The air you exhale is lower in oxygen and higher in carbon dioxide than it was when you inhaled it. Some of this air remains in your windpipe, throat, and mouth after you exhale. When you inhale, you breathe the air from this dead air space before any fresh air enters your lungs.

Divers whose breathing pattern is too shallow do not inhale enough fresh air after inhaling the air from this dead air space. As a result, their carbon dioxide levels continue to increase. This signals the body to breathe more often, and can lead to breathlessness or feelings of air starvation.

For this reason, you should maintain a deeper breathing pattern when diving. This allows more efficient delivery of oxygen to your lungs, and helps maintain a safe carbon dioxide level in your bloodstream.