Q. How do kidneys participate in the blood volume control? How is the blood volume of the body related to the arterial pressure?

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 major groups of antihypertensive drugs is the diuretics. Doctors frequently prescribe diuretics for the hypertensive patients to excrete more water and thus lower their blood pressure.