Renin is an enzyme which catalyses the conversion of 

JGA is a special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent
arteriole at the location of their contact. Columnar tubule cells in this region are crowded together, and so are known as the macula densa. Along with the macula densa, the wall of the afferent arteriole contains modified smooth muscle fibers called juxtaglomerular cells (JG). A fall in GFR can activate the JG cells to release renin which can stimulate the glomerular blood flow and thereby the GFR back to normal.