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The lungs (Fig. 6.1)
costals increases the A-P diameter of the upper thorax; contraction of
• The lungs provide an alveolar surface area of approximately 40 m
the lower external intercostals increases the transverse diameter of the
for gaseous exchange.
lower thorax; and contraction of the diaphragm increases the vertical
• Each lung has: an apex which reaches above the sternal end of the 1st 2 • During normal inspiration: contraction of the upper external inter-
rib; a costovertebral surface which underlies the chest wall; a base length of the internal thorax. These changes serve to increase lung vol-
overlying the diaphragm and a mediastinal surface which is moulded to ume and thereby result in reduction of intrapulmonary pressure causing
adjacent mediastinal structures. air to be sucked into the lungs. In deep inspiration the sternocleidomas-
• Structure: the right lung is divided into upper, middle and lower toid, scalenus anterior and medius, serratus anterior and pectoralis
lobes by oblique and horizontal fissures. The left lung has only an major and minor all aid to maximize thoracic capacity. The latter are
oblique fissure and hence no middle lobe. The lingular segment repres- termed collectivelyathe accessory muscles of respiration.
ents the left sided equivalent of the right middle lobe. It is, however, an • Expiration is mostly due to passive relaxation of the muscles of inspira-
anatomical part of the left upper lobe. tion and elastic recoil of the lungs. In forced expiration the abdominal
Structures enter or leave the lungs by way of the lung hilum which, musculature aids ascent of the diaphragm.
as mentioned earlier, is ensheathed in a loose pleural cuff (see Fig. 5.1).
• Blood supply: the bronchi and parenchymal tissue of the lungs are The chest X-ray (CXR) (Fig. 6.2)
supplied by bronchial arteriesabranches of the descending thoracic The standard CXR is the postero-anterior (PA) view. This is taken with
aorta. Bronchial veins, which also communicate with pulmonary veins, the subject’s chest touching the cassette holder and the X-ray beam
drain into the azygos and hemiazygos. The alveoli receive deoxy- directed anteriorly from behind.
genated blood from terminal branches of the pulmonary artery and oxy- Structures visible on the chest X-ray include the:
genated blood returns via tributaries of the pulmonary veins. Two • Heart borders: any significant enlargement of a particular chamber
pulmonary veins return blood from each lung to the left atrium. can be seen on the X-ray. In congestive cardiac failure all four cham-
• Lymphatic drainage of the lungs: lymph returns from the periphery bers of the heart are enlarged (cardiomegaly). This is identified on the
towards the hilar tracheobronchial groups of nodes and from here to PA view as a cardiothoracic ratio greater than 0.5. This ratio is calcu-
mediastinal lymph trunks. lated by dividing the width of the heart by the width of the thoracic cav-
• Nerve supply of the lungs: a pulmonary plexus is located at the root ity at its widest point.
of each lung. The plexus is composed of sympathetic fibres (from the • Lungs: the lungs are radiolucent. Dense streaky shadows, seen at the
sympathetic trunk) and parasympathetic fibres (from the vagus). lung roots, represent the blood-filled pulmonary vasculature.
Efferent fibres from the plexus supply the bronchial musculature and • Diaphragm: the angle made between the diaphragm and chest wall is
afferents are received from the mucous membranes of bronchioles and termed the costophrenic angle. This angle is lost when a pleural effu-
from the alveoli. sion collects.
• Mediastinal structures: these are difficult to distinguish as there is
The mechanics of respiration considerable overlap. Clearly visible, however, is the aortic arch
• A negative intrapleural pressure keeps the lungs continuously par- which, when pathologically dilated (aneurysmal), creates the impres-
tially inflated. sion of ‘widening’ of the mediastinum.

The lungs 17