The pharynx conducts air into the trachea, a tube reinforced with C-shaped rings of cartilage to prevent its collapse (you can feel these rings if you press your fingers gently against the front of your throat). Cilia in the lining of the trachea move impurities up toward the throat, where they can be eliminated by swallowing or by expectoration. At the top of the trachea is the larynx. The larynx is shaped by nine cartilages, the most prominent of which is the thyroid cartilage at the front that forms the "Adam's apple." The opening between the vocal cords is the glottis. The small leaf-shaped cartilage at the top of the larynx is called the epiglottis. When one swallows, the epiglottis covers the opening of the larynx and helps to prevent food from entering the respiratory tract.
The larynx contains the vocal cords, folds of tissue that are important in speech production. Vibrations produced by air passing over the vocal cords form the basis for voice production, although portions of the throat and mouth are needed for proper articulation of speech.
The trachea is contained in a region known as the mediastinum, which consists of the space between the lungs together with the organs contained in this space. In addition to the trachea, the mediastinum contains the heart, esophagus, large vessels, and other tissues.
At its lower end, the trachea divides into a right and a left main stem bronchus that enter the lungs. The right bronchus is shorter and wider; it divides into three secondary bronchi that enter the three lobes of the right lung. The left bronchus divides into two branches that supply the two lobes of the left lung. Further divisions produce an increasing number of smaller tubes that supply air to smaller subdivisions of lung tissue. As the air passageways progress through the lungs, the cartilage in the walls gradually disappears and is replaced by smooth (involuntary) muscle.
The smallest of the conducting tubes, the bronchioles, carry air into the microscopic air sacs, the alveoli, through which gases are exchanged between the lungs and the blood. It is through the ultrathin walls of the alveoli and their surrounding capillaries that oxygen diffuses into the blood and carbon dioxide diffuses out of the blood for elimination.
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