Explain the renin-angiotensin mechanism.

The renin-angiotensin mechanism is the most important trigger for the release of aldosterone. It is mediated by the juxtaglomerular (JG) apparatus of the renal tubules. The JG apparatus consists of modified smooth muscle cells that are stimulated by low blood pressure within the afferent arteriole or changes in solute content of the filtrate. The JG cells respond to these changes by releasing renin into the blood. Renin catalyzes reactions that lead to angiotensin II production, which then acts directly on the blood vessels to cause vasoconstriction as well as aldosterone release. Aldosterone then causes the reabsorption of sodium and water, leading to increased blood volume and blood pressure.