将下列英语段落翻译成汉语：When flying in high altitudes without pressurization, especially above 4000 meters, people will feel a certain hypoxia (lack of oxygen), or even lose consciousness. The pressurized areas are the cockpit, the cabin, the cargo hold and avionics bay. The pressurization of the modern aircraft is regulated by the compressed air. One of the most common sources of compressed air is bleed air from the engines. Another source of compressed air is the bleed air from APU (Auxiliary Power Unit).