Macula densa

In the kidney, the macula densa is an area of closely packed specialized cells lining the wall of the cortical thick ascending limb of the loop of Henle, at the transition to the distal convoluted tubule.

The cells of the macula densa are sensitive to the concentration of sodium chloride in the distal convoluted tubule . 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 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.

As such, an increase in sodium chloride concentration would result in vasoconstriction of afferent arterioles, and reduced paracrine stimulation of juxtaglomerular cells. This demonstrates the macula densa feedback, where compensatory mechanisms act in order to return GFR to normal.

The release of renin is an essential component of the renin–angiotensin–aldosterone system (RAAS), which regulates blood pressure and volume.

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).

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.

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.

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.

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