Whys and wherefores of juxtaglomerular apparatus function

The juxtaglomerular apparatus is an anatomic structure which links the distal end of the thick ascending limb at the macula densa with the glomerular vascular pole. Specialized interstital cells and renin containing granular cells are located in the vascular hilum at this site. Evidence has accumulated that this connection is critical for local regulation of renin secretion and glomerular vascular tone via the tubuloglomerular feedback mechanism. The tubuloglomerular feedback mechanism maintains a constant chloride concentration at the macula densa at a set point determined by the volume state of the animal, a effect probably important for adjustment of renin secretion to changing salt balance. Evidence supporting these two regulatory roles is reviewed here.