The sympathetic nervous system initiates the fight-or-flight response each time we have a thought of potential or actual danger or pain. The text lists numerous physiological changes that help increase power, speed, and strength, including increased central nervous system activity and mental activity; increased secretion of adrenaline, noradrenaline, and cortisol; increased cardiac output; increased oxygen consumption; increased muscle contractions; and many more.The parasympathetic nervous system returns the physiology to a state of homeostasis after the threat, danger, or potential pain is no longer perceived to be imminent. The parasympathetic branch is responsible for counterbalancing the body's sympathetic activity, which restores calm, promotes relaxation, and facilitates digestive functions, energy storage, and tissue repair and growth. Breathing is slow, as is the heart rate. Blood pressure and body temperature drop. In general, muscle tension decreases.