Diuretics, as a first-line drug for the treatment of hypertension, have been widely used. They can excrete sodium and diuresis by inhibiting the reabsorption of sodium and water by renal tubules, so that the excretion of sodium and water in human body exceeds the intake, so that the blood volume and extracellular fluid volume are reduced, and the cardiac output is reduced, thus achieving the antihypertensive effect. However, after 3 ~ 4 weeks of continuous medication, the blood volume and cardiac output have gradually recovered, but the blood pressure has remained persistently reduced. This is because there is a slight sodium deficiency in the body at this time, and there is a lack of sodium in the arteriole wall cells, which reduces the reactivity of arteriole smooth muscle to vasoconstrictors such as norepinephrine, thus causing vasodilation, reducing peripheral vascular resistance, and leading to a drop in blood pressure. At present, diuretics used to reduce blood pressure in clinic are divided into the following three categories according to their effects. A strong diuretic such as furosemide; Intermediate diuretics, such as thiazide diuretic dihydrochlorthiazide; Weak diuretics, such as potassium-preserving diuretics, triamterene, spironolactone, etc. The latter mainly forms competitive antagonism with aldosterone, which weakens its effect and leads to sodium excretion and potassium preservation. Hydrochlorothiazide is a diuretic with antihypertensive effect, so it is also called diuretic antihypertensive drug. Drugs such as cyclopentthiazide and chlorthiazone also belong to this category. Hydrochlorothiazide has a strong diuretic and detumescence effect by inhibiting the reabsorption of sodium and water by renal tubules. Is there any relationship between its antihypertensive effect and diuretic effect? Is hypotensive action the result of diuretic action? In the early years, it was thought that the hypotensive effect of hydrochlorothiazide was the result of diuresis. Because the drug can reduce the blood volume through diuresis, and the blood pressure can decrease when the blood volume decreases, and it also has the effect of expelling sodium, and the retention of sodium in the body can also increase the blood pressure. However, many experimental observations since then have gradually denied this view. For example, after taking hydrochlorothiazide, the blood volume lost due to diuresis is supplemented with dextran, which is a blood volume expander, and still shows antihypertensive effect. After taking hydrochlorothiazide for a long time, the blood volume gradually returned to the level before administration, but the antihypertensive effect still existed.