A fall in glomerular filtration rate (GFR) activates

Juxtaglomerular apparatus consists of macula densa and juxtaglomerular cells. The apparatus becomes active when there is decrease in renal blood pressure or decrease in glomerular filtration rate. There is increased absorption of $\mathrm{NaCl}$ in ascending limb or loop of Henle. The decreased $\mathrm{NaCl}$ content of filtrate activates cells of macula densa to increase secretion of renin from juxta glomerular cells. Renin convert angiotensinogen to angiotensin peptides.