JOHN BAVOSI / SCIENCE PHOTO LIBRARY JOHN BAVOSI / SCIENCE PHOTO LIBRARY
Illustration of lung bronchioles and alveoli. The lung consists of many branched air passages called bronchioles (upper left), which end in groups of tiny balloon-like sacs called alveoli. It is in each alveolus that gases are exchanged during re- spiration. The bronchiole walls are lined spirally with smooth muscle (here, coloured orange). The alveolar sacs are thin-walled and supplied with a capillary network of blood vessels. Pulmonary veins (blue) bring deoxygenated blood to the lungs which is oxygenated by gas exchange across the alveolar walls (see lower left). Arteries (red) transport this oxygenated blood to the body. There are about 300 million alveoli in a human lung.
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