{HEAD ~ The vent} acts to prevent ambient air and thus, oxygen from entering the inner passageway of the pipes of a system when positive pressure, such as that caused by high temperature and fire, is present in the inner passageway of {TAIL ~ the pipes} 
{ HEAD ~ The vent } acts to prevent ambient air and thus , oxygen from entering the inner passageway of the pipes of a system when positive pressure , such as that caused by high temperature and fire , is present in the inner passageway of { TAIL ~ the pipes }
HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL