The given blank can be filled with the autonomic nervous system.
The ANS or autonomic nervous system is a part of the peripheral nervous system, which supplies glands and smooth muscles and thus affects the activity of internal organs.
The ANS refers to a control system, which functions majorly instinctively and monitors the functions of the body like digestion, heart rate, pupillary reaction, respiratory rate, sexual arousal, and urination. This the main mechanism system in control of the fight-or-flight reaction. Thus, it helps one to flee or fight from dangerous conditions.