After air has passed down the trachea to the lungs, it enters two branches: one branch enters the right lung and the other branch enters the left lung. These two branches are called primary bronchi. One branch is called a bronchus.Bronchi also have cartilage around them, but the amount decreases as the bronchi extend in to the lung. Soon after the primary bronchi enter the lungs, each bronchus branches again. These new branches are called secondary bronchi. Each secondary bronchus enters into one of the lobes (parts) of the lung. There are three lobes in the right lung and two lobes in the left lung. Thus there are five secondary bronchi. In the lobes of the lung, the secondary bronchi branch into tertiary bronchi (tert - means three)The tertiary bronchi within the lung branch into small tubes called bronchioles. Although these bronchioles are quite small in diameter, they are very important in regulating the amount of air that enters the lungs. The bronchioles have a thin layer of smooth muscle that contracts when less air is needed and dilates when more air is needed.Eventually, the air that is carried by the bronchioles enters clusters of alveoli. Alveoli are very tiny bubbles with very thin walls. One individual "bublle" is called an alveolus. Alveoli cluster around a bronchiole, just like grapes cluster around a stem; therefore, all alveoli in a cluster connect with a single bronchiole. The alveoli are where the blood drops off carbon dioxide anf picks up oxygen. Your lungs are packed with them.In fact, each lung contains about 150 million alveoli. Despite all of the tissues yor lungs contain, they are very lightweight. They resemble a sponge.The walls of each alveolus are comprised of elastin fibers. Recall that elastin is also found in skin. Elastin allows skin as well as the wal of each alveolus to stretch when you breathe in and return to its original size when you breathe out.
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