Craig C. Freudenrich
Have you ever been startled? Your heart races. You breathe rapidly. Your muscles tense. These are aspects of the fight-or-flight response. This response literally prepares your body to fight a danger or run from it. The fight-or-flight response is the first stage of a coordinated response called the stress response. The stress response involves the sympathetic nervous system, hypothalamus, pituitary gland, adrenal gland, liver and thyroid gland, which all communicate with each other by chemical messages called hormones.
In stage one, also known as fight or flight, your sympathetic nervous system sends signals to multiple organs. First, they stimulate your heart and blood vessels by secreting a chemical called norepinephrine. Norepinephrine increases your heart rate, blood pressure, and blood flow to various organs, especially skeletal muscles. Second, they stimulate the lungs to expand the airways and increase the rate of breathing, which brings more oxygen into your body. Next, they stimulate the adrenal gland to secrete a hormone called epinephrine, which stimulates the heart, expands the blood vessels, and breaks down glycogen stores in the liver for energy. But stage one cannot be kept up for long.
During tage two, also known as resistance, the hypothalamus sends signals to the pituitary to secrete several hormones. Adrenocorticotropic hormone acts on the adrenal gland to secrete cortisol. Human growth hormone acts on the liver to secrete insulin-like growth factors. Thyroid stimulating hormone acts on the thyroid gland to secrete thyroid hormones. These latter hormones cause the liver to break down glycogen and release glucose, fat tissue to break down fats and release fatty acids, and skeletal muscle to break down protein and release amino acids. The glucose, fatty acids and amino acids travel through the blood to many other cells, which use them for energy. This stage provides the energy for sustained responses.
And finally, during stage three, also known as exhaustion, prolonged exposure to cortisol (or repeated exposures) depletes your body’s energy stores, wastes muscles away and suppresses the immune system. Your body can no longer respond to stress; it becomes weakened susceptible to disease. Typically, most everyday stresses do not reach stage three.
The fight-or-flight response happens the moment you find yourself in a tense situation. In the brain, your hypothalamus begins the chain reaction that will enable you to either "run away" from the danger or turn to "fight" it, metaphorically speaking (or sometimes literally!). The hypothalamus controls the pituitary gland. This gland links the nervous system (communications) with the endocrine system (hormone central, in our bodies). Once these two systems are speaking to each other, you will feel an adrenaline rush because the endocrine system releases hormones.
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