The adrenal medulla develops from neural tissue and secretes two hormones, epinephrine and norepinephrine. These two hormones are secreted in response to stimulation by sympathetic nerve, particularly during stressful situations. A lack of hormones from the adrenal medulla produces no significant effects.
While the sinoatrial node typically regulates the heartbeat during normal sinus rhythm, PACs occur when another region of the atria depolarizes before the sinoatrial node and thus triggers a premature heartbeat.
There are a variety of triggers that can cause leukocytoclastic vasculitis, though in many cases, the problem arises spontaneously with no identifiable cause. Often it appears to be caused by a misguided immune response, where the body mistakenly sees its own blood vessels as an intruder and attacks them.