The kidneys and the hormones that act upon them are the main physiological regulators of the total blood volume of the body. As more water is resorbed in the nephron tubules the more the blood volume increases; as more water is excreted in urine the more the blood volume lowers.
The blood volume in its turn has a direct relation to blood pressure. The blood pressure increases when the blood volume increases and it lowers when the blood volume lowers. That is the reason why one of the main groups of antihypertensive drugs is the diuretics. Doctors often prescribe diuretics for the hypertensive patients to excrete more water and thus lower their blood pressure.
