Final answer:
Walter Cannon is the American Physiologist who developed the four postulates of homeostasis and coined the term "fight or flight" response. He detailed this concept in his work on the physiology related to stress and described how it helps maintain homeostasis. The response is a fundamental part of the sympathetic nervous system's way of dealing with threats to the organism's internal balance.
Step-by-step explanation:
The American Physiologist who developed four postulates of homeostasis and coined the term "fight or flight" response is Walter Cannon. Working at Harvard in 1915, Cannon expanded upon French ideas of homeostasis and the functioning of the sympathetic nervous system. He identified the fight-or-flight response as an animal's instinct to respond to threats by standing to fight or to run away, a concept he explained in his book on the physiology of pain, hunger, fear, and rage. This adaptive mechanism is a part of the sympathetic nervous system's response, enabling organisms to stabilize their internal environment, such as blood pressure and temperature, in the face of stressors.
The fight-or-flight response is a physiological reaction to stress, where the body prepares for a quick, energetic response required for survival. This has evolved as a way to maintain homeostasis when faced with environmental threats, which may not be large predators in the modern world, but can still trigger the same autonomic nervous system response. While Cannon's phrase has been broadened in recent discussions about the sympathetic nervous system to include states such as fright or freeze, his contribution remains a fundamental concept in understanding physiological responses to stress.