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.
Kidneys
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)