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.
However, there are different ways cancer can cause SVCS. A tumor in the chest may press on the superior vena cava. A tumor may grow into the superior vena cava, causing a blockage. If cancer spreads to the lymph nodes surrounding the superior vena cava, the lymph nodes may enlarge and press on or block the vein.