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Other Terms: Lungs, Set of lungs, Lungs pair, Pulmones, Poumon
The lungs are the essential organs of respiration; there are two lungs in the human body, placed one on either side within the thorax, and separated from each other by the heart and other contents of the mediastinum. The substance of the lung is of a light, porous, spongy texture; it floats in water, and has a popping sound when handled, owing to the presence of air in the alveoli; it is also highly elastic; hence the retracted state of these organs when they are removed from the closed cavity of the thorax. The surface is smooth, shining, and marked out into numerous polyhedral areas, indicating the lobules of the organ: each of these areas is crossed by numerous lighter lines. At birth the lungs are pinkish white in color; in adult life the color is a dark slaty gray, mottled in patches; and as an age advances, this mottling assumes a black color. The coloring matter consists of granules of a carbonaceous substance deposited in the loose connective tissue near the surface of the organ. It increases in quantity as age advances, and is more abundant in males than in females. As a rule, the posterior border of the lung is darker than the anterior. The right lung usually weighs about 625 gm. The left 567 gm., but much variation is met with according to the amount of blood or serous fluid they may contain. The lungs are heavier in the male than in the female, their proportion to the body being, in the former, as 1 to 37, in the latter as 1 to 43. Each lung is conical in shape, and presents for examination an apex, a base, three borders, and two surfaces.
APEX (apex pulmonis)
The apex is rounded, and extends into the root of the neck, reaching from 2.5 to 4 cm. above the level of the sternal end of the first rib. A sulcus produced by the subclavian artery as it curves in front of the pleura runs upward and lateralward immediately below the apex.
The base is broad, concave, and rests upon the convex 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 stomach, and the spleen. Since the diaphragm extends higher on the right than on the left side, the concavity on the base of the right lung is deeper than that on the left. Laterally and behind, the base is bounded by a thin, sharp margin which projects for some distance into the phrenicocostal sinus of the pleura, between the lower ribs and the costal attachment of the diaphragm. The base of the lung descends during inspiration and ascends during expiration.
The two surfaces of the lungs are the costal surface end the mediastinal surface.The costal surface is smooth, convex, of considerable extent, and corresponds to the form of the cavity of the chest, being deeper behind than in front. It is in contact with the costal pleura, and presents, in specimens which have been hardened in situ, slight grooves corresponding with the overlying ribs. The mediastinal surface is in contact with the mediastinal pleura. It presents a deep concavity, the cardiac impression, which accommodates the pericardium; this is larger and deeper on the left than on the right lung, on account of the heart projecting farther to the left than to the right side of the median plane. Above and beyond this concavity is a triangular depression named the hilum, where the structures which form the root of the lung enter and leave the organ. These structures are invested by pleura, which, below the hilum and behind the pericardial impression, forms the pulmonary ligament. On the right lung immediately above the hilum, is an arched furrow which accommodates the azygos vein; while running upward, and then arching lateral. Some little distance below the apex, is a wide groove for the superior vena cava and right brachiocephalic vein; behind this, and nearer the apex, is a furrow for the brachiocephalic artery. Behind the hilum and the attachment of the pulmonary ligament is a vertical groove for the esophagus; this groove becomes less distinct below, owing to the inclination of the lower part of the esophagus to the left of the mid line. Anterior and to the right of the inferior part of the esophageal groove is a deep concavity for the extrapericardiac portion of the thoracic part of the inferior vena cava. On the left lung immediately above the hilum, is a well-marked curved furrow produced by the aortic arch, and running upward from this toward the apex is a groove accommodating the left subclavian artery; a slight impression anterior to the latter and close to the margin of the lung lodges the left brachiocephalic vein. Behind the hilum and pulmonary ligament is a vertical furrow produced by the descending aorta, and anterior to this , near the base of the lung, the lower part of the esophagus causes a shallow impression.
The borders of the lung include: inferior, posterior, and anterior.The inferior border is thin and sharp where it separates the base from the costal surface and extends into the phrenicocostal sinus; medially where it divides the base from the mediastinal surface it is blunt and rounded. The posterior border is broad and rounded, and is received into the deep concavity on either side of the vertebral column. It is much longer than the anterior border, and projects, inferior, into the phrenicocostal sinus. The anterior border is thin and sharp, and overlaps the front of the pericardium. The anterior border of the right lung is almost vertical, and projects into the costomediastinal sinus; that of the left presents, inferior, an angular notch, the cardiac notch, in which the pericardium is exposed. Opposite this notch the anterior margin of the left lung is situated some little distance lateral to the line of reflection of the corresponding part of the pleura.
FISSURES AND LOBES OF THE LUNGS
The left lung is divided into two lobes, an upper and a lower, by an oblique fissure, which extends from the costal to the mediastinal surface of the lung both above and below the hilum. As seen on the surface, this fissure begins on the mediastinal surface of the lung at the upper and posterior part of the hilum, and runs posterior and superior to the posterior border, which it crosses at a point about 6 cm. below the apex. It then extends inferior and anterior over the costal surface, and reaches the lower border a little behind its anterior extremity, and its further coarse can be followed superior and posterior across the mediastinal surface as far as the lower part of the hilum. The superior lobe lies superior and anterior to this fissure, and includes the apex, the anterior border, and a considerable part of the costal surface and the greater part of the mediastinal surface of the lung. The inferior lobe, the larger of the two, is situated inferior and posterior to the fissure, and comprises almost the whole of the base, a large portion of the costal surface, and the greater part of the posterior border.
The right lung is divided into three lobes, superior, middle, and inferior, by two interlobular fissures. One of these, the oblique fissure, separates the inferior from the middle and superior lobes, and corresponds closely with the fissure in the left lung. Its direction is, however, more vertical, and it cuts the lower border about 7.5 cm. behind its anterior extremity. The other fissure, the horizontal fissure, separates the superior from the middle lobe. It begins in the previous fissure near the posterior border of the lung, and running horizontally forward, cuts the anterior border on a level with the sternal end of the fourth costal cartilage; on the mediastinal surface it may be traced posterior to the hilum. The right lung although shorter by 2.5 cm. than the left, in consequence of the diaphragm rising higher on the right side to accommodate the liver, is broader, owing to the inclination of the heart to the left side; its total capacity is greater and it weighs more than the left lung. The middle lobe , the smallest lobe of the right lung, is wedge-shaped, and includes the lower part of the anterior border and the anterior part of the base of the lung.
THE ROOT OF THE LUNG (RADIX PULMONIS)
A little above the middle of the mediastinal surface of each lung, and nearer its posterior than its anterior border, is its root, by which the lung is connected to the heart and the trachea. The root is formed by the bronchus, the pulmonary artery, the pulmonary veins, the bronchial arteries and veins, the pulmonary plexuses of nerves, lymphatic vessels, bronchial lymph glands, and loose connective tissue, all of which are enclosed by a reflection of the pleura. The root of the right lung lies posterior to the superior vena cava and part of the right atrium, and inferior to the azygos vein. That of the left lung passes inferior to the aortic arch and anterior to the descending aorta, the phrenic nerve, the pericardicophrenic artery and vein, and the anterior pulmonary plexus, lie in front of each, and the vagus and posterior pulmonary plexus behind each; below each is the pulmonary ligament.The chief structures composing the root of each lung are arranged as follows: the upper of the two pulmonary veins anterior; the pulmonary artery in the middle; and the bronchus, together with the bronchial vessels, posterior. From superior to inferior, on the two sides, their arrangement differs, thus: on the right side their position is –eparterial bronchus, pulmonary artery, hyparterial bronchus, pulmonary veins, but on the left side their position is –pulmonary artery, bronchus, pulmonary veins. The lower of the two pulmonary veins, is situated inferior to the bronchus, at the apex or lowest part of the hilus.
The lungs are composed of an external serous coat, a subserous connective tissue and the pulmonary substance or parenchyma. The serous coat is the pulmonary pleura ;it is thin, transparent, and invests the entire organ as far as the root. The subserous connective tissue contains a large proportion of elastic fibers; it invests the entire surface of the lung, and extends inward between the lobules. The parenchyma is composed of secondary lobules, which vary in size; those on the surface are large , of pyramidal form, the base turned toward the surface; those in the interior smaller, and of various forms. Each secondary lobule is composed of several primary lobules, the anatomical units of the lung. The primary lobule consists of an alveolar duct, the air spaces connected with it and their blood vessels, lymphatics and nerves. The intrapulmonary bronchi divide and subdivide throughout the entire organ, the smallest subdivisions constituting the bronchioles. The larger divisions consist of an outer coat of fibrous tissue in which are found at intervals irregular plates of hyaline cartilage, most developed at the points of division; internal to the fibrous coat, a layer of circularly disposed smooth muscle fibers, the bronchial muscle; most internally, the mucous membrane, lined by columnar ciliated epithelium resting on a basement membrane. The corium of the mucous membrane contains numerous elastic fibers running longitudinally, and a certain amount of lymphoid tissue; it also contains the ducts of mucous glands, the acini of which lie in the fibrous coat. The bronchioles differ from the larger tubes in containing no cartilage and in the fact that the ciliated epithelial cells are cubical in shape. Each bronchiole divides into two or more respiratory bronchioles, with scattered alveoli, and each of these again divides into several alveolar ducts, with a greater number of alveoli connected with them. Each alveolar duct is connected with a variable number of irregularly spherical spaces, which also possess alveoli, the atria. With each atrium a variable number of alveolar sacs are connected which bear on all parts of their circumference alveoli or air sacs. The alveoli are lined by a delicate layer of simple squamous epithelium, the cells of which are united at their edges by cement substance. Outside the epithelial lining is a little delicate connective tissue containing numerous elastic fibers and a close net-work of blood capillaries, and forming a common wall to adjacent alveoli.
VESSELS AND NERVES
Blood supply and drainage
The pulmonary artery conveys the venous blood to the lungs; it divides into branches which accompany the bronchial tubes and end in a dense capillary net-work in the walls of the alveoli. In the lung the branches of the pulmonary artery are usually superior and anterior to a bronchial tube, the vein inferior. The pulmonary capillaries form plexuses which lie immediately beneath the lining epithelium, in the walls and septa of the alveoli and of the infundibula. In the septa between the alveoli the capillary network forms a single layer. The capillaries form a very minute network, the meshes of which are smaller than the vessels themselves; their walls are also exceedingly thin. The arteries of neighboring lobules are independent of each other, but the veins freely anastomose. The pulmonary veins commence in the pulmonary capillaries, the radicles coalescing into larger branches which run through the substance of the lung, independently of the pulmonary arteries and bronchi. After freely communicating with other branches they form large vessels, which ultimately come into relation with the arteries and bronchial tubes and accompany them to the hilum of the organ. Finally they open into the left atrium of the heart, conveying oxygenated blood to be distributed to all parts of the body by the aorta. The bronchial arteries supply blood for the nutrition of the lung; they are derived from the thoracic aorta or from the upper aortic intercostals arteries, and accompanying the bronchial tubes, are distributed to the bronchial glands and upon the walls of the larger bronchial tubes and pulmonary vessels. Those supplying the bronchial tubes form a capillary plexus in the muscular coat, from which branches are given off to form a second plexus in the mucous coat; this plexus communicates with small venous trunks that empty into the pulmonary veins. Others are distributed in the interlobular loose connective tissue, and end partly in the deep, partly in the superficial, bronchial veins. Lastly some ramify upon the surface of the lung, beneath the pleura, where they form a capillary network. The bronchial vein is formed at the root of the lung, receiving superficial and deep veins corresponding to branches of the bronchial artery. It does not however, receive all the blood supplied by the artery, as some of it passes into the pulmonary veins. It ends on the right side in the azygos vein, and on the left side in the highest intercostal or in the accessory hemiazygos vein.
The lungs are supplied from the anterior and posterior pulmonary plexuses, formed chiefly by branches from the sympathetic and vagus. The filaments from these plexuses accompany the bronchial tubes, supplying efferent fibers to the bronchial muscle and afferent fibers to the bronchial mucous membrane and to the alveoli of the lung. Small ganglia are found upon these nerves.