The stage of the human response to a life-threatening situation characterized by muscle tightening, quickening pulse, and adrenaline flow is called:

The stage of the human response to a life-threatening situation characterized by muscle tightening, quickening pulse, and adrenaline flow is best described by the term often referred to as the "fight or flight" response. This physiological reaction prepares an individual to either confront or flee from a perceived threat.

During this response, the body's sympathetic nervous system is activated, leading to various physical changes: muscle tension increases, heart rate accelerates, and there is a surge of adrenaline, also known as epinephrine, which enhances physical abilities and heightens awareness. This response is instinctual and has evolved to enhance survival by enabling quick reactions in dangerous situations.

The term "reaction" might seem relevant but lacks the specificity and recognition of the physiological and psychological changes that occur in such life-threatening scenarios. Panic, while it may involve heightened emotional responses to a threat, does not accurately encapsulate the organized physical responses present in the fight or flight scenario. Lastly, "calm" does not correspond to the intense physiological responses that occur in life-threatening situations.