What is "physiologic dead space"

n. 1 (context obsolete English) The volume of the conducting airways of the lung. This definition became inadequate once the ''alveolar dead space'' was described. 2 (context physiology English) That volume of air in the lung which does not partake in gas exchange. 3 (context physiology English) That volume of air in the lung that does not eliminate carbon dioxide, as defined functionally by the http://en.wikipedia.org/wiki/Bohr%20equation. 4 (context physiology English) The sum of the anatomic plus the alveolar dead spaces. 5 (context physiology English) The sum of the parallel and the serial dead spaces.