To what special feature does macula densa refer?

Answer 1

In the kidney, the macula densa is an area of closely packed specialized cells lining the wall of the distal tubule at the point of return of the nephron to the vascular pole of its parent glomerulus glomerular vascular pole. The cells of the macula densa are sensitive to the ionic content and water volume of the fluid in the tubule. If low water volume is detected by these cells, they will produce molecular signals that promote renin secretion by other cells of the juxtaglomerular apparatus. The release of renin is an essential component of the renin-angiotensin-aldosterone system (RAAS), which regulates blood pressure and volume. http://en.wikipedia.org/wiki/Macula_densa