The trachea carries the air toward the lungs. Before reaching the lungs, the trachea splits into two tubes called the primary bronchi. Each tube enters one lung. Within the lungs, the primary bronchi divide into smaller and smaller tubes, finally branching into extremely tiny tubes called bronchioles. The bronchioles end in hundreds of millions of thin-walled structures called alveoli or air sacs. The alveoli give the lungs a tremendously extended surface area. If the air sacs were flattened out, the lungs would cover from 55 to 90 square metres.