In the kidney, the macula densa is an area of closely packed specialized cells lining the wall of the distal tubule where it touches the glomerulus.

A decrease in sodium chloride concentration initiates a signal from the macula densa that has two effects: (1) it decreases resistance to blood flow in the afferent arterioles, which raises glomerular hydrostatic pressure and helps return the glomerular filtration rate (GFR) toward normal, and (2) it increases renin release from the juxtaglomerular cells of the afferent and efferent arterioles, which are the major storage sites for renin.

The close proximity and prominence of the nuclei cause this segment of the distal tubule wall to appear darker in microscopic preparations,[5] hence the name macula densa.

This results in a smaller capillary hydrostatic pressure, which causes an increased absorption of sodium ions into the vasa recta at the proximal tubule.

Damage to the macula densa would impact blood flow to the kidneys because the afferent arterioles would not dilate in response to a decrease in filtrate osmolarity and pressure at the glomerulus would not be increased.