Which substance is secreted by the kidneys to initiate the renin-angiotensin-aldosterone system?

The correct answer is that renin is the substance secreted by the kidneys to initiate the renin-angiotensin-aldosterone system (RAAS). Renin is an enzyme released by the juxtaglomerular cells of the kidney in response to several stimuli, including low blood pressure, low sodium concentration, and sympathetic nervous system activation.

Once renin is secreted into the bloodstream, it catalyzes the conversion of angiotensinogen (a protein produced by the liver) into angiotensin I. This is a critical first step in the RAAS, as it sets off a cascade of reactions that ultimately lead to the production of aldosterone, which increases sodium and water reabsorption in the kidneys, promoting an increase in blood pressure.

Erythropoietin, although a hormone produced by the kidneys, primarily regulates red blood cell production and does not play a role in the RAAS. Angiotensinogen, while important in the RAAS, is not secreted by the kidneys; rather, it is produced in the liver. Aldosterone is the final product of the RAAS and acts on the kidneys to increase sodium reabsorption, but it is not involved in the initial signaling of the system