Light micrograph of a section through a normal human lung, showing the smooth muscle & connective tissue walls (red) of the alveoli (air sacs). The alveoli represent the terminal branches of the tree of airways. Its trunk, the trachea (windpipe) splits into the left and right bronchi, which divides many times into numerous bronchioles in each lung. The bronchial wall is covered by a simple epithelial layer of ciliated columnar cells supported by a layer of smooth muscle. Bronchioles divide into alveolar ducts and alveoli, where gaseous exchange occurs through surrounding blood capillaries. Magnification: x25 at 6x6cm size.
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