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.
However, if PVCs occur too frequently (more than several times per minute), the heart is unable to pump an adequate volume of blood to the body, which can cause symptoms such as weakness, fatigue, palpitations, or low blood pressure. Other types of rhythm problems can have similar effects. Problems may occur when the heart beats too fast, too slow, or with an irregular rhythm. But whether the heart is beating too fast, too slow, or too irregularly, the effects are often the same as one or more of those described above.