To understand acute respiratory distress syndrome (ARDS), it is helpful to understand how your lungs work.

Normal Lung Function

A slice of normal lung looks like a pink sponge¡ªfilled with tiny bubbles or holes. Around each bubble is a fine network of tiny blood vessels. These bubbles, which are surrounded by blood vessels, give the lungs a large surface to exchange oxygen (into the blood where it is carried throughout the body) and carbon dioxide (out of the blood). This process is called gas exchange. Healthy lungs do this very well.

Here's how normal breathing works:

• You breathe in air through your nose and mouth. The air travels down through your windpipe (trachea) through large and small tubes in your lungs called bronchial (BRON-kee-ul) tubes. The larger tubes are bronchi (BRONK-eye), and the smaller tubes are bronchioles (BRON-kee-oles). Sometimes, we use the word "airways" to refer to the various tubes or passages that air uses to travel from the nose and mouth into the lungs. The airways in your lungs look something like an upside-down tree with many branches.
• At the ends of the small bronchial tubes, there are groups of tiny bubbles called air sacs or alveoli. The bubbles have very thin walls, and small blood vessels called capillaries are next to them. Oxygen passes from the air sacs into the blood in these small blood vessels. At the same time, carbon dioxide passes from the blood into the air sacs.

Effects of ARDS

In ARDS, the tiny blood vessels leak too much fluid into the lungs. This results from toxins (poisons) that the body produces in response to the underlying illness or injury. The lungs become like a wet sponge, heavy and stiffer than normal. They no longer provide the effective surface for gas exchange, and the level of oxygen in the blood falls. If ARDS is severe and goes on for some time, scar tissue called fibrosis may form in the lungs. The scarring also makes it harder for gas exchange to occur.

People who develop ARDS need extra oxygen and may need a breathing machine to breathe for them while their lungs try to heal. If they survive, ARDS patients may have a full recovery. Recovery can take weeks or months. Some ARDS survivors take a year or longer to recover, and some never completely recover from having ARDS