During normal breathing oxygen and substances in the air are brought into your lungs. The oxygen goes into the blood cells, which then carry the oxygen to cells throughout the body. As oxygen goes into these cells, waste (carbon dioxide) is removed by the blood cells and carried back to your lungs through the bloodstream. This waste is removed when you breathe out.
Blood cells pick up oxygen and deposit waste within air sacs in your lungs called alveoli. The alveoli are located at the end of the bronchial tubes. During normal breathing, the bronchial tubes in your lungs are wide open and air moves freely in and out of your lungs. The muscles surrounding each of these air tubes are relaxed, and the walls are free of extra mucus.