In the human body, what is the principal physiological function of the enzyme renin?

The enzyme renin is secreted by pericytes in the vicinity of the afferent arterioles and similar microvessels of the kidney from specialized cells of the juxtaglomerular apparatus—the juxtaglomerular cells, in response to three stimuli:

1) A decrease in arterial blood pressure (that could be related to a decrease in blood volume) as detected by baroreceptors (pressure-sensitive cells). This is the most direct causal link between blood pressure and renin secretion (the other two methods operate via longer pathways).
2) A decrease in sodium load delivered to the distal tubule. This load is measured by the macula densa of the juxtaglomerular apparatus.
3) Sympathetic nervous system activity, which also controls blood pressure, acting through the beta 1 adrenergic receptors.
Human renin is secreted by at least 2 cellular pathways: a constitutive pathway for the secretion of the precursor prorenin and a regulated pathway for the secretion of mature renin.