Dead space

In physiology, dead space is air that is inhaled by the body in breathing, but does not partake in gas exchange.

Not all the air we breath in is able to be used for the exchange of oxygen and carbon dioxide. About a third of every resting breath is exhaled exactly as it came into the body.

Dead space can be divided into two components:

• Anatomical dead space - the gas in the conducting areas of the respiratory system, such as the mouth and trachea, where the air doesn't come to the alveoli of the lungs.
• Alveolar dead space - the area in the alveoli that does get air to be exchanged, but there is not enough blood flowing through the capillaries for exchange to be effective.

Alveolar dead space is normally very small (less than 5 mL) in healthy individuals. It can increase dramatically in some lung diseases.

Anatomical dead space is normally about 150 mL (or 2.2 mL per kilogram of body weight). This is about a third of the resting tidal volume (500 mL).

Because of dead space, taking deep breaths more slowly (e.g. ten 500 mL breaths per minute) is more effective that taking shallow breaths quickly (e.g. twenty 250 mL breaths per minute). Although the amount of gas per minute is the same (5 L/min), a large proportion of the shallow breaths is dead space, and does not allow oxygen to get into the blood.

Dead space can be enlarged (and better envisaged) by breathing into a long tube. Even though one end of the tube is open to the air, when one inhales, it is mostly the carbon dioxide from expiration. Using a snorkel increases a diver's dead space in the airways (though usually not significantly).