The juxtaglomerular apparatus. Integration of the regulated secretion of renin is carried out at the JGA. There are three major pathways regulating the secretion of renin by granular cells at the JGA: the baroreceptor, the macula densa mechanism, and direct stimulation by the sympathetic nervous system. The renal baroreceptor monitors renal perfusion pressure and signals an increase in renin when renal perfusion pressure falls. In the macula densa mechanism, macula densa cells sense the decrease in chloride ions in the filtrate in the distal tubule, thereby stimulating release of renin. Increased activity of renal sympathetic nerves directly stimulates renin release via activation of β adrenergic receptors. Sympathetic innervation also modulates both the baroreceptor and macula densa mechanisms.