A fall in GFR activates

Solution:

A fall in Glomerular Filtration Rate (GFR) indicates reduced blood flow to the kidneys. This is detected by the juxtaglomerular apparatus (JGA). Specifically, the macula densa cells sense the decreased flow and signal the juxtaglomerular (JG) cells (also called granular cells) in the afferent arteriole. The JG cells then release the enzyme renin. Renin initiates the Renin-Angiotensin-Aldosterone System (RAAS), which helps to restore blood pressure and GFR. Note that the hormone released is renin, not rennin (which is a milk-curdling enzyme). Therefore, JG cells release renin upon a fall in GFR.