Final answer:
The brain prompts a person to breathe again to prevent a dangerous buildup of carbon dioxide, not oxygen, which is more closely monitored to maintain blood gas and pH homeostasis.
Step-by-step explanation:
When a person has been holding his or her breath for a long time, the brain triggers an automatic response to resume breathing. This response is not due primarily to the lack of oxygen, but rather to the buildup of carbon dioxide (CO2) in the bloodstream. The accumulation of CO2 is closely monitored by cells in the brain that control breathing through the autonomic nervous system. When CO2 levels rise, these cells signal for the breathing rate to increase, allowing the body to expel the excess CO2 and restore blood gas and pH balance. As such, before reaching a dangerous level of CO2, the human body will involuntarily start breathing again to prevent harmful effects.