Aerodynamic Pressure Loads in FEA of Lifting Surfaces - Query
There is fuel inside the wing, and there is definitely a connection with the outside atmosphere for venting while fuel is flowing into the engines. The fuel tanks are sealed from the outside airflow. In a document about the B737NG from Smartcockpit.com (now via Scribd) it says:


  The purpose of the fuel vent system is to prevent damage to the wings due to excessive buildup of positive or negative pressure inside the fuel tanks and to provide ram air pressure within the tanks. The tanks are vented into surge tanks which vent through a single opening at each wing tip.


A recent development, long recommended by the NTSB but deemed too expensive to mandate by the FAA, from this site:


  Boeing has developed a Nitrogen Generating System (NGS) which decreases the flammability exposure of the center wing tank to a level equivalent to or less than the main wing tanks. The NGS is an onboard inert gas system that uses an air separation module (ASM) to separate oxygen and nitrogen from the air. After the two components of the air are separated, the nitrogen enriched air (NEA) is supplied to the center wing tank and the oxygen enriched air (OEA) is vented overboard. NEA is produced in sufficient quantities, during most conditions, to decrease the oxygen content to a level where the air volume ... will not support combustion. The FAA Technical Center has determined that an oxygen level of 12% is sufficient to prevent ignition,