In the RAAS system, what substance is converted into angiotensin I?

Angiotensinogen is the precursor protein that is converted into angiotensin I in the renin-angiotensin-aldosterone system (RAAS). When blood pressure drops or there is decreased blood flow to the kidneys, the juxtaglomerular cells of the kidneys release renin, an enzyme that acts on angiotensinogen, which is produced mainly by the liver. The conversion process transforms angiotensinogen into angiotensin I, which is then further processed by the enzyme angiotensin-converting enzyme (ACE) into angiotensin II.

Recognizing the correct sequence in the RAAS is essential for understanding its physiological role in regulating blood pressure and fluid balance. Other options, such as renin and angiotensin II, are active components within the system but do not serve as the initial substance that is converted. Aldosterone is a hormone produced downstream as a result of angiotensin II's action, but it is not involved in the initial conversion of angiotensinogen. Understanding this hierarchy of interactions helps clarify how the RAAS functions as a crucial system in cardiovascular and renal physiology.