the immune surveillance of
secreting goblet cells. There are fewer cilia in terminal and respiratory
the epithelium. Clusters of lymphocytes sometimes lie beneath non-
bronchioles, and the cells are reduced in height to low columnar or
ciliated epithelial cells of the microfold (M-cell) type.
cuboidal. The epithelium of smaller bronchi and bronchioles is folded
into conspicuous longitudinal ridges, which allow for changes in Mast cells
luminal diameter (see Fig. 54.11). The epithelium in the respiratory
Mast cells are present within the basal regions of the epithelium. They
bronchioles progressively reduces in height towards the alveoli, and is
resemble connective tissue mast cells, and their cytoplasmic histamine-
eventually composed of cuboidal, non-ciliated cells. Respiratory bron-
containing granules are released in response to irritants, including
chioles have lateral pouches in their walls, which are lined with squa-
inhaled allergens.
mous cells, so providing an accessory respiratory surface.
Six distinct types of cell have been described in the conducting
Submucosal glands
airways, namely: ciliated columnar, goblet, Clara, basal, brush and neu-
roendocrine (see Fig. 54.11). Lymphocytes and mast cells migrate into
the epithelium from the underlying connective tissue. Tubuloacinar, seromucous glands are present in the submucosa of the
trachea and bronchi and, to a lesser extent, in the larger bronchioles.
Ciliated columnar cells They contain separate mucous and serous cells and are an important


Ciliated columnar cells are the driving force of the mucociliary rejection source of the mucus at the surface of the ciliated respiratory epithelium.
current (escalator) in the bronchial tree. They vary from low to tall Their secretions include mucins; bacteriostatic substances such as lyso-
columnar, and each has up to 300 cilia (Ch. 1, see Fig. 1.17) projecting zyme and lactoferrin; secretory antibodies (immunoglobulin A (IgA))
from the apical surface. The cilia extend into a watery fluid secreted by produced by plasma cells in the submucosal connective tissue; and
serous cells of the submucosal glands, but their tips are in contact with protease inhibitors (e.g. α-antitrypsin), important for neutralizing
1
a more superficial layer of thicker mucus secreted by surface goblet cells leukocyte-derived proteases in the respiratory tract, particularly elastase.
and mucous cells in the submucosal glands. The rate of ciliary beating Deficiency of α-antitrypsin causes chronic obstructive pulmonary
1
is usually 12–16 per second; mechanical stimulation of the epithelial disease, by inducing panacinar emphysema and bronchiectasis. The
surface and inflammatory mediators increase the rate. In addition to secretory acini and tubules are surrounded by myoepithelial cells,
tight junctions, which seal the apical intercellular space from the airway which are innervated by autonomic fibres (see above).
lumen, the ciliated cells are coupled by gap junctions, which allow
a change in rate of beating to spread from stimulated cells to their Connective tissue and muscle
neighbours (probably via calcium signalling) so that their metachronal
coordination remains intact. Primary ciliary dyskinesia is a ciliary Broad, longitudinal bands of elastin within the submucosa (Fig. 54.15)
abnormality inherited as an autosomal recessive disorder and may be follow the course of the respiratory tree and connect with the elastin
a feature of Kartagener’s syndrome, which is a combination of bron- networks of the interalveolar septa (see Fig. 54.11). This elastic frame-


chiectasis, sinusitis and situs inversus. work is a vital mechanical element of the lung and is responsible for
Goblet cells elastic recoil during expiration.
In the trachea and extrapulmonary bronchi, the smooth muscle is
Goblet cells are present from the trachea (6000–7000 per mm2) distal
mainly confined to the posterior, non-cartilaginous part of the tracheal
to the smaller bronchi, but are normally absent from bronchioles. They
tube (Fig. 54.16). Along the entire intrapulmonary bronchial tree,
contain an apical region full of large secretory vacuoles filled with
smooth muscle forms two opposed helical tracts, which become thinner
mucinogen. When the epithelium is irritated, e.g. by tobacco smoke,
and finally disappear at the level of the alveoli. The tone of these muscle
there is an overall increase in the number of goblet cells, and they also
fibres is under nervous and hormonal control; groups of muscle cells
extend into the bronchioles.
are coupled by gap junctions to spread excitation within fascicles.
Clara cells Muscle cell contraction narrows the airway, while relaxation permits
bronchodilation. Some tone normally exists in the muscular bands,
Clara cells are cuboidal, non-ciliated cells with apices that bulge into
which relax slightly during inspiration and contract during expiration,
the lumen. They contain numerous electron-dense secretory granules
thereby assisting the tidal flow of air. Abnormal contraction may be
and many lysosomes. Clara cells produce surfactant lipoprotein and
caused by circulating smooth muscle stimulants or by local release of
thus share functional similarities with the type II alveolar cell of pul-
excitants such as 5-HT (5-hydroxytryptamine, serotonin), histamine
monary alveoli, although their secretory granules differ in structure and
and leukotrienes, which produces bronchospasm. Numerous mast cells
composition. They may also regulate ion transport.
are present in the connective tissue of the respiratory tree, especially
Basal cells towards the bronchioles.
Basal cells are small, rounded cells that occur in parts of the airway
lined by pseudostratified respiratory epithelium; they are mitotic stem
cells for other epithelial cell types. Basal cells contact the basal lamina


and are most frequent in the larger conducting passages.


Pleura, lungs, trachea and bronchi
968.e2
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Fig. 54.15 A large bronchiole stained for elastin (black), showing smooth
muscle in the bronchiolar wall and a few small seromucous glands
(arrows).
M
S
C
Fig. 54.16 A micrograph of the trachea of a child, sectioned transversely.
An incomplete cartilage ring (C) is joined posteriorly by muscular tissue
(M). Seromucous submucosal glands (S) are concentrated in this region
and between adjacent cartilages. Note the thyroid tissue (bottom). (With
permission from Stevens A, Lowe JS 1996 Human Histology, 2nd ed.
London: Mosby.)


trachea and bronchi
969
45
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Anatomy of coughing
Coughing is an important defensive reflex. It enhances clearance
of secretions and particulates from the airways and protects from
aspiration of foreign materials (occurring as a consequence of aspira-
tion or inhalation of particulate matter, pathogens, accumulated
secretions, postnasal drip, inflammation and mediators associated with
inflammation).
Under normal conditions, cough serves an important protective role
in the airways and lungs, but in some conditions it may become exces-
sive, non-productive and potentially harmful to the airway mucosa.
The mechanical events of a cough can be divided into three phases. A B
An initial deep inhalation generates the volume necessary for an effec-
tive cough and is followed by a compression phase, when forceful Fig. 54.17 A virtual CT bronchoscopic view obtained by planar
contraction of the muscles of the chest wall, diaphragm and abdominal reconstruction of spiral CT. A, Left principal bronchus. B, Carina.
wall against a closed glottis results in a rapid rise in intrathoracic pres- (Courtesy of GE Medical Systems.)
sure. Subsequent glottal opening causes a rapid peak expiratory airflow
that dislodges and expels mucus from the airways; forceful airway com-
pression produces the coughing sound.
Bonus e-book images
BRONCHOSCOPY
Fig. 54.9 A lung window axial view showing an intralobar
Bronchoscopy allows the direct visualization of the vocal cords, trachea
sequestration in the left lower lobe of the lung in a 15-month-old
and major airways as far as the first division of the subsegmental airway. girl.
Occasionally, bronchoscopy can also provide some information about
structures adjacent to the airways, e.g. significant subcarinal lymphaden- Fig. 54.10 The systemic arterial blood supply from the abdominal
opathy may cause splaying or widening of the carina. Bronchoscopy aorta to an intralobar sequestration in the left lower lobe of the lung
enables the acquisition of samples (e.g. bronchial lavage, bronchial in a 15-month-old girl.
brushings, bronchial and transbronchial biopsies); provides informa-


tion about staging in lung cancer; and facilitates therapeutic procedures Fig. 54.15 A large bronchiole stained for elastin, showing smooth
(e.g. foreign body removal, ablation of tumours and insertion of airway muscle in the bronchiolar wall and a few small seromucous glands.
stents). The information obtained by volume CT scanning can be recon-
structed to provide three-dimensional images that can be used to create Fig. 54.16 A micrograph of the trachea of a child, sectioned
endobronchial views (virtual bronchoscopy, Fig. 54.17) or to demon- transversely.
strate the pulmonary arteries.


Pleura, lungs, trachea and bronchi
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Coughing is initiated when mechanically and chemically sensitive expiratory stimulation (sometimes termed the expiratory reflex), pro-
vagal afferents innervating the airways from the upper respiratory tract tecting the airway from aspiration; stimulation distal to the larynx
to the terminal bronchioles and lung parenchyma are activated (Morice causes a more prominent inspiratory phase to generate the airflow
and Gepetti 2004). Data from experimental studies suggest that the necessary to remove the stimulus.
receptors innervated by these afferents may be broadly divided into During vigorous coughing, intrathoracic pressures may reach
three groups: namely, slowly (SAR) and rapidly (RAR) adapting stretch 300 mmHg (with an abrupt rise in pleural pressure from 6.5 to 13 kPa)
receptors and bronchopulmonary unmyelinated C fibres. The vagal and expiratory velocities approach 500 miles per hour (Ford et al
afferents terminate centrally in largely non-overlapping regions of the 2007). While these pressures and velocities are responsible for the
caudal half of the nucleus tractus solitarius. Second-order neurones beneficial effects of cough on mucus clearance, they are also the cause
from this nucleus terminate on respiratory-related regions of the pons, of many of the complications of cough, including exhaustion, self-
medulla and spinal cord. Efferent impulses travel via the vagus, phrenic consciousness, insomnia, headache, dizziness, musculoskeletal pain,
and spinal motor nerves to the inspiratory and expiratory muscles, hoarseness, excessive perspiration, urinary incontinence and cough-
larynx, diaphragm and abdominal wall muscles. Airway receptors in the induced multiple rib fractures.
external auditory canals, eardrums, paranasal sinuses, pharynx, dia- Lesions that compress the upper airway, including arteriovenous
phragm, pleura, pericardium and stomach are probably mechanorecep- malformations and retrotracheal masses, may present with chronic
tors, stimulated by triggers such as touch or displacement (Polverino cough. Cough can also be a symptom of tracheobronchomalacia, which
et al 2012). results from the loss of rigid support of the large airways and inspiratory


collapse, usually seen in conjunction with obstructive lung disease in
Rapidly adapting receptors (RAR) RARs are dynamic receptors patients with a history of cigarette smoking.
that respond to changes in airway mechanical properties (e.g. diameter, Chronic dry cough may be caused by irritation of the external audi-
length and interstitial pressures). Functional studies of RARs suggest tory canal by impacted foreign bodies or cerumen (Jegoux et al 2002).
that they terminate within or beneath the epithelium mainly of the The aetiology of the ‘ear-cough’ (otorespiratory reflex) is related to
intrapulmonary airway (Widdicombe 2003). They may be differenti- stimulation of the auricular branch of the vagus nerve (Arnold’s nerve)
ated from other airway afferents by their rapid adaptation (in 1–2 (Feldman and Woodworth 1993). Holmes–Adie syndrome is a rare
seconds) to sustained lung inflation (Schelegle and Green 2001). RARs cause of coughing when autonomic dysfunction affects the vagus nerve.
are sporadically active throughout the respiratory cycle, activated by the Patients present with anisocoria, abnormal deep tendon reflexes and
dynamic mechanical forces that accompany lung inflation and defla- patchy areas of hyperhidrosis or anhidrosis (Kimber et al 1998).
tion, and become more active as the rate and volume of lung inflation Differences among several sites from which cough stimuli can origi-
increase. Their activation initiates reflex bronchospasm and mucus nate may result in variations in the sounds and patterns of coughing.
secretion through parasympathetic pathways. Laryngeal stimulation produces a choking type of cough without a
preceding inspiration. Inadequate mucociliary clearance mechanisms
Slowly adapting stretch receptors (SARs) SARs are highly (as in bronchiectasis or cystic fibrosis) may produce a pattern of cough-
sensitive to the mechanical forces that act on the lung during breathing. ing with less violent acceleration of air and a sequence of interrupted
Their activity increases during inspiration and peaks just prior to the expirations without any intervening inspiration.
initiation of expiration. They are thus thought to be the afferent fibres Since cough is an important defensive reflex required to maintain


involved in the Hering–Breuer reflex, which terminates inspiration and the health of the lungs, people who do not cough effectively are at risk
initiates expiration when the lungs are adequately inflated. SARs appear of atelectasis, recurrent pneumonia and chronic airways disease from
to terminate primarily in the intrapulmonary airways. SAR activation aspiration and retention of secretions. Many disorders can impair the
results in the central inhibition of respiration and the inhibition of the ability to cough effectively, which may result in persistent cough. The
cholinergic drive to the airways, leading to decreased phrenic nerve elderly, the newborn, lung transplant recipients and patients with para-
activity and decreased airway smooth muscle tone (Canning et al lysis or neuromuscular disorders have a poorly developed and/or com-
2001b); most of these SARs are associated with alveoli or bronchioles. promised cough reflex, and are rendered highly susceptible to infective
and aspiration pneumonia (Polverino et al 2012). Patients with paraly-
C fibres The majority of afferent nerves innervating the airways and sis or neuromuscular disorders (including rib fractures) and chest wall
lungs are unmyelinated C fibres, similar to the unmyelinated somatic deformities may not generate the flows necessary for effective clearance
sensory nerves that innervate the skin, skeletal muscle, joints and bones of secretions due to defective ‘pump’ mechanisms. Individuals with
and that respond to noxious chemical and mechanical stimuli. In addi- reduced function of the abdominal wall musculature are particularly at
tion to their slow conduction velocity (<2 m/s), airway vagal afferent C risk of ineffective cough. Patients with tracheobronchomalacia (‘floppy’
fibres are distinguished from RARs and SARs by their relative insensitiv- airways) or with obstructive airways diseases often do not generate the
ity to mechanical stimulation and lung inflation. They are generally high flow rates needed for effective clearance of secretions. Individuals
quiescent throughout the respiratory cycle (Lee and Pisarri 2001). with laryngeal disorders, including those with tracheostomies, may not
The specific pattern of the cough depends on the site and type of achieve sufficient laryngeal closure to generate the increased intrathor-


stimulation. Mechanical laryngeal stimulation results in immediate acic pressures necessary for an effective cough.


Pleura, lungs, trachea and bronchi
969.e2
7
noitces
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Asgharian B, Menache MG, Miller FJ 2004 Modeling age-related particle Lee LY, Pisarri TE 2001 Afferent properties and reflex functions of broncho-
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Belvisi MG 2002 Overview of the innervation of the lung. Curr Opin Phar- Morice AH, Geppetti P 2004 Cough. 5: The type 1 vanilloid receptor: a
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Canning BJ, Reynolds SM, Mazzone SB 2001b Multiple mechanisms of reflex E132–5.
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Am 41:453.


CHAPTER
55
Diaphragm and phrenic nerves
The diaphragm is a domed musculofibrous sheet that separates the side, the costal part arises from the internal surfaces of the lower six
thoracic and abdominopelvic cavities (Fig. 55.1). Its mainly convex costal cartilages and their adjoining ribs, interdigitating with transversus
superior surface faces the thorax, and its concave inferior surface is abdominis (see Fig. 53.15).
directed towards the abdomen. The positions of the domes or cupulae The lumbar part arises from two aponeurotic arches – the medial
of the diaphragm are extremely variable; they are influenced by body and lateral arcuate ligaments (sometimes termed lumbocostal arches)
build (often higher in the short and obese compared to the tall and – and asymmetrically from the upper lumbar vertebral bodies via two
asthenic), position and ventilatory phase. Conditions resulting in lung pillars or crura. The lateral arcuate ligament covers quadratus lumbo-
over-inflation, e.g. emphysema, cause marked diaphragmatic depres- rum, arching across the upper part of that muscle, and attaching medi-
sion. Usually, after forced expiration, the right cupula is level anteriorly ally to the anterior aspect of the transverse process of the first lumbar
with the fourth costal cartilage and the right nipple, whereas the left vertebra, and laterally to the inferior margin of the midpoint of the
cupula lies approximately one rib lower. On full inspiration, the cupula twelfth rib. The medial arcuate ligament covers the upper part of psoas
will descend by as much as 10 cm, and on a plain chest radiograph the major. Medially, it is continuous with the lateral tendinous margin of
right dome coincides with the anterior end of the sixth rib. The dia- the corresponding crus, and is thus attached to the side of the body of
phragm lies more superiorly in the supine compared to the erect posi- the first or second lumbar vertebra. Laterally, it is fixed to the anterior
tion, and the dependent half of the diaphragm will be considerably aspect of the transverse process of the first lumbar vertebra.
higher than the uppermost one in the decubitus position. The crura are tendinous at their attachments and blend with the


anterior longitudinal ligament. The broader and longer right crus arises
from the anterolateral surfaces of the bodies and intervertebral discs of
ATTACHMENTS AND COMPONENTS
the upper three lumbar vertebrae. The left crus arises from the corre-
sponding parts of the upper two lumbar vertebrae. The medial tendi-
The muscle fibres of the diaphragm arise from the highly oblique cir- nous margins of the crura meet in the midline to form an often poorly
cumference of the thoracic outlet; the attachments are low posterolater- defined arch, the median arcuate ligament, which crosses anterior to
ally and high anteriorly (Fig. 55.2). Although it is a continuous sheet, the aorta at the level of the intervertebral T12/L1 disc. The crura vary
the muscle can be considered in three parts: sternal, costal and lumbar. greatly in length, thickness and ratio of muscle to tendon (Loukas et al
The sternal part arises by two fleshy slips from the posterior aspect 2015a).
of the xiphoid process. The attachment varies; it may be a broad, con- From these circumferential attachments, the muscle fibres of the
tinuous band but is not always present (Loukas et al 2015a). On each diaphragm converge into a central tendon. Fibres from the xiphoid
Xiphoid process
Inferior phrenic veins Internal thoracic artery and vein
Central tendon
Oesophageal hiatus; oesophagus,
anterior and posterior vagal trunks
Inferior vena cava opening
Left phrenic nerve,
phrenico-abdominal branch
Right phrenic nerve,
phrenico-abdominal branch
Inferior phrenic artery
Greater splanchnic nerve
Median arcuate ligament
Hemiazygos vein
Right crus
Lesser splanchnic nerve
Lateral arcuate ligament
Aortic hiatus; abdominal aorta;
Twelfth rib thoracic duct
Quadratus lumborum
Transverse process, first lumbar vertebra
Psoas major
Medial arcuate ligament
Sympathetic trunk
Azygos vein
Fig. 55.1 The abdominal aspect of the diaphragm. (With permission from Waschke J, Paulsen F (eds), Sobotta Atlas of Human Anatomy, 15th ed,
970 Elsevier, Urban & Fischer. Copyright 2013.)


Apertures
971
55
RETPAHC
Fig. 55.2 A coronal CT
scan depicting the
diaphragmatic domes
(arrows) superior to the
liver and spleen. Notice
the attachment of the
diaphragm at the
lumbar vertebrae.
(Courtesy of Professor
Loukas.)
A
process are short, run almost horizontally and are occasionally aponeu-
rotic. Fibres originating from the medial and lateral arcuate ligaments,
and particularly the osteochondral surfaces of the ribs, are much longer.
They arise almost vertically at first and then curve towards their central
attachment. Fibres from the crura diverge, the most lateral becoming
even more lateral as they ascend to the central tendon. Medial fibres of B
the right crus embrace the oesophagus where it passes through the
Fig. 55.3 Two diaphragms removed from the thoracic cavity to show the
diaphragm, the more superficial fibres ascend on the left, and deeper central tendon and muscular components. Note the different proportions
fibres cover the right margin. Sometimes, a fleshy fasciculus from of the central tendon and the muscular portions between A and B. (With
the medial side of the left crus crosses the aorta and runs obliquely permission from Loukas M, du Plessis M, Shah S, Tubbs RS, Raffi E,
through the fibres of the right crus towards the caval opening; this Polo L, Shoja MM. The clinical anatomy of the musculotendinous part of
fasciculus does not continue upwards around the oesophageal hiatus the diaphragm. Surgical and Radiologic Anatomy, in press, 2015a.)
on the right side.
The central tendon of the diaphragm is a thin but strong aponeurosis
of closely interwoven fibres situated near the centre of the muscle, but
closer to the anterior wall of the thorax, so that the posterior muscular specified in clinical descriptions. The right hemidiaphragm is found at
fibres are longer (Fig. 55.3). In the centre, it lies immediately inferior the anterior end of the sixth rib on a properly inspired posteroanterior


to the pericardium, with which it is partially blended. It is trifoliate in chest radiograph, the left hemidiaphragm 1.5–2.5 cm lower (see Fig.
shape. The middle folium, which projects anteriorly, has the form of an 56.16). Unilateral paralysis may be seen as a raised hemidiaphragm on
equilateral triangle with the apex directed towards the xiphoid process. a chest radiograph, but this sign should not be relied on uncritically in
The right and left folia are tongue-shaped and curve posterolaterally, clinical practice.
the left being a little narrower.
The central area of the tendon consists of four well-marked diagonal Ultrasound imaging The posterolateral aspects of the diaphragm
bands that fan out from a thick central node where compressed tendi- may be visualized using ultrasound. It is typically identified by its
nous strands decussate anterior to the oesophagus and to the left of the curved morphology, deep location and specific echotexture. It can be
inferior vena cava. Although always present, their morphology varies; visualized as a thin, muscular hypoechoic line wedged between two
in some individuals, the right leaflet of the central tendon is smaller hyperechoic layers consisting of peritoneum and pleura. During inspira-
than the left or the anterior leaflets. The ratio between muscle and tion, the hypoechoic line thickens as the muscle contracts, making it
tendon varies disproportionately, such that larger diaphragms may have more visible. This change can be used diagnostically to identify an
a small central tendon, while smaller diaphragms may have only a small atrophic diaphragm, when the size of the hypoechoic line does not
amount of muscle contributing to their overall size (Loukas et al change. Ultrasound investigation of the diaphragm is best done with
2015a). the patient in the supine position, limiting veiling by other organs, as
well as the risk of misdiagnosis attributable to underlying pulmonary
pathologies. The left side poses more of a challenge in visualization
PROFILE AND RELATIONS
(Sarwal et al 2013).
The superior surface of the diaphragm is related to three serous mem-
APERTURES
branes. On each side, the pleura separates it from the base of the cor-


responding lung, and the pericardium is interposed between the middle
folium of the central tendon and the heart. The middle folium, the A number of structures pass between the thorax and abdomen via
‘cardiac plateau’, is almost flat and extends more to the left than the apertures in the diaphragm. There are three large openings, for the
right. In anteroposterior view, the superior profile of the diaphragm aorta, oesophagus and inferior vena cava, and a number of smaller ones
rises on either side of the cardiac plateau to a smooth convex dome or (see Fig. 55.1).
cupula. The right cupula is higher and slightly broader than the left. The aortic hiatus is the most posteroinferior of the large openings,
Most of the inferior surface is covered by peritoneum. The right side is and is found at the level of the lower border of the twelfth thoracic
moulded over the convex surface of the right lobe of the liver, right vertebra and the adjacent intervertebral disc, slightly to the left of
kidney and suprarenal gland. The left side conforms to the left lobe of the midline. It is actually an osseo-aponeurotic opening defined by the
the liver, gastric fundus, spleen, left kidney and suprarenal gland. In diaphragmatic crura laterally, the vertebral column posteriorly and the
view of these differences in the profile and anatomical relationships of median arcuate ligament anteriorly; the aorta therefore passes posterior
the right and left sides of the diaphragm, the side should always be to the diaphragmatic musculature and is not affected by its contraction


DiAPHRAgm AnD PHREniC nERvEs
972
7
nOiTCEs
(Nason et al 2012). Occasionally, some tendinous fibres from the Congenital hernia Congenital diaphragmatic defects may cause
medial parts of the crura also pass posterior to the aorta, converting the abdominal organs, usually the stomach and/or the small bowel, colon,
osseo-aponeurotic opening into a fibrous ring. The aortic hiatus trans- liver and spleen, to herniate through the diaphragm into the thorax;
mits the aorta, thoracic duct, lymphatic trunks from the lower posterior they are described on page 907.
thoracic wall and, sometimes, the azygos and hemiazygos veins.
The oesophageal hiatus is located at the level of the tenth thoracic Eventration Eventration is a focal bulge, usually in the anteromedial
vertebra, anterosuperior to, and a little to the left of, the aortic hiatus. segment of the muscular portion of the diaphragm, that is caused by
It transmits the oesophagus, vagal trunks and gastric nerves, oesopha- congenital thinning.
geal branches of the left gastric vessels and some lymphatic vessels. The
elliptical opening has a slightly oblique long axis, and is bounded by Diaphragmatic trauma
muscle fibres that originate from the medial part of the right crus and
cross the midline, forming a ‘chimney’ approximately 2.5 cm long, that
Available with the Gray’s Anatomy e-book
accommodates the terminal part of the oesophagus. The outermost
fibres run in a craniocaudal direction, and the innermost fibres are
arranged circumferentially. There is no direct continuity between the Median arcuate ligament (Dunbar’s) syndrome
oesophageal wall and the muscle around the oesophageal hiatus. The
fascia on the inferior surface of the diaphragm, rich in elastic fibres, is Available with the Gray’s Anatomy e-book
continuous with the transversalis fascia and extends superiorly into the
opening as a flattened cone to blend with the oesophageal wall 2–3 cm
superior to the gastro-oesophageal (squamocolumnar) junction. Some VASCULAR SUPPLY AND LYMPHATIC DRAINAGE


of its elastic fibres penetrate to the submucosa of the oesophagus. This
peri-oesophageal areolar tissue, the phreno-oesophageal ligament,
ARTERIES
connects the oesophagus flexibly to the diaphragm, thus permitting
freedom of movement during swallowing and ventilation while syn-
The profuse arterial supply of the diaphragm is derived from anasto-
chronously limiting superior oesophageal displacement.
moses between the lower five intercostal and subcostal arteries, the
The caval opening, the most superior of the three large openings,
superior and inferior phrenic arteries, and the pericardiacophrenic and
lies at about the level of the intervertebral disc between the eighth and
musculophrenic arteries.
ninth thoracic vertebrae. It is quadrilateral, with aponeurotic margins,
located at the junction of the right leaf with the central area of the
Intercostal and subcostal arteries
tendon. It is traversed by the inferior vena cava, which adheres to the
margin of the opening, and by some branches of the right phrenic The lower five intercostal and subcostal arteries supply the costal
nerve. During inspiration, dilation of the caval hiatus, combined with margins of the diaphragm (Ch. 53).
increased intra-abdominal pressure, increases cardiac venous return
Inferior phrenic arteries
(Nason et al 2012).
There are two lesser apertures within each crus; one transmits the The major arterial supply to the diaphragm is derived from the right
greater, and the other, the lesser, splanchnic nerve. The ganglionated and left inferior phrenic arteries (Fig. 55.4). Most commonly, the right
sympathetic trunks usually enter the abdominal cavity posterior to the inferior phrenic artery arises from the coeliac trunk or separately from
diaphragm, deep to the medial arcuate ligaments. Openings for minute the aorta, less commonly it arises from the right renal artery, and occa-
veins frequently occur in the central tendon. sionally it arises from the left gastric artery or the hepatic artery proper.
On each side of the diaphragm, there are small areas where the The left inferior phrenic artery usually arises from the coeliac trunk or


muscle fibres are replaced by areolar tissue. One, between the sternal separately from the aorta; occasionally, it may arise from the left renal
and costal parts, contains the superior epigastric branch of the internal artery, left gastric artery or the hepatic artery proper (Loukas et al
thoracic artery and some lymph vessels from the abdominal wall and 2008).
convex surface of the liver. The other, between the costal part and the Each artery ascends anterolateral to the diaphragmatic crus, near
fibres that spring from the lateral arcuate ligament, is less constant; the medial border of the suprarenal gland. The left passes posterior to
when it is present, the posterosuperior surface of the kidney is separated the oesophagus and then runs anteriorly on the left side of its
from the pleura only by areolar tissue. Additional, smaller fascial spaces diaphragm atic opening. The right inferior phrenic artery passes poste-
may also be found throughout the lateral aspects of the diaphragmatic rior to the inferior vena cava, and then along the right side of its
domes without evidence of vascular penetration. Although these spaces opening. Each phrenic artery divides into medial and lateral branches
are found on both left and right sides, they appear to be more numer- near the posterior border of the central tendon. The medial branch
ous on the left (Loukas et al 2015a). curves anteriorly to anastomose with its contralateral fellow anterior to
the central tendon and with the musculophrenic and pericardiaco-
Oesophageal reflux Reflux of gastric contents into the oesophagus, phrenic arteries. The lateral branch approaches the thoracic wall and
with risk of pulmonary aspiration, is normally prevented by a physio- anastomoses with the inferior posterior intercostal and musculo-
logical antireflux barrier located at the gastro-oesophageal junction. The phrenic arteries. The lateral branch of the right inferior phrenic artery
major components of this barrier consist of the specialized smooth supplies the inferior vena cava. The left sends ascending branches to
muscle of the inferior oesophageal wall and the encircling crural fibres. the oesophagus.
The diaphragm acts as a ‘physiological’ sphincter to prevent gastro-
Superior phrenic arteries
oesophageal reflux by constricting during inspiration (Nason et al


2012) (see also Ch. 64). The superior phrenic arteries supply the superior portion of the dia-
phragm (Figs 55.5–55.6). The right superior phrenic artery most com-
Hiatus hernia The diaphragm lends additional power to all expulsive monly arises from either the thoracic aorta, the proximal (pre-intercostal
efforts: sneezing, coughing, laughing, crying, urination, defecation and space) segment of the tenth intercostal artery or its distal segment. The
uterine fetal expulsion are all preceded by a deep inspiration. Similarly, left superior phrenic artery most commonly arises from the thoracic
a deep inspiration followed by closure of the glottis is a common pre- aorta or the proximal segment of the tenth intercostal artery; occasion-
liminary to powerful recruitment of the trunk muscles in lifting heavy ally, it arises from the distal segment of the tenth intercostal artery
objects because the increased intra-abdominopelvic pressure pneumati- (Loukas et al 2005).
cally braces the vertebral column.
Repeated stress may eventually compromise the integrity of the
VEINS
hiatus with widening of the muscular hiatal tunnel. Concomitant laxity
of the phreno-oesophageal ligament allows the gastro-oesophageal
Phrenic veins
junction to migrate into the thorax; this is usually termed a sliding, or
type I, hiatus hernia. Sliding hernias are typically acquired, commonly
occur in the fifth decade of life, and are found in more than 50% of The superior surface of the diaphragm is drained by tributaries of the
patients with gastro-oesophageal reflux. The latter induces tonic con- musculophrenic and pericardiacophrenic veins that run alongside the
traction of the longitudinal oesophageal muscle and this further exac- corresponding arteries. The inferior surface of the diaphragm is drained
erbates the hiatus hernia. When the stomach herniates into the thorax by tributaries of the right and left inferior phrenic veins. The right infe-
adjacent to the oesophagus, it is termed a para-oesophageal, or type II, rior phrenic vein drains into the inferior vena cava – almost always
hiatus hernia. inferior, but occasionally superior, to the diaphragm – or to the right


Diaphragm and phrenic nerves
972.e1
55
RETPAHC
On imaging, it appears as a sharp transition between adjoining parts In up to a quarter of individuals, the median arcuate ligament lies
and may be mistaken for a diaphragmatic hernia. It is identified and significantly inferiorly, positioning itself anterior to the coeliac axis. This
becomes more pronounced with increased abdominal pressure and may cause compression and downward arching of the coeliac artery,
has been linked to obesity. Eventration is typically found on the left; leading to mesenteric angina and pancreaticoduodenal aneurysms as a
the affected part shows diminished or paradoxical movement during consequence of resulting collateral flow. Most patients will present with
fluoroscopic sniff testing while the posterior aspect of the ipsilateral postprandial pain, nausea and often vomiting, and many will avoid
dome acts normally (Nason et al 2012). eating, resulting in major weight loss. During expiration, concurrent
movement of the abdominal aorta and the median arcuate ligament
Closed or penetrating thoraco-abdominal injuries may result in rupture compresses the coeliac artery; the position, angle and level of the coeliac
or laceration of the diaphragm. With closed injuries and diaphrag- artery are also major aetiological factors. Diagnosis is based mainly on
matic rupture, there may be subsequent herniation of the abdominal exclusion of other causes, e.g. compression of the vessel as a result of
contents into the thorax. Spiral CT with planar reformation should be fibrotic or neoplastic changes in the sympathetic plexus and coeliac
the primary investigation (Shanmuganathan et al 2000). Magnetic ganglion, as well as atherosclerotic changes in the vessel itself. Defini-
resonance imaging (MRI) is usually performed when other imaging tive diagnosis can only be made with multiple imaging modalities.
modalities have produced equivocal findings. Early operative repair is
recommended because untreated cases may develop gastrointestinal
obstruction or perforation. Patients with penetrating injuries may
require additional assessment by thoracoscopy (Lowdermilk and
Naunheim 2000).
Anterior branch
of RIPA
LIPA
Lateral branch
of RIPA
Coeliac trunk
Medial branch Superior
of RIPA suprarenal artery
Superior suprarenal
artery Left kidney
SMA
Right kidney


Fig. 55.4 A posterior cadaveric dissection showing the origin and course of the right (RIPA) and left (LIPA) inferior phrenic arteries. The right inferior
phrenic artery arises from the superior mesenteric artery (SMA), and the left inferior phrenic artery arises from the coeliac trunk. (With permission from
Loukas M, Hullett J, Wagner T. Clinical anatomy of the inferior phrenic artery. Clin Anat. 2005;18(5):357–65.)
Aortic hiatus
Right crus
Left crus
Diaphragm
Left superior Right superior
phrenic artery phrenic artery
Inferior
Aorta Lateral Medial
Superior
Fig. 55.5 A cadaveric dissection showing the origin of the right and left superior phrenic arteries from the thoracic aorta and supplying portions of the
right and left diaphragmatic crura. (With permission. Loukas M, Louis RG Jr, Wartmann CT, Tubbs RS, Esmaeili E, Bagenholm AC, Merbs W, Curry B,
Jordan R. Superior phrenic artery: an anatomic study. Surg Radiol Anat. 2007;29(1):97–102.)


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Central tendon
Left superior
phrenic artery
Aorta
Endothoracic fascia
Oesophagus
Inferior Medial
Phrenic nerve
Lateral Superior
Fig. 55.6 A lateral view of the left superior phrenic artery arising from the aorta. (With permission. Loukas M, Louis RG Jr, Wartmann CT, Tubbs RS,
Esmaeili E, Bagenholm AC, Merbs W, Curry B, Jordan R. Superior phrenic artery: an anatomic study. Surg Radiol Anat. 2007;29(1):97-102.)


innervation
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RETPAHC
hepatic vein. The left inferior phrenic vein drains into the inferior Right phrenic nerve
vena cava – commonly inferior, or rarely superior, to the diaphragm. It
The right phrenic nerve is shorter and more vertical than the left. Sepa-
may also drain into the left suprarenal vein, the left renal vein, the
rated at the root of the neck from the second part of the right subclavian
left hepatic vein or, rarely, the left suprarenal renal vein and the infe-
artery by scalenus anterior, the nerve then descends lateral to the right
rior vena cava (Loukas et al 2005).
brachiocephalic vein, the superior vena cava and the fibrous pericar-
dium covering the right surface of the right atrium and inferior vena
cava, and divides just superior to or at the level of the diaphragm.
LYMPHATIC DRAINAGE
Left phrenic nerve
The diaphragm is drained by anterior, middle and posterior groups of At the root of the neck, the left phrenic nerve commonly leaves the
lymph nodes that lie on its superior surface; they also drain the superior medial edge of scalenus anterior and passes anterior to the first part of
portion of the liver, the gastro-oesophageal junction and the abdominal the left subclavian artery and posterior to the thoracic duct. However,
surface of the diaphragm. In addition, lymph drains anterosuperiorly sometimes the right and left nerves are symmetrical in their cervical
to parasternal and anterior mediastinal nodes, and posterosuperiorly to course, in which case the left phrenic nerve may cross anterior to the
posterior mediastinal and brachiocephalic nodes. second part of the left subclavian artery, from which it is separated by
scalenus anterior, at the level of the thoracic inlet. Thereafter, the left
Hepatocellular carcinoma phrenic nerve crosses anterior to the left internal thoracic artery and
descends across the medial aspect of the apex of the left lung and its
Available with the Gray’s Anatomy e-book pleura to the first part of the subclavian artery, which it crosses obliquely
to reach a groove between the left common carotid and subclavian


arteries. It passes anteromedially, superficial to the left vagus nerve just
INNERVATION superior to the aortic arch and posterior to the left brachiocephalic vein,
then descends superficial to the aortic arch and the left superior inter-
The diaphragm receives its motor supply via the phrenic nerves (Fig. costal vein, anterior to the left pulmonary hilum, to lie between the
55.7). Sensory fibres are distributed to the peripheral part of the muscle fibrous pericardium covering the surface of the left ventricle and the
by the lower six or seven intercostal nerves. The fibres of the right crus mediastinal pleura (see Fig. 57.3).
of the diaphragm divide to the right and left of the oesophagus and are
innervated by both right and left phrenic nerves. There is some evidence Diaphragmatic relationships The right phrenic nerve passes
that the crural fibres contract slightly before the costal part, and this through the central tendon of the diaphragm, either by the caval hiatus
may be functionally significant for non-respiratory tasks. or just lateral to it. The left phrenic nerve passes through the muscular
part of the diaphragm anterior to the central tendon, just lateral to the
Phrenic nerve left cardiac surface and more anterior than the right phrenic nerve (see
Fig. 55.7). At the diaphragm or slightly superior to it, each phrenic nerve
supplies fine branches to the parietal pleura above the central dia-
The phrenic is a mixed nerve that provides the sole motor supply to its
phragm, and the parietal peritoneum below. Typically, the trunk of each
hemidiaphragm. Derived mostly from the fourth cervical ramus, it also
nerve then divides into three branches as it passes through the dia-
receives contributions from the third and fifth cervical ventral rami (see
phragm. With some variation, these constitute an anterior (sternal)
Figs 29.17, 29.18). The course of the cervical part of the phrenic nerve
branch, which runs anteromedially towards the sternum and connects
is described on page 464. Within the thorax, the phrenic nerve descends
with its fellow; an anterolateral branch, which runs to the lateral leaf


anterior to the pulmonary hilum between the fibrous pericardium and
of the central tendon; and a short posterior branch. The latter divides
mediastinal pleura, accompanied by the pericardiacophrenic vessels. In
into a posterolateral ramus that courses posterior to the lateral leaf and
its thoracic course, each phrenic nerve supplies sensory branches to the
a posterior ramus that supplies the crural fibres. Posterolateral and
mediastinal pleura, fibrous pericardium and parietal serous pericar-
posterior crural rami may arise as separate branches from the phrenic
dium. The right and left phrenic nerves differ in their intrathoracic
nerve.
relationships (Rajanna 1947).
These main branches may be submerged in diaphragmatic muscle
or lie inferior to it. They supply motor fibres to the muscle and sensory
fibres to the peritoneum and pleura related to the central part of the
diaphragm. They also relay proprioceptive fibres from the musculature.
Scalenus anterior The right phrenic nerve supplies the part of the right crus that lies
to the right of the oesophagus, and the left phrenic nerve supplies
Phrenic nerve the left crus and the part of the right crus that lies on the left of the
oesophagus.
Right subclavian
artery and vein Phrenic ganglia The right phrenic nerve continues its course inferior
First rib
to the diaphragm and frequently connects to the right phrenic ganglion.
An additional, smaller, right accessory phrenic ganglion is sometimes
Superior vena cava
located proximally along the phrenic nerve, closer to its caval foraminal
exit. The right phrenic ganglion exhibits plexiform communications
with the coeliac and aorticorenal ganglia and with the right suprarenal
gland (Fig. 55.8). The left phrenic nerve usually continues its course
Pericardium inferior to the diaphragm. A left phrenic ganglion is less common than
Pericardial branch of
its right counterpart; when present, it exhibits plexiform communica-
phrenic nerve
Diaphragm tions to several autonomic ganglia. Both phrenic nerves and ganglia are


closely related to the diaphragmatic crura; sutures for crural approxima-
tion in repair of hiatal hernia must be placed superior to the ganglia in
order to avoid iatrogenic injuries to the autonomic supply of the dia-
phragm and abdomen (Loukas et al 2015b).
Phrenic nerve, Lesions of the phrenic nerve An appreciation of the location of
phrenico-abdominal the main branches of the phrenic nerves is important to avoid iatrogenic
branch surgical damage. Radial incisions in the diaphragm from the costal
margin to the oesophageal hiatus lead to diaphragmatic paralysis,
whereas thoraco-abdominal incisions in a circumferential manner in
the periphery of the diaphragm do not involve any significant branches
of the phrenic nerves and preserve diaphragmatic function. Similarly,
incisions of the central tendon are safe.
Fig. 55.7 The distribution of the right and left phrenic nerves. (With Division of the phrenic nerve in the neck completely paralyses the
permission from Waschke J, Paulsen F (eds), Sobotta Atlas of Human corresponding half of the diaphragm, which atrophies. If an accessory
Anatomy, 15th ed, Elsevier, Urban & Fischer. Copyright 2013.) phrenic nerve exists, section or crushing of the main nerve as it lies on


Diaphragm and phrenic nerves
973.e1
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RETPAHC
Successful control of hepatocellular carcinoma often requires chemo-
embolization of the hepatic artery, as well as any extrahepatic collater-
als; the right inferior phrenic artery is often found to be associated with
hepatocellular carcinoma as a major collateral pathway. Transcatheter
oily chemoembolization regresses the tumour with increased survival
rates. The inferior phrenic vein is one of the major sources of collateral
venous drainage in hepatocellular carcinoma (and in retroperitoneal
malignant disease). As well as conventional angiography, the use of 64+
slice computed tomography (CT) scanners enables clear visualization
of the right and left inferior phrenic arteries, making CT a reliable
method of diagnosing thrombosis and aneurysm of these vessels
(Loukas et al 2008).
Inferior phrenic artery
Coeliac ganglion
Crural branch of
phrenic nerve
Inferior vena cava
Phrenic nerve
Aorticorenal ganglion
Right phrenic ganglion
Medial
Suprarenal gland
Superior Inferior
Lateral
Fig. 55.8 A cadaveric dissection showing a right phrenic ganglion close to the right inferior phrenic artery and right diaphragmatic crus with plexiform
communications to the coeliac ganglion, aorticorenal ganglion and suprarenal gland (arrows). (With permission from Loukas M, Joseph S, Louis RG Jr.,
Wartmann CT, Tubbs RS. The surgical anatomy of the subdiaphragmatic phrenic nerve and phrenic ganglia. Clinical Anatomy, in press, 2015b.)


DiAPHRAgm AnD PHREniC nERvEs
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scalenus anterior will not produce complete paralysis. The phrenic
nerve may be involved in traumatic lesions of the upper brachial plexus. First rib
Historically, it was deliberately injured in order to collapse, and hence
rest, the lung in patients with pulmonary tuberculosis. Cardiac surgery
is one of the most common iatrogenic causes of phrenic nerve injury,
Left lung
especially as a result of the instillation of saline slush for myocardial Right lung
Pressure decreases
preservation. Surgery for tumours of the lung or mediastinum may also
cause damage, as well as infections such as poliomyelitis, obstetric
injury during traumatic childbirth, and radiation therapy (Nason et al
2012). Respiratory muscle weakness of rapid onset is a feature of
Guillain–Barré syndrome.
Phrenic nerve damage leads to paradoxical movement of the dia-
phragm that is best observed fluoroscopically, with the patient first in
Eighth rib
the upright position (diaphragm unloaded), and then supine with a Diaphragm
small weight on the abdomen (diaphragm loaded). Diaphragmatic Pressure increases
paralysis can also be assessed by ultrasound examination with a sniff
manœuvre. The ‘gold standard’ for assessment of the phrenic nerve/
diaphragm unit is electrical or magnetic stimulation of the phrenic
nerve with recording of the compound muscle action potential and/
or the transdiaphragmatic pressure. Electrical stimulation of the dia-
phragm, by ‘pacing’ of one or both phrenic nerves, has been used with
some success in infants with central alveolar hypoventilatory syndrome
(‘Ondine’s curse’) and in patients with high cervical lesions of the spinal Zone of apposition Zone of apposition
cord, in whom the diaphragm is paralysed but the lower motor neu-
rones are intact. Electrodes are placed adjacent to the nerves, sometimes
in the neck but more usually in the chest, and a ventilatory rhythm is
established by trains of stimuli delivered by an implanted device. As Fig. 55.9 Inspiratory movements: pressure changes during inspiration.


this is a non-physiological way of recruiting the muscle, the fibres must (Adapted from Drake RL, Vogl AW, Mitchell A (eds), Gray’s Anatomy for
be ‘conditioned’ during the initial period of stimulation, so that they Students, 2nd ed, Elsevier, Churchill Livingstone. Copyright 2010.)
acquire the necessary resistance to fatigue.
Referred pain Diaphragmatic pain is frequently felt at the tip of the than those of the left. The balance between descent of the diaphragm,
shoulder, reflecting common nerve root origins in the neck. It usually protrusion of the abdominal wall (‘abdominal breathing’), and eleva-
occurs when there is inflammation of the diaphragmatic pleura, e.g. in tion of the ribs (‘thoracic breathing’) varies in different individuals and
basal pneumonia, pleural effusions or malignant disease, or with irrita- with the phase and depth of ventilation, the degree of distension of the
tion of the peritoneal pleura, as with subphrenic abscess, where invol- stomach and intestines, and the size of the liver. The thoracic element
untary diaphragmatic contraction (hiccups) may also occur. is usually more marked in females, but increases in both sexes during
deep inspiration.
Accessory phrenic nerve Radiographs show that the height of the diaphragm within the
thorax also varies considerably with posture. It is highest when the body
is supine, when it performs the greatest ventilatory excursions with
The accessory phrenic nerve is described on page 464.
normal breathing. When the body is erect, the diaphragm is lower and
its ventilatory movements become smaller. The diaphragmatic profile
is still lower in the sitting posture and ventilatory excursions are small-
ANATOMY OF BREATHING
est under these conditions. When the body is horizontal and on one
side, the two halves of the diaphragm do not behave in the same way.
Breathing is a highly coordinated abdominal and thoracic process. The The uppermost half sinks to a lower level than that seen when sitting,
diaphragm is the major muscle of inspiration, responsible for approxi- and moves little with ventilation. The lower half rises higher in the
mately two-thirds of quiet breathing in healthy humans. The external thorax than it does even in the supine position, and its ventilatory


intercostal muscles are most active in inspiration, and the less strong excursions are considerably greater. Changes in the level of the dia-
internal intercostals in expiration. Increasing the vertical, transverse and phragm with alterations in posture explain why patients with severe
anteroposterior dimensions of the chest increases the volume of the dyspnoea are most comfortable, and least short of breath, when
pleural space, and the resulting decrease in intrapleural pressure draws sitting up.
air into the lungs. During expiration, the diaphragm relaxes and moves Diaphragmatic excursion is typically 1.5 cm in quiet breathing.
superiorly. Air is expelled from the lungs and the elastic recoil of the During deep ventilation, the maximum movement ranges from 6 to
lung creates a subatmospheric pressure that returns the lateral and 10 cm. After a forced inspiration, e.g. when breathing is partially
anteroposterior dimensions of the thorax to normal (De Troyer and obstructed, the right cupula of the diaphragm may descend to about
Estenne 1988, Celli 1998). During inspiration, the most inferior ribs the level of the eleventh thoracic vertebra, while the left cupula may
are fixed and contraction of the diaphragm draws the central tendon reach the level of the body of the twelfth thoracic vertebra. After forced
inferiorly, scarcely altering diaphragmatic curvature. The cupulae move expiration, the right cupula of the diaphragm is level anteriorly with
inferiorly and a little anteriorly, almost parallel to their original posi- the fourth costal cartilage, laterally with the fifth, sixth and seventh ribs,
tions. The associated inferior displacement of the abdominal viscera is and posteriorly with the eighth rib; the left cupula is a little lower.
permitted by the extensibility of the abdominal wall, but the limit of The role of the abdomen in breathing is often underestimated. If,
this extensibility is soon reached. The central tendon, its motion for example, the anterolateral wall were made of steel and linked the
arrested by the abdominal viscera, then becomes a fixed point from pelvic rim rigidly to the costal margins, inspiration would be impossi-
which the fibres of the diaphragm continue to contract. This causes the ble; the diaphragm could not descend (because the abdominal contents


second to tenth ribs to be elevated and the inferior portions of the ribs are incompressible), and the ribs could not rise (because the links
are turned outwards as a result of direct transmission of pressure to the pelvis would be inextensible). During normal breathing, the
through the zone of apposition (Fig. 55.9). The medial aspect of the abdomen relaxes as the diaphragm contracts. It is possible to oppose
rib is elevated and this increases the transverse dimension of the chest this motion by tensing the abdomen, as in the ‘beach posture’ adopted
in the same manner as a bucket handle swinging outwards; this effect to exaggerate the size of the chest. In this case, the abdominal contents
is most evident in the inferior seventh to tenth ribs. Movements at the fix the central tendon of the diaphragm, so that it raises the ribcage as
costovertebral joints cause elevation of the anterior ends of the ribs that it contracts, but it is a condition of that manœuvre that the gap between
push the body of the sternum and the upper ribs anteriorly in a ‘pump the ribs and the pelvic rim widens.
handle’ movement; this effect is most evident in the superior second to The ventilatory muscles must also work during sleep, when the
sixth ribs, and increases the anteroposterior dimension of the thorax. pharyngeal muscles relax and upper airway resistance increases. It is
The right cupula of the diaphragm, which lies on the liver, has a greater now appreciated that in some people, particularly the obese, this relaxa-
resistance to overcome than the left, which lies over the stomach, and tion can lead to periodic apnoea and marked hypoxia during sleep,
so the right crus and the fibres of the right side are more substantial implying that the pharyngeal muscles play an important ventilatory role


Anatomy of breathing
975
55
RETPAHC
in waking life. It is also clear that although ventilatory muscles rarely structures. The diaphragmatic and costomediastinal regions expand
tire in normal life, they do fatigue when placed under abnormal loads, most of all. Most of the volumetric change during ventilation occurs
e.g. in chronic obstructive pulmonary disease. in the alveoli.
The different pulmonary regions do not all move equally in venti- The primary role of the intercostal muscles is to stiffen the chest
lation. In quiet ventilation, the juxtahilar part of the lung scarcely wall, preventing paradoxical motion during inspiratory diaphragmatic
moves and the middle region moves only slightly. The superficial parts descent. This becomes most obvious immediately after high spinal
of the lung expand the most, and the mediastinal surface, posterior injury, when there is flaccid paralysis of the entire trunk and only the
border and apex move less because they are related to less movable diaphragm remains functional.
Bonus e-book images
Fig. 55.4 A cadaveric dissection showing phrenic arteries from the thoracic aorta and Fig. 55.8 A cadaveric dissection showing a
the origin and course of the right and left supplying portions of the right and left right phrenic ganglion close to the right
inferior phrenic arteries. diaphragmatic crura. inferior phrenic artery and right
diaphragmatic crus with plexiform
Fig. 55.5 A cadaveric dissection showing Fig. 55.6 A lateral view of the left superior communications to the coeliac ganglion,
the origin of the right and left superior phrenic artery arising from the aorta. aorticorenal ganglion and suprarenal gland.


Diaphragm and phrenic nerves
975.e1
55
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In a healthy adult with a vital capacity of 4.5 litres, some 3 litres is
accounted for by diaphragmatic excursion. Immediately after high
spinal injury, the vital capacity decreases to about 300 ml, even though
the diaphragm is moving maximally, because some 2.7 litres is lost by
paradoxical incursion of the flaccid chest wall as the diaphragm
descends. Usually after several weeks, the paralysis becomes spastic,
stiffening the chest wall, and the vital capacity increases towards its
phrenic limit of about 3 litres. In the same way, high spinal injury
reveals the role of the abdomen in inspiration and expiration. The
abdominal musculature plays a major role in active expiration in
humans. During the flaccid stage of high spinal paralysis, the only
mechanisms available for returning the relaxed diaphragm into the
thorax on expiration are passive recoil of the lungs and chest wall, and
the weight of the abdominal viscera. The latter is the most important
and operates only when patients are lying down. If they are sat up or
raised upright, they are unable to exhale; trussing the abdomen with
an elastic binder can be helpful. Conversely, when paralysis becomes
spastic, the stiff abdominal wall opposes inspiration.


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REFERENCES
Celli B 1998 The diaphragm and respiratory muscles. Chest Surg Clin N Am Lowdermilk GA, Naunheim KS 2000 Thoracoscopic evaluation and treat-
8:207–24. ment of thoracic trauma. Surg Clin North Am 80:1535–42.
De Troyer A, Estenne M 1988 Functional anatomy of the respiratory muscles. Nason LK, Walker CM, McNeeley MF et al 2012 Imaging of the diaphragm:
Clin Chest Med 9:175–93. anatomy and function. RadioGraphics 32:E51–E70.
Loukas M, Louis RG Jr, Hullett J et al 2005 An anatomical classification of Rajanna MJ 1947 Anatomical and surgical considerations of the phrenic and
the variations of the inferior phrenic vein. Surg Radiol Anat 27: accessory phrenic nerves. J Int Coll Surg 60:42–52.
566–74. Sarwal A, Walker FO, Cartwright MS 2013 Neuromuscular ultrasound for
Loukas M, Diala el-Z, Tubbs RS et al 2008 A review of the distribution of evaluation of the diaphragm. Muscle Nerve; 47:319–29.
the arterial and venous vasculature of the diaphragm and its clinical Shanmuganathan K, Killeen K, Mirvis SE et al 2000 Imaging of diaphrag-
relevance. Folia Morphol (Warsz) 67:159–65. matic injuries. J Thorac Imaging 15:104–11.
Loukas M, du Plessis M, Shah S et al 2015a The clinical anatomy of the
musculotendinous part of the diaphragm. Surg Radiol Anat. In press.
Loukas M, Joseph S, Louis RG Jr et al 2015b The surgical anatomy of the
subdiaphragmatic phrenic nerve and phrenic ganglia. Clin Anat. In
press.


SUBSECTION: Heart and mediastinum
CHAPTER
56
Mediastinum
Strictly speaking, the mediastinum is the visceral compartment between trunk, left common carotid and subclavian arteries (the vertebral artery
the two lungs and includes the mediastinal pleura. However, it is com- sometimes arises from the aortic arch between them), the internal
monly defined as the region between the two pleural sacs, bounded thoracic arteries, the left superior intercostal, hemiazygos, internal tho-
anteriorly by the sternum and posteriorly by the thoracic vertebral racic and inferior thyroid veins, and numerous lymph nodes, including
column, and extending vertically from the thoracic inlet to the dia- those from the tracheobronchial, paratracheal and brachiocephalic
phragm. It communicates with the neck via the superior thoracic aper- groups. Laterally, the superior mediastinal structures have the appear-
ture and with the extraperitoneal, extrapleural and epidural spaces. ance of being ‘shrink-wrapped’ by the tightly applied pleura. The supe-
Beyond each pulmonary hilum, the mediastinum merges into the lung rior mediastinum transmits the sympathetic and parasympathetic
interstitium, incorporating the bronchial tree with its accompanying cardiac nerves, the phrenic nerve laterally, the vagus nerves medially
neurovascular bundles and the pulmonary vasculature. and the left recurrent laryngeal nerve. The terminal part of the thoracic
The posterior mediastinal boundary is longer as a result of the duct emerges from the posterior aspect of the aorta and oesophagus,
oblique disposition of the superior thoracic aperture and the diaphragm- ascending between the left subclavian and carotid arteries. The left and
atic curvature posteroinferiorly. With respect to the median sagittal right brachiocephalic veins form the superior vena cava. The superior
plane, the mediastinum appears asymmetrical. (extrapericardial) half of the superior vena cava and its major tributaries
The mediastinum is divided into superior and inferior mediastina are situated on a more superficial plane posterior to the manubrium


by a plane that crosses the manubriosternal joint and the lower surface and the origins of sternothyroid inferiorly, and sternohyoid superiorly.
of the fourth thoracic vertebra (Fig. 56.1). The inferior mediastinum is An enlarged thyroid gland may extend inferiorly between the sternum
subdivided into anterior, middle and posterior parts. and the brachiocephalic veins; its surgical excision requires due dili-
Detailed accounts of mediastinal contents are included with descrip- gence from possible venous adherence. The origins of longus colli
tions of the respiratory organs (Ch. 54) and the heart (Ch. 57). (inferior oblique and vertical intermediate portions) are the deepest
elements of the superior mediastinum.
SUBDIVISIONS OF THE MEDIASTINUM
INFERIOR MEDIASTINUM
SUPERIOR MEDIASTINUM
Anterior mediastinum
The superior mediastinum lies between the manubrium sterni anteri-
orly and the upper four thoracic vertebrae posteriorly, and is bounded The anterior mediastinum lies between the sternal body and pericar-
laterally by the mediastinal pleurae (see Fig. 56.1). The reflection of the dium (see Fig. 56.1). It narrows above the fourth costal cartilages
costomediastinal pleurae follows a line that passes from the sternocla- where the pleural sacs converge, and contains loose connective tissue,
vicular joints in an inferomedial direction, to join the midline on the the sternopericardial ligaments, a few lymph nodes, the mediastinal
posterior aspect of the sternal angle. The resultant triangular space is branches of the internal thoracic artery, and sometimes part of the
occupied by the thymus or its remnants. The superior mediastinum thymus gland or its degenerated remains. The paired intervals between
accommodates the trachea, oesophagus, aortic arch, brachiocephalic the sternal and costal muscular slips of the diaphragm are filled with
areolar tissue and are crossed by the superior epigastric vessels and by
some lymph vessels from the abdominal wall and liver; they are rare
Left common sites of abdominal herniation. The pericardium and the heart are rou-


carotid artery Trachea tinely approached by either a complete median or partial sternotomy.
C7 The sternopericardial ligaments and the pleural reflections are easily
SUPERIOR Left subclavian separated by blunt dissection. In case of pericardial tamponade, the
MEDIASTINUM T1 artery
pericardial cavity can be easily drained through a subxyphoid approach,
Line of first rib T2 either surgically or via needle pericardiocentesis.
Thymus Aortic arch
T3 Middle mediastinum
Manubrium Pulmonary
T4 artery (right)
Sternal angle The middle mediastinum is the broadest part of the inferior mediasti-
T5
num (see Fig. 56.1) and contains the pericardium, heart and ascending
Main bronchus
aorta, the inferior, intrapericardial half of the superior vena cava (receiv-
ANTERIOR T6 (left)
ing the azygos venous arch posteriorly), the tracheal bifurcation and
MEDIASTINUM
Heart main bronchi, the pulmonary trunk, right and left pulmonary vascula-
T7
MIDDLE ture, phrenic nerves, the deep part of the cardiac plexus and the tracheo-
MEDIASTINUM bronchial lymph nodes. The short thoracic part of the inferior vena
T8
cava, both extra- and intrapericardial segments, extends between the
vena caval aperture of the diaphragm and its termination in the right
T9
atrium. The structures forming the pulmonary root converge at the level
of the hilum. The mediastinal pleura is continuous with the visceral
T10
Diaphragm pleura at the level of the hilum, which is where the lateral boundary of
Inferior vena the middle mediastinum is conventionally located. The fibrous pericar-
cava T11 dium lies on, and is fused with, the anterior two-thirds of the central
Oesophagus tendon of the diaphragm.
POSTERIOR
MEDIASTINUM T12
Posterior mediastinum
Aorta
L1
The posterior mediastinum is the lengthiest and narrowest portion of
976 Fig. 56.1 The mediastinal divisions. the inferior mediastinum (see Fig. 56.1). It is bounded anteriorly by the


Subdivisions of the mediastinum
977
65
RETPAHC
tracheal bifurcation, pericardium and pulmonary vessels, and posteri-
orly by the bodies of the fifth to the twelfth thoracic vertebrae. There is
no strict demarcation with the superior mediastinum; numerous struc-
tures travel through both regions. The arched posterior third of the
central portion of the diaphragm constitutes the anteroinferior limit of
the posterior mediastinum laterally as the mediastinal pleurae come
close together. Two pleural recesses, the interaortico-oesophageal and Superior
interazygo-oesophageal recesses, intercalate from the left and right sides intercostal
vein
between the aorta and oesophagus and the azygos vein and oesophagus,
respectively.
The posterior mediastinum contains the descending thoracic aorta
(on the left side of the spine), the oesophagus (median, but positioned
anterior to the aorta inferiorly) and, more posteriorly, the azygos and
hemiazygos venous systems, the thoracic duct, lymph nodes, right and
left sympathetic chains and thoracic splanchnic nerves. The vagal trunks Azygos vein
course adjacent to the oesophagus; the anterior trunk is constituted
Right lung:
mainly from the left vagus nerve and the posterior mainly from the outline of root Accessory
right vagus nerve. The descending thoracic aorta gives off the intercostal hemiazygos
arteries and the largest anterior segmental medullary artery, the ‘great vein
radicular artery of Adamkiewicz’ (Yoshioka et al 2003) (p. 770).
There are a number of communications between the posterior
mediastinum and the abdomen. These are: the oesophageal aperture
(also transmits the vagal trunks) at the level of the tenth thoracic ver-
tebra; the aortic aperture (also transmits the thoracic duct and occasion-
ally the azygos vein) at the level of the twelfth thoracic vertebra;
apertures within the diaphragmatic crura that transmit the thoracic
splanchnic nerves; apertures deep to the medial lumbosacral arches for
the sympathetic chains; and minute openings in the central tendon of
Hemiazygos


the diaphragm that transmit small veins. All openings represent poten- vein
tial communication sites for suppurative or neoplastic processes.
Descending
thoracic aorta
MEDIASTINAL COMMUNICATIONS WITH THE NECK
Anatomical pathways exist between the oral cavity and the thorax via
the parapharyngeal space and along other fascial planes of the neck.
The former is more likely to be infected than any of the other potential
Fig. 56.2 A common course followed by the intrathoracic azygos,
tissue spaces in the head and neck, communicating with the retropha-
hemiazygos and accessory azygos veins. Outlines of the root of the right
ryngeal and pretracheal spaces, so reaching the superior mediastinum
lung and ascending thoracic aorta are included.
and then the anterior part of the inferior mediastinum (see Chs 29 and
31). The plane between the buccopharyngeal and prevertebral fasciae is
a highway for spread of air and gastric contents between the neck and Brachiocephalic vein Internal jugular vein
mediastinum after oesophageal injury (Wind and Valentine 2013). The
carotid sheath, containing the carotid arteries, jugular veins and vagus Subclavian vein
nerves, represents another potential route of communication.
First posterior intercostal vein
Superior
Brachiocephalic vein
VASCULAR SUPPLY AND LYMPHATIC DRAINAGE vena cava
Superior intercostal vein
Azygos vein
The mediastinum contains the great vessels (ascending aorta, aortic arch Accessory hemiazygos vein
and its branches, descending thoracic aorta, superior vena cava and the Azygos vein
Hemiazygos vein
terminal part of the inferior vena cava), the pulmonary arteries and
veins, the internal thoracic and posterior intercostal arteries and veins, Subcostal vein Lumbar
the azygos and hemiazygos venous systems, and the thoracic duct. First azygos veins
lumbar vein Alternative channel
Great vessels of the superior mediastinum Ascending
Ascending lumbar veins
The aortic arch, descending thoracic aorta, pulmonary trunk and supe- lumbar vein Second
lumbar vein
rior vena cava are described in Chapter 57. Inferior
vena cava Third
lumbar vein