These short tubes enter the lungs. Bronchioles - The bronchi branch again and again to form bronchioles. These tubes spread through the lungs like twigs on a tree. Alveoli - These are the tiny air sacs in the lungs. This is where oxygen is absorbed and carbon dioxide is removed.
Normal breathing is carried out by movement of the diaphragm.
The diaphragm is a tough sheet of muscle separating the organs of the chest from those of the abdomen. The respiratory system works like a simple air pump, with the diaphragm acting as the plunger and the rib cage acting as the outer casing. As the diaphragm contracts, it flattens out, so air rushes into the lungs to fill the extra space. When the diaphragm relaxes, it regains its former domed shape. This squeezes the lungs slightly, forcing the air out again.