When water and sodium build up in the body, the body (especially the extremities) swells up, causing a condition known as edema. The excess water and salt also play a part in constricting blood vessels, hampering the passage of oxygen-rich blood. Diuretics are sometimes prescribed as a means of prompting a brief period (generally, several hours) of increased urination, ridding the body of the unneeded water and sodium. This relaxes the blood pressure walls, lowering blood pressure. It's an especially effective means of offering relief to those who have swelling in their legs. Diuretics are usually the first thing doctors reach for when treating high blood pressure.