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The Respiratory System / The Lungs
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The lungs are the main organs of our respiratory system. The right lung is composed of 3 lobes and weighs a little over a pound. The left lung weighs a little under a pound and consists of 2 lobes. The trachea enters the lungs and branches out into bronchi, and then bronchioles (about .01 inches in diameter), and eventually into 250 million air sacs called alveoli. Capillaries bring blood low in oxygen and high in carbon dioxide to the lungs. The carbon dioxide diffuses into the alveoli for removal and oxygen is transferred to the capillaries.
 Air travels to the lungs through a series of air tubes and passages. It enters the body through the nostrils or the mouth, passing down the throat to the larynx, or voice box, and then to the trachea, or windpipe. In the chest cavity the trachea divides into two branches, called the right and left bronchi or bronchial tubes, that enter the lungs.
In the adult human, each lung is 25 to 30 cm (10 to 12 in) long and roughly conical. The left lung is divided into two sections, or lobes: the superior and the inferior. The right lung is somewhat larger than the left lung and is divided into three lobes: the superior, middle, and inferior. The two lungs are separated by a structure called the mediastinum, which contains the heart, trachea, esophagus, and blood vessels. Both right and left lungs are covered by an external membrane called the pleura. The outer layer of the pleura forms the lining of the chest cavity.
 The branches of the bronchi eventually narrow down to tubes of less than 1.02 mm (less than 0.04 in) in diameter. These tubes, called bronchioles, divide into even narrower tubes, called alveolar ducts. Each alveolar duct ends in a grapelike cluster of thin-walled sacs, called alveoli (a single sac is called an alveolus). From 300 million to 400 million alveoli are contained in each lung. The air sacs of both lungs have a total surface area of about 93 sq m (about 1000 sq ft), nearly 50 times the total surface area of the skin.
In addition to the network of air tubes, the lungs also contain a vast network of blood vessels. Each alveolus is surrounded by many tiny capillaries, which receive blood from arteries and empty into veins. The arteries join to form the pulmonary arteries, and the veins join to form the pulmonary veins. These large blood vessels connect the lungs with the heart.
 Though the right lung has three lobes, the left lung, with a cleft to accommodate the heart, has only two. The two branches of the trachea, called bronchi, subdivide within the lobes into smaller and smaller air vessels. They terminate in alveoli, tiny air sacs surrounded by capillaries. When the alveoli inflate with inhaled air, oxygen diffuses into the blood in the capillaries to be pumped by the heart to the tissues of the body, and carbon dioxide diffuses out of the blood into the lungs, where it is exhaled.
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