In physiology, dead space is the volume of air which is inhaled that does not take part in the gas exchange, either because it (1) remains in the conducting airways, or (2) reaches alveoli that are not perfused or poorly perfused. In other words, not all the air in each breath is available for the exchange of oxygen and carbon dioxide.

Benefits do accrue to a seemingly wasteful design for ventilation that includes dead space.[1][citation needed] Fish waste none of their ventilation because they exchange gases from a continuous unidirectional stream of water that crosses their gills. Mammals breathe in and out of their lungs, wasting that part of the inspiration which remains in the conducting airways where no gas exchange can occur.

Carbon dioxide is retained, making a bicarbonate-buffered blood and interstitium possible. Inspired air is brought to body temperature, increasing the affinity of hemoglobin for oxygen, improving O2 uptake. Particulate matter is trapped on the mucus that lines the conducting airways, allowing its removal by mucociliary transport. Inspired air is humidified, improving the quality of airway mucus.[1] About a third of every resting breath has no change in O2 and CO2 levels. In adults, it is usually in the range of 150 mL.

Dead space can be increased (and better envisioned) by breathing through a long tube, such as a snorkel. Even though one end of the snorkel is open to the air, when the wearer breathes in, they inhale a large quantity of carbon dioxide that remained in the snorkel from the previous exhalation. Thus, a snorkel increases the person's dead space by adding even more "airway" that doesn't participate in gas exchange.

Anatomic dead space[]

A different maneuver is employed in measuring anatomic dead space: the test subject breathes all the way out, inhales deeply from a 0% nitrogen gas mixture (usually 100% oxygen) and then breathes out into equipment that measures nitrogen and gas volume. This final exhalation occurs in three phases. The first phase has no nitrogen, and is the air that entered the lung only as far as the conducting airways. The nitrogen concentration then rapidly increases during the brief second phase and finally reaches a plateau, the third phase. The anatomic dead space is equal to the volume exhaled during the first phase plus half that exhaled during the second phase. (The Bohr equation is used to justify the inclusion of half the second phase in this calculation.[2][3]


  1. Williams, R; Rankin, N; Smith, T; Galler, D; Seakins, P (1996 Nov). "Relationship between the humidity and temperature of inspired gas and the function of the airway mucosa.". Critical Care Medicine 24 (11): 1920–9. PMID 8917046
  2. Fowler W.S. (1948). "Lung Function studies. II. The respiratory dead space". Am. J. Physiol 154: 405–416.
  3. Heller H, Könen-Bergmann M, Schuster K (1999). "An algebraic solution to dead space determination according to Fowler's graphical method". Comput Biomed Res 32 (2): 161–7. doi:10.1006/cbmr.1998.1504. PMID 10337497