lung (L) [AS, lungen] , one of a pair of light, spongy organs in the thorax, constituting the main component of the respiratory system. The two highly elastic lungs are the main mechanisms in the body for inhaling (inspiring) air, from which oxygen is extracted for the arterial blood system, and for exhaling (expiring) carbon dioxide dispersed from the venous system. The right lung is divided into three lobes; the left lung, two lobes. Each lung is conical and has an apex, a base, three borders, and two surfaces. The apex is rounded and extends into the root of the neck about 4 cm above the first rib. The base of the lung is broad and concave, rests on the convex surface of the diaphragm, and with the diaphragm moves up during expiration and down during inspiration. Each lung is composed of an external serous coat, a subserous layer of areolar tissue, and the parenchyma. The serous coat comprises the thin, visceral pleura. The subserous areolar tissue contains many elastic fibers and invests the entire surface of the organ. The parenchyma is composed of secondary lobules divided into primary lobules, each of which consists of blood vessels, lymphatics, nerves, and an alveolar duct connecting with air spaces. The surfaces of the lungs are partially concave, with a cardiac impression that cradles the heart. The bronchial arteries supply blood to nourish the lungs and are derived from the ventral side of the thoracic aorta or from the aortic intercostal arteries. The bronchial vein is formed at the root of the lung. Most of the blood supplied by the bronchial arteries is returned by the pulmonary veins to the left atrium of the heart. The lungs are pinkish white at birth and darken in later life. The coloring is from carbon granules deposited in the areolar tissue near the surface of the lung. The carbon deposits increase with age. The lungs of men are usually heavier than those of women and usually have a greater capacity. The quantity of air that can be exhaled from the lungs after the deepest inspiration, the vital capacity, averages 3700 cc.
