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.
Causes of Peripheral Vascular Disease. The most common cause of PVD is peripheral artery disease, which is due to atherosclerosis. Fatty material builds up inside the arteries and mixes with calcium, scar tissue, and other substances.