Pleura Anatomy
The thoracic cavity
lies within the walls of the thorax and is separated from the abdominal cavity
by the diaphragm. The cavity contains the right and left lungs, each surrounded
by a serous membrane called the pleura. Between the lungs is a central
partition, the mediastinum, which includes the heart and great vessels, the
trachea and the oesophagus. Superiorly, numerous mediastinal structures enter
or leave the root of the neck through the superior thoracic aperture (p. 322).
Inferiorly, important structures including the aorta, inferior vena cava and
oesophagus pass between the mediastinum and the abdomen through openings in the
diaphragm (p. 203).
The pleura surrounds the lungs and
lines the walls of the thoracic cavity and is subdivided into visceral and
parietal parts. The visceral layer covers the surface of the lung and is
continuous with the parietal layer around the mediastinal attachment of the
lung at the lung root. The parietal layer covers the lateral aspect of the
mediastinum, the upper surface of the diaphragm and the inner aspect of the
chest wall (Fig. 2.17). Although the parietal and visceral layers are normally in contact, a
space, the pleural cavity (Fig. 2.18), exists between them and contains a thin
film of serous fluid. The fluid ensures close apposition of the two pleural
surfaces and reduces friction during respiratory movements. Injury or disease
may produce an accumulation of air (pneumothorax) or fluid (pleural effusion)
within the pleural cavity, causing the lung to collapse.
The parietal pleura is named according
to the surfaces it covers. Thus, the mediastinal pleura conforms to the
contours of the structures forming the lateral surface of the mediastinum and
is innervated by sensory branches of the phrenic nerve. Inferiorly, the
diaphragmatic pleura clothes the upper surface of the diaphragm. The central
portion receives sensory branches from each phrenic nerve, while the periphery
is innervated by lower intercostal nerves. The pleura covering the inner
surface of the thoracic wall is called the costal pleura and is innervated
segmentally by the intercostal nerves (Fig. 2.17).
The periphery of the diaphragm slopes
steeply downwards towards its attachment to the thoracic wall, creating a narrow gutter, the
costodiaphragmatic recess. Within this recess, which is particularly deep laterally
and posteriorly, the costal and diaphragmatic parts of the pari- etal pleura
lie in mutual contact.
The parietal pleura extending into the
root of the neck is called the cervical pleura and is innervated by the first intercostal
nerve. It is applied to the undersurface of a firm fascial layer, the
suprapleural membrane, which prevents upward movement of the apex of the lung
and pleura during ventilation (Fig. 7.15).
Surface markings of the parietal
pleura Because the parietal
pleura is reflected from the thoracic wall onto both the mediastinum and the
diaphragm, a line of pleural reflection can be mapped out on the body surface.
Traced from its upper limit, approximately 2.5 cm above the medial third of the
clavicle, this line descends behind the sternoclavicular joint.
This cervical pleura may be
accidentally perforated when a central venous line is being placed in the
subclavian vein. The pleural reflection line approaches the midline at the
level of the manubriosternal joint. On the right, the pleural reflection
descends vertically to the level of the sixth costal cartilage, while on the
left, the heart displaces the pleura laterally (Fig. 2.17) so that from the
fourth to the sixth costal cartilages, the line of reflection lies just lateral
to the edge of the sternum. This displacement exposes part of the pericardium
underlying the medial ends of the fourth and fifth intercostal spaces. Traced
laterally from the sixth costal cartilage, the surface marking is the same on
each side, crossing the eighth rib in the midclavicular line and the tenth rib
in the midaxillary line.
Posteriorly, the parietal pleura
continues horizontally, crosses the twelfth rib 5 cm from the midline and
continues medially for a further 2.5 cm. Thus, a small area of parietal pleura
lies below the level of the twelfth rib.
The visceral pleura (Fig.
2.18) is continuous with
the mediastinal parietal pleura around the root of the lung. Structures
entering or leaving the hilum of the lung occupy the upper part of this pleural
sleeve, the lower part consisting of an empty fold of pleura, the pulmonary
ligament (Fig. 2.25). The visceral pleura firmly adheres to the surface of the
lung and extends into the depths of the fissures. Unlike the parietal layer,
the visceral pleura does not have a somatic innervation.
Surface markings of the visceral pleura Since the visceral pleura covers the surface of
the lung, its surface markings coincide with those of the lung (p. 41).
