<P> The cells of the macula densa are taller and have more prominent nuclei than surrounding cells of the distal straight tubule (cortical thick ascending limb). </P> <P> The close proximity and prominence of the nuclei cause this segment of the distal tubule wall to appear darker in microscopic preparations, hence the name macula densa . </P> <P> Macula densa cells sense changes in sodium chloride level, and will trigger an autoregulatory response to increase or decrease reabsorption of ions and water to the blood (as needed) in order to alter blood volume and return blood pressure to normal . </P> <P> A decrease in afferent arteriole diameter causes a decrease in the GFR (glomerular filtration rate), resulting in a decreased concentration of sodium and chloride ions in the filtrate and / or decreased filtrate flow rate . Reduced blood pressure means decreased venous pressure and, hence, a decreased peritubular capillary pressure . This results in a smaller capillary hydrostatic pressure, which causes an increased absorption of sodium ions into the vasa recta at the proximal tubule . </P>

As a consequence of reduced blood volume the macula densa