Renin is a component of the Renin-Angiotensin-Aldosterone system. It is secreted from the kidneys, under a variety of stimuli. In the plasma, it splits angiotensin I off of angiotensinogen, which then undergoes further cleavage by angiotensin converting enzyme or ACE which is mostly in the lungs. This cleavage produces angiotensin II, a powerful vasoconstrictor.

In the kidneys, renin is secreted by the juxtaglomerular cells of the juxtaglomerular apparatus. This secretion can be caused by The Renal Sympathetic Nerves
These directly innervate the juxtaglomerular cells, and when active, cause secretion of renin. These nerves respond mostly to a decrease in cardiovascular pressures. The other two parts are totally contained within the kidneys. The Juxtaglomerular Cells sit in the wall of the afferent arteriole to a glomerular capillary. They have a very good sampling of systemic plasma, and are very sensitive to the pressure in the afferent arteriole. If the pressure is too low, indicating too little plasma, they secrete renin. Renin causes retention of sodium, and where sodium goes, water follows. The Macula Densa, however, are closer to the nephrons, where they sit near the ascending loops of Henle, in order to sense salt concentration. Too little salt is indicative of not enough filtration (GFR is too low). This would occur if arterial pressure was too low. Again, renin is released to help ensure that sodium is retained and therefore blood pressure is maintained. FYI, renin is a hormone.
These are my interpretation of my lecture notes, but I may have used some references from Hole's Anatomy and Physiology (Shier, Butler, Lewis) and Human Physiology (Vander, Sherman, Luciano)

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