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.
Premature ventricular contractions (PVCs, PVC) are premature heartbeats originating from the ventricles of the heart. Premature ventricular contractions are premature because they occur before the regular heartbeat. The heart has four chambers. The upper two chambers are the atria, and the lower two chambers are the ventricles.
The reason PACs or PVCs can sometimes be felt as a skipped beat, is that the heart gets a premature signal to squeeze before it has had time to fill with blood. The resulting heart contraction does not produce blood flow to the body. A pause is felt, and the following heart beat is more forceful than usual. If the PAC or PVC is less premature, and the heart has had time to fill with blood before receiving the early signal to squeeze, an extra beat will be felt rather than a skipped beat. Premature beats are common in healthy people of all ages.