Breathing
To help astronauts survive the forbidding environment of space, the Apollo spacecraft were designed with many safety features. The command and lunar modules protected the astronauts against such hazards as cosmic radiation, extremes of heat and cold, and micro-meteoroids. The Environmental Control Systems and the Crew Life Support Systems in the spacecraft provided the crews with oxygen, water, and food.
The atmosphere in the Apollo spacecraft was 100% oxygen, at a pressure of five pounds per square inch. The oxygen system constantly added fresh oxygen to the cabin to replenish that breathed by the crew.