of subassembly 12 answers this need valve 14 responds to an opening relatively low pressure differential of approximately 0 5 cm h 2 o in the particular embodiment illustrated and it functions completely independent of any fluid present in either the fluid collection chambers or the u tube the full line position of fig1 is the normal operating condition of the system assuming some air is being evacuated from the patient 39 s chest cavity fluids mostly blood enter the system through drainage tube 66 when they are collected in chambers 26 after having passsed through the u tube the air in turn while passing through the fluid in the u tube bypasses any fluid collected in the fluid collection chambers and instead exits the latter through check valve 14 directly into air chamber 20 under normal operating conditions a negative pressure environment will exist in chamber 20 due to the vacuum being drawn therein by vacuum line 22 or alternatively this line may be left open to the atmosphere for gravity operation positive pressure relief valve 16 in the top of chamber 20 will normally remain closed when maintaining a sub atmosphereic pressure in chamber 20 the fluids and air are aspirated from the patient without he or she having to exert.