The Respiratory System
The respiratory system starts at the nose and mouth and continues through the airways to the lungs. Air enters the respiratory system through the nose and mouth and passes down the throat (pharynx) and through the voice box, or Adam's apple (larynx). The entrance to the larynx is covered by a small flap of tissue (epiglottis) that automatically closes when swallowing, thus preventing food or drink from entering the airways.
The largest airway is the windpipe (trachea), which branches into two smaller airways: the left and right bronchi, which lead to the two lungs. The left lung is a little smaller than the right lung because it shares space in the left side of the chest with the heart. Each lung is divided into sections (lobes): three in the right lung and two in the left lung.
 See the figure Inside the Lungs and Airways.
The bronchi themselves divide many times before branching into smaller airways (bronchioles). These are the narrowest airways--as small as one half of a millimeter across. The larger airways resemble an upside-down tree, which is why this part of the respiratory system is often called the bronchial tree. The airways are held open by flexible, fibrous connective tissue called cartilage. Circular airway muscles can dilate or constrict the airways, thus changing the size of the airway.
At the end of each bronchiole are thousands of small air sacs (alveoli). Together, the millions of alveoli of the lungs form a surface of more than 100 square meters. Within the alveolar walls is a dense network of tiny blood vessels called capillaries. The extremely thin barrier between air and capillaries allows oxygen to move from the alveoli into the blood and allows carbon dioxide to move from the blood in the capillaries into the alveoli.
The pleura is a slippery membrane that covers the lungs as well as the inside of the chest wall. It allows the lungs to move smoothly during breathing and as the person moves around. Normally, the two layers of the pleura have only a small amount of lubricating fluid between them. The two layers glide smoothly over each other as the lungs change size and shape.
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