Final answer:
In an open-circuit scuba system, divers use approximately 5% of the oxygen from the tank with each breath, releasing the rest into the water, but actual oxygen usage varies based on depth and exertion.
Step-by-step explanation:
In an open-circuit scuba diving system, the exhaled gas is released into the environment and not reused. Exhaled air from a diver at rest typically contains about 16% oxygen, down from the roughly 21% oxygen concentration found in atmospheric air. Given that atmospheric air is commonly used in scuba tanks, this suggests that approximately 5% of the oxygen is being utilized. However, due to variations in depth, exertion levels, and individual breathing rates, the actual percentage of oxygen used from a tank can vary. It’s also important to consider the air is not composed solely of oxygen; only about 21% of the tank's volume is oxygen, with the majority being nitrogen.