Lungs and Pleura
The lungs are the essential organs of respiration and are responsible for the uptake of oxygen into the blood and the removal of carbon dioxide. The functional design of the thorax facilitates this complex process. The
muscles of respiration and the diaphragm, acting together, increase the intrathoracic volume, creating a negative pressure within the pleural space which surrounds the lung and causing expansion of the lung. The
resultant reduction in intra-alveolar pressure prompts the conduction of air through the upper respiratory tract into the trachea and airways and thence into the alveoli, where gaseous exchange occurs.
LUNGS
The lungs are the essential organs of respiration.They are situated on either side of the heart and other mediastinal contents. Each lung is free in its pleural cavity, except for its attachment to the heart and trachea at the hilum and pulmonary ligament respectively. When removed from the thorax, a fresh lung is spongy, can float in water, and
crepitates when handled, because of the air within its alveoli. It is also highly elastic and so it retracts on removal from the thorax. Its surface is smooth and shiny and is separated by fine, dark lines into numerous small polyhedral domains, each crossed by numerous finer lines, indicating the areas of contact between its most peripheral lobules and the pleural surface.
At birth the lungs are pink, but in adults they are dark grey and patchily mottled. As age advances, this maculation becomes black, as granules of inhaled carbonaceous material are deposited in the loose connective tissue near the lung surface. Darkening is often more marked in men than women, in those who have dwelt in industrial areas and in smokers. The posterior pulmonary border is usually darker than the anterior. Lungs from fetuses or stillborn infants who have not respired differ from those of infants who have
taken a breath in that they are firm, non-crepitant and do not float in water.
The adult right lung usually weighs 625 g, and the left 565 g, but the range of wet weights is considerable, not least because it reflects the amount of blood or serous fluid contained within the lungs when weighed.In proportion to body stature, the lungs are heavier in men than in women.
PULMONARY SURFACE FEATURES
Each lung has an apex, base, three borders and two surfaces (base of lungs are inferior surface, too). In shape,each lung approximates to half a cone.
Apex
The apex, the rounded upper extremity, protrudes above the thoracic inlet where it contacts the cervical pleura, and is covered in turn by the suprapleural membrane. As a consequence of the obliquity of the inlet,
the apex rises 3–4 cm above the level of the first costal cartilage; it is level posteriorly with the neck of the first rib.Its summit is 2.5 cm above the medial third of the clavicle. The apex is therefore in the root of the neck. It
has been claimed that, because the apex does not rise above the neck of the first rib, it is really intrathoracic, and that it is the anterior surface that ascends highest in inspiration. The subclavian artery arches up and laterally over the suprapleural membrane, grooves the anterior surface of the apex near its summit and separates it from scalenus anterior. Scalenus medius is lateral, the brachiocephalic artery, right brachiocephalic vein and trachea are adjacent to the right medial surface of the lung, and the left subclavian artery and left brachiocephalic vein are adjacent to the left medial surface of the apex of the lung.
Base
The basal surface is semilunar and concave, and rests upon the superior surface of the diaphragm, which separates the right lung from the right lobe of the liver and the left lung from the left lobe of the liver, the gastric fundus and spleen. Since the diaphragm extends higher on the right than on the left, the concavity is deeper on the base of the right lung. Posterolaterally, the base has a sharp margin that projects a little into
the costodiaphragmatic recess.
Costal surface
The costal surface of the lung is smooth and convex, and its shape is adapted to that of the thoracic wall, which is vertically deeper posteriorly. It is in contact with the costal pleura; in specimens that have beenpreserved in situ, this surface exhibits grooves that correspond with the overlying ribs.
Medial surface
The medial surface has a posterior vertebral and anterior mediastinal part. The vertebral part lies in contact with the sides of the thoracic vertebrae and intervertebral discs, the posterior intercostal vessels and the splanchnic nerves. The mediastinal area is adapted to the heart at the cardiac impression, which is much larger and deeper on the left lung where the heart projects more to the left of the median plane. Posterosuperior to this concavity is the somewhat triangular hilum, where various structures enter or leave the lung, collectively surrounded by a sleeve of pleura that
also extends below the hilum and behind the
cardiac impression as the pulmonary
ligament.
Other impressions on the lung surface
In addition to these pulmonary features,
cadaveric lungs that have been preserved in
situ can show a number of other impressions
that indicate their relations with surrounding
structures .On the right lung the cardiac impression is related to the anterior surface of the right auricle, the anterolateral surface of the right atrium and partially to the anterior surface of the right ventricle. The impression ascends anterior to the hilum as a wide groove for the superior vena cava and the terminal portion of the right brachiocephalic vein. Posteriorly this groove is joined by a deep sulcus which arches forwards above the hilum and is occupied by the azygos vein. The right side of the oesophagus makes
a shallow vertical groove behind the hilum and the pulmonary ligament. Towards the diaphragm it inclines left and leaves the right lung, and therefore does not reach the lower limit of this surface. Posteroinferiorly the cardiac impression is confluent with a short wide groove adapted to the inferior vena
cava. Between the apex and the groove for the azygos, the trachea and right vagus are close to the lung, but do not mark it.
On the left lung (Fig.) the cardiac impression is related to the anterior and lateral surfaces of the left ventricle and auricle. The anterior infundibular surface and adjoining part of the right ventricle is also related to the
lung as it ascends in front of the hilum to accommodate the pulmonary trunk. A large groove arches over the Hilum, and descends behind it and the pulmonary ligament, corresponding to the aortic arch and descending aorta. From its summit a narrower groove ascends to the apex for the left subclavian artery. Behind this, above the aortic groove, the lung is in contact with oesophagus. In front of the subclavian groove there is a faint linear depression for the left brachiocephalic vein.
