Human lungs take in oxygen from the air to keep the body alive.
When a person inhales, air travels through the nose or mouth, down the trachea, and through branching bronchi into the lungs. At the ends of the smallest airways are millions of microscopic air sacs called alveoli. Oxygen crosses the thin alveolar walls into nearby capillaries and enters the bloodstream.
Red blood cells then transport most of that oxygen by binding it to hemoglobin. Cells use oxygen during aerobic respiration, a process that helps release energy from nutrients. The main waste gas, carbon dioxide, travels in the opposite direction—from tissues to blood, then across the alveoli to be exhaled.
Air is mostly nitrogen, but the lungs do not use nitrogen as the body’s life-sustaining fuel. Humans inhale nitrogen along with oxygen, and most of it passes back out unchanged. The lungs also do more than simply inflate: their enormous alveolar surface and thin blood-air barrier make rapid gas exchange possible.