What is the initial substance in the renin-angiotensin-aldosterone system (RAAS)?

In the renin-angiotensin-aldosterone system (RAAS), the initial substance is angiotensinogen. This is a precursor protein produced and released by the liver. When blood pressure drops or there is a decrease in sodium chloride concentration detected by the kidneys, renin is released from the juxtaglomerular cells of the kidneys. Renin then catalyzes the conversion of angiotensinogen into angiotensin I.

Angiotensin I itself is an inactive precursor and must be converted into angiotensin II by the action of angiotensin-converting enzyme (ACE) primarily in the lungs. Angiotensin II is the active component of the system and has multiple effects on blood vessels and fluid balance, ultimately leading to increased blood pressure.

Understanding this sequence is essential because it highlights the role of angiotensinogen as the starting point for the activation of the RAAS, which is critical for regulating blood pressure and fluid homeostasis in the body.