The chest or thorax is a cage of bone, cartilage, and muscle used to expand the lungs.
Muscles of respiration are the diaphragm and intercostal muscles of the rib cage.
There are three major divisions of the chest, the right and left pleural cavities each containing a lung and the medistinum located between the lungs containing the heart. The trachea and bronchi form a tree like structure that transports air from the environment to the alveoli. The trachea branches into bronchi and then into bronchioles terminating in the alveoli where the oxygen and carbon dioxide exchange takes place. The bronchioles have smooth muscle wrapped around them and are lined by a mucous membrane. The pleura are serous membranes that line the thoracic cavity and cover the lungs.