Our lungs bring fresh oxygen into our bodies and remove carbon dioxide and other gaseous waste products. As we breathe air in, we use the muscles of our rib cage and especially the major muscle called the diaphragm (pronounced DY uh fram), to pull air into our lungs. As we breathe air in, the diaphragm contracts or tightens and flattens, allowing air to be sucked into the lungs.

The diaphragm and the rib cage muscles relax and air is expired passively (in other words, the muscles do no work when we breathe out.)

Air, containing the oxygen our bodies need, is inhaled through the mouth and the nose. The mucus membranes in our mouth and nose warm and moisten the air, as well as trap particles of foreign matter. The air passes through the throat into the trachea or windpipe.

The trachea divides into the left and right bronchi. Like a branch, each bronchus divides again and again, becoming narrower and narrower.


The smallest airways end in the alveoli, small, thin air sacs that are arranged in clusters like bunches of balloons. When you breathe in by enlarging the chest cage, the "ballons" expand as air rushes in to fill the vacuum. When you breathe out, the "balloons" relax and air moves out of the lungs.

Tiny blood vessels surround each of the 300 million alveoli in the lungs. Oxygen moves across the walls of the air sacs, is picked up by the blood and carried to the rest of the body. Carbon dioxide or waste gas passes into the air sacs from the blood and is breathed out.

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