What triggers the release of renin from the juxtaglomerular cells?

The release of renin from the juxtaglomerular cells is primarily triggered by the macula densa sensing low sodium chloride (NaCl) concentration. The juxtaglomerular cells are located in the walls of the afferent arterioles in the kidneys and are sensitive to changes in blood flow and sodium concentration. When the macula densa detects a decrease in NaCl—as would occur with a reduction in renal blood flow or low volume states—it sends signals that stimulate the juxtaglomerular cells to release renin.

Renin is an enzyme crucial for the renin-angiotensin-aldosterone system (RAAS), which helps regulate blood pressure and fluid balance. By releasing renin, the body initiates a cascade leading to the formation of angiotensin II, which causes vasoconstriction and stimulates aldosterone secretion, ultimately restoring blood pressure and enhancing sodium reabsorption.

While sympathetic nervous system activity can also stimulate renin release, the primary and more direct trigger in this context is the macula densa's response to decreased sodium chloride concentration. This mechanism emphasizes the importance of sodium sensing in the regulation of blood pressure and demonstrates how the kidneys adapt to changes in hemodynamics.