Blood Volume Regulation
One way in which the kidneys maintain blood pressure is through the regulation of the volume of blood in the body. As the American Heart Association explains, one of the major roles of the kidneys is maintaining the proper levels of electrolytes (such as sodium and potassium) in the body.
The amount of electrolytes in the body influences the amount of fluid in the body. When electrolyte levels are high, the body retains more water, which in turn increases the volume of the blood. More blood volume results in higher blood pressure. Thus, the kidneys maintain blood pressure by indirectly controlling the amount of blood in the body.
The kidneys also regulate blood pressure hormonally. In order to do this, the kidneys must directly monitor the blood pressure, which they do by measuring the amount of blood flow that the kidneys receive. As the Cardiovascular Physiology Concepts site explains, this function is performed by special renal cells known as the juxtaglomerular cells. These cells are located in the arteries that feed into the kidneys. When blood flow to the kidney is reduced, a hormone called renin is excreted. This system can inadvertently lead to high blood pressure if the arteries leading to the kidney get narrowed because the juxtaglomerular cells will interpret this as low blood pressure even though blood pressure throughout the body is normal (or even elevated).
Renin is a hormone that is produced by the kidneys and it acts to elevate the blood pressure. Renin is a protein which interacts with another protein, called angiotensinogen. When renin is secreted, it turns angiotensinogen into angiotensin I. Angiotensin I is then converted in the lungs into angiotensin II. Angiotensin II causes blood vessels to constrict, which raises the blood pressure. It also causes the kidneys to retain more sodium and water, which raises the volume of the blood.