S Your new gas furnace may have been installed in one of two. ways, as a direct-vent (2-pipe-Fig. 2) application or as a. non-direct vent (1-pipe-Fig. 3) application. S In a direct-vent (2-pipe) application,your furnace uses air. from outside the home for combustion and ventsflue gas to the. outdoors.
For the biological concept, see Anti-predator adaptation. A defence mechanism is an unconscious psychological mechanism that reduces anxiety arising from unacceptable or potentially harmful stimuli. Sigmund Freud was one of the first proponents of this construct.
The body possesses many mechanisms that impart nonspecific defense. The objectives of these mechanisms are to prevent microorganisms from gaining a foothold in the body and to destroy them if they penetrate to the deeper tissues. Mechanical barriers. Mechanical barriers at the portal of entry represent the first line of defense for the body.