Final answer:
The macula densa is a cluster of modified epithelial cells in the distal convoluted tubule or juxtaglomerular cells that monitor fluid composition and release paracrine signals in response to changes in flow rate and sodium concentration. It is part of the juxtaglomerular apparatus (JGA) and plays a role in regulating the glomerular filtration rate and blood pressure.
Step-by-step explanation:
The cluster of modified epithelial cells in the distal convoluted tubule or juxtaglomerular cells is known as the macula densa. These cuboidal epithelial cells monitor the fluid composition in the tubule and release paracrine signals, such as adenosine triphosphate (ATP) and adenosine, in response to changes in flow rate and sodium concentration. The macula densa, along with the juxtaglomerular cells, form the juxtaglomerular apparatus (JGA), which plays a role in regulating the glomerular filtration rate and blood pressure.