Which organ primarily regulates blood pressure by controlling fluid volume and the RAAS system?

• A. Kidneys
• B. Heart
• C. Brain
• D. Lungs

Answer: (A)

The kidneys are the main regulator of blood pressure through fluid volume control and the RAAS system. When they sense reduced blood flow or sodium delivery, the juxtaglomerular cells release renin, triggering the RAAS cascade. Renin converts angiotensinogen to angiotensin I, which is converted to angiotensin II (primarily by ACE in the lungs). Angiotensin II causes vasoconstriction and stimulates aldosterone release, promoting sodium and water retention in the distal nephron. This increases extracellular fluid volume and raises blood pressure, providing the long-term regulation of BP. While the heart, brain, and lungs influence BP in other ways (cardiac output, autonomic tone, and ACE activity), the kidneys are the key organ governing fluid volume and RAAS-driven BP control.