Complex sentence: The highly selective aspartic protease renin is secreted from the juxtaglomerular apparatus, which is the only source of active renin,, although its precursor, prorenin, can be secreted by other tissues, such as the salivary glands, brain, heart and blood vessels.
Split into simpler sentences: The highly selective aspartic protease renin is secreted from the juxtaglomerular apparatus. The juxtaglomerular apparatus is the only source of active renin,, although its precursor, prorenin can be secreted by other tissues, such as the salivary glands, brain, heart and blood vessels.