22nd June 2023

The RAAS

Renin is an enzyme synthesised and stored in an inactive form (prorenin) in juxtaglomerular cells in the juxtaglomerular apparatus (JGA), a structure adjacent to the glomerulus which regulates the function of each nephron. The JGA is made up of the macula densa (part of the DCT of the nephron), juxtaglomerular cells, and extraglomerular mesangial cells.

Prorenin is cleaved to form renin is released in response to:

• Decreased sodium delivery to the DCT (detected by the macula densa).
• Decreased perfusion pressure to the kidney (detected by baroreceptor in the afferent arteriole).
• Sympathetic stimulation of the juxtaglomerular apparatus.

Renin acts enzymatically on angiotensinogen (a precursor protein produced in the liver) to form angiotensin I. Angiotensin I is converted to angiotensin II by angiotensin converting enzyme (ACE). ACE is largely produced in the endothelium of lung vessels and thus this conversion largely occurs in the lungs. ACE is produced in much smaller quantities in the renal endothelium.