In order to understand ARDS, it is important first to review how the
lung works. Air, which contains oxygen, is inhaled through the nose and mouth.
It then passes into the windpipe (trachea). From the trachea the air flows
through tubes called bronchi. These bronchial tubes go to the microscopic air sacs
called alveoli. Very small blood vessels (capillaries) sit next to these air sacs.
Oxygen passes out of the air sacs into the bloodstream and carbon dioxide passes
from the bloodstream into the air sacs. The carbon dioxide is then exhaled.
The inflammation caused by ARDS sometimes can be mild but is often
very severe. ARDS usually involves both lungs. In the early stages of ARDS,
the inflammation can start in one lung, but often spreads to involve the other lung as
well. When this inflammation occurs, it can cause a great deal of lung damage,
specifically to the alveoli and the capillaries.
When alveoli are damaged, they can collapse and lose the ability to
receive oxygen. When capillaries are damaged, they leak fluid (edema) into the lungs
and alveoli. With some alveoli collapsed and others filled with fluid, it becomes
very difficult for the lungs to absorb oxygen and get rid of carbon dioxide. If this
inflammation continues, the lung, like any other part of the body, can become scarred as
it tries to heal itself. When that happens, much of the fluid in the lungs becomes
replaced by scar tissue (fibrosis). If too much fibrosis occurs, it will also
interfere with the exchange of oxygen and carbon dioxide.