The lungs fill most of the thorax. The right lung is larger than the left. The heart takes up more room on the left side of the lung. The lungs are held in an inflated state by surface tension. It is created by fluid produced by a thin lining around the lungs and chest wall. This holds the lungs open. When the chest expands the lungs are pulled out and the air is taken into the alveoli. As the body exhales the rib muscles relax gradually. If the body relaxes completely, the lungs will spring back rapidly.
When air gets between the lungs and chest wall the surface tension breaks and the lungs collapse. The lung is connected to the trachea by the bronchus and to the pulmonary vessels. In normal lungs, oxygen from the air is transferred to the capillaries that surround each alveolus. A person who has a lung disorder experiences air constriction. Here are some useful definitions:
Pneumonia - air sac are filled with fluid.
Emphysema - the walls of the air sacs break down.
Asthma - muscular walls of the bronchioles are narrowed.
Bronchitis - bronchus fills with mucus.