The lungs, with the body's left lung made transparent in this illustration so we can see the bronchi inside The body needs a continuous supply of oxygen. This invisible gas makes up about one-fifth of the air around us. Oxygen is used by cells to release energy from food. This process also releases waste carbon dioxide. The respiratory (breathing) system draws fresh air into the body, absorbs the vital oxygen from it into the blood, and then pushes out "stale" air containing carbon dioxide. The parts of the system where oxygen is absorbed are the lungs, which are two large, spongy organs, made up almost entirely of a latticework of tubes.
The respiratory system
Breathing muscles stretch the lungs to make them larger and suck in air. These muscles are the diaphragm below the lungs, and the intercostal muscles between the ribs. Fresh air passes in through the nose and mouth, down the pharynx (throat) and trachea (windpipe), into the lungs. The lung airways, called bronchi, divide many times and become thinner, ending in terminal bronchioles, narrower than human hairs.
A person may breathe in on average about 18 kg (40 lb) of dust during their lifetime.
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