The story of oxygen transport to body cells is not complete without a look at the respiratory system, which brings oxygen from the air into the body, transfers it to the blood, and then rids the body of the waste products of cellular energy. When you breathe, the organs of your res- piratory system perform the physical job of bringing air into the body and expelling it. The same organs are the site of the more complex biochemical process of respiration, the oxygenation of blood at a cellu- lar level.
When you inhale air, it passes down your trachea, into the tubular bronchi that branch into your lungs, and through a system of subdivid- ing air passages that end deep in lung tissue as microscopic tubes called bronchioles. The bronchioles open into tiny, elastic air sacs called alveoli.
Parallel to these branching air passages, a network of blood vessels brings blood into lung tissue. Minute capillaries cover the surface of the alveoli, and through the walls of these capillaries oxygen passes from the air sacs into the blood. Carbon dioxide molecules, carried in the blood from body tissues, pass into the alveoli. The oxygen-laden blood ?ows back into the heart, where it then can be circulated throughout the body, while the carbon dioxide moves back through the lungs to be exhaled.

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When you inhale, you bring air into your lungs via the trachea, or windpipe. In the lungs, branching air passages (bronchi) end deep in lung tissue in microscopic clusters of air sacs called alveoli. In these clusters, networks of tiny blood vessels (capillaries) cover the air sacs. Oxygen exchange takes place through the walls of the alveoli and capillaries, as oxygen passes from lung tissue into the bloodstream and waste products (such as carbon dioxide) pass from the bloodstream into the lungs to be exhaled out of your body.