Here's some rough neuroscience: the brain is composed of three regions, each corresponding to a stage of our species' evolution. Let's just say we have three brains: a reptilian brain, a mammalian brain, and a human brain. The reptilian brain generates basic instinctual impulses, the mammalian brain adds emotion and memory into the mix [1] , and the human brain gives us all that make us unique amongst mammals, of which the most commonly discussed feature is our capacity for abstraction and symbolic information processing.
While the entire brain is involved in the experience of fear, it's the limbic system, or the mammalian brain, that triggers a range of physiological and neurological changes when your being perceives a threat. These changes include activation of your sympathetic nervous system, which is hardware dedicated to saving your ass. It slows digestion to save energy and redirects blood to major muscle groups; it draws blood away from the surface of your skin to avoid blood loss in case of injury;