The blood pressure becomes elevated when larger than normal amounts of water (and sodium) are held in the body, a situation that invariably occurs during the alarm reaction to stress. In this case, the quantity of blood plasma, or the blood volume, increases. On the other hand, arteries can become smaller when tension causes the muscular walls to contract or when they are plugged with cholesterol, compressed in beds of fat, or shrunk by scar tissue that may be calcified. Most persistent high blood pressure results from a combination of these factors.