High blood pressure, also called hypertension, describes a condition that occurs when the pressure of the blood against the walls of the blood vessels increases to a level that can cause damage to the arteries and eventually the organs. The American Heart Association reports that a systolic pressure of 140 mmHg and a diastolic pressure of 90 mmHg indicates high blood pressure and requires treatment. When changes in diet and exercise fail to lower blood pressure alone, doctors prescribe medications, such as diuretics, which induce the kidneys to remove excess water and salts from the body.
Thiazide Diuretics
Doctors often recommend a thiazide diuretic as the first type of medication to try to lower blood pressure, according to MayoClinic.com. Thiazide diuretics, such as chlorthalazone and metolazone, inhibit the reabsorption of sodium in the distal tubule part of the kidney. Although this section only reabsorbs 5 percent of the sodium, inhibition here can effectively decrease blood pressure. In addition to the wanted effect of inhibit sodium reabsorption, thiazide diuretics also inhibit potassium reabsorption. This leads to a decrease in potassium levels that can lead to hypokalemia---a condition of low potassium that can negatively affect the heart, kidneys and blood pressure.
Loop Diuretics
Loop diuretics inhibit the absorption of salts in the portion of the kidney known as the thick ascending limb that follows the loop of henle. Because 25 percent of sodium reabsorption takes place in this section, loop diuretics remove more salts, and therefore water, than thiazide diuretics. Loop diuretics induce large amounts of salt and water to be excreted in a short amount of time, which can cause side effects such as weakness, lethargy, cramps and dizziness. Therefore, doctors typically only prescribe loop diuretics for hypertension in patients who have lost more than 50 percent of their kidney function, according to the Merck Manual.
Potassium-Sparing Diuretics
Potassium-sparing diuretics provide the advantage of not affecting the level of potassium. These medications, such as amiloride and triamterene, interfere with the functions of aldosterone---a hormone---in the kidney which allows more sodium to pass into the collecting duct to be excreted. Although they keep potassium levels up, they are not as effective in lowering blood pressure as the other types of diuretics. Therefore, doctors often prescribe potassium-sparing diuretics along with another diuretic to increase the effect on blood pressure while keeping an adequate level of potassium in the blood.
