The kidneys regulate blood pressure primarily by controlling fluid volume and driving the RAAS system. They manage how much sodium and water are reabsorbed or excreted, directly shaping extracellular fluid volume and thus blood pressure. The renin-angiotensin-aldosterone system starts with the kidneys: juxtaglomerular cells release renin in response to low renal perfusion, low salt delivery to the macula densa, or sympathetic stimulation. Renin cleaves angiotensinogen to angiotensin I, which is converted to angiotensin II (largely by ACE in the lungs). Angiotensin II causes vasoconstriction and stimulates aldosterone release from the adrenal cortex. Aldosterone promotes sodium and water reabsorption in the distal tubules, increasing circulating volume and raising blood pressure. While the heart and brain influence blood pressure through cardiac output and autonomic regulation, and the lungs contribute ACE for angiotensin II formation, the kidneys are the central organ controlling both fluid volume and the RAAS, making them the primary regulator in this context.