What is Macula Densa?

The macula densa is a specialized structure located in the kidneys, specifically in the juxtaglomerular apparatus (JGA). The JGA is found at the point where the afferent arteriole, which brings blood to the glomerulus, comes into contact with the distal convoluted tubule (DCT) of the nephron.

The macula densa consists of a group of closely packed specialized cells within the wall of the DCT. These cells are densely packed with nuclei, giving rise to its name "macula densa," which means "dense spot" in Latin. The macula densa cells are columnar epithelial cells that are taller and narrower than the adjacent DCT cells.

The primary function of the macula densa is to sense changes in the concentration of sodium chloride (NaCl) in the fluid passing through the DCT. They act as chemoreceptors that monitor the composition of the filtrate and play a crucial role in regulating renal blood flow and glomerular filtration rate (GFR).

When the macula densa cells detect a decrease in the NaCl concentration, they release chemical signals that interact with the nearby juxtaglomerular cells, which are specialized smooth muscle cells in the wall of the afferent arteriole. This interaction triggers the release of renin, an enzyme involved in the renin-angiotensin-aldosterone system (RAAS). The activation of the RAAS system ultimately leads to vasoconstriction, increased blood pressure, and increased reabsorption of sodium and water in the kidneys.

Overall, the macula densa plays a critical role in the regulation of renal function, maintaining proper fluid balance, and influencing blood pressure.