The juxtaglomerular cells (JGC) secretes an enzyme rennin in response to a variety of stimuli such as decreased flow of filtrate and NaCl to macula densa, decrease in glomerular filtration rate (GFR), decreased stretch of JGC, and increased frequency of nerve impulses in the renal sympathetic nerves.
Atrial Natriuretic Factor (ANF) decreases blood pressure as it is a powerful vasodilator.
Angiotensin II is a powerful vasoconstrictor.
The counter-current mechanism involves the exchange of water and electrolytes between blood flowing in vasa recta and filtrate in Loop of Henle.