In the RAAS response to low blood pressure, what is the initial release in the kidney?

• A. Antidiuretic hormone is released from the kidneys in response to low BP.
• B. Aldosterone is released directly in response to low BP.
• C. Juxtaglomerular cells release renin.
• D. Angiotensin II is produced first in the RAAS cascade.

Answer: (C)

When blood pressure drops, the kidney’s juxtaglomerular apparatus responds first by releasing renin. This makes renin the initiating step of the RAAS. Juxtaglomerular (JG) cells sense lower perfusion pressure and, along with signals from the macula densa that detect reduced NaCl flow, trigger renin secretion into the bloodstream. Renin then converts angiotensinogen (from the liver) into angiotensin I, which is quickly converted to angiotensin II. Angiotensin II acts to raise blood pressure by constricting blood vessels and stimulating aldosterone release from the adrenal cortex, leading to increased Na+ and water reabsorption. Antidiuretic hormone is released from the posterior pituitary, not the kidney, and aldosterone release is a downstream effect rather than the initial response. Angiotensin II is produced after renin acts, so it’s not the first trigger.