hey, after reading ineligible's post, I realised I actually did know something about this from my psychology class.......The ANS (autonomic nervous system) has two "parts" - the parasympathetic and the sympathetic. When placed under stress, the sympathetic system comes into action and accelerates the heart rate, slows down digestion and other processes that can be withheld for a later date, constricts blood vessels, and raises blood pressure. It bascially, as Ineligible said, diverts more blood to the muscles that would need it.... and in turn it neglects other things including the penis, which is why people have sex difficulties when under alot of stress!!!