A good question! Having served on modern nuclear submarines for 20 years, I'm not as well versed on WWII diesel boats as some people may be, but I'll give it my best shot: The submarines of WWII were really more like temporarily submersible surface ships. They were designed to run on the surface using her diesels from point A to point B and then quickly submerge to either attack the enemy or evade the enemy. Submerged operations were conducted on batteries and were of very limited duration. Air was generally taken in while on the surface and once submerged, what was inside the hull was all they had available. Some provision were made for emergency chemical air purification to remove carbon dioxide, but that was pretty much it. Once the snorkel was put into use, it became feasable for diesel boats to run their diesels while submerges at periscope depth, as well as ventilate the sub underwater. Modern nuclear subs make all the oxygen they need from the sea water by electrolysis and store it on oxygen banks for use. Also, the air is continually purified to remove carbon monoxide and carbon dioxide. They never have to surface for air.