Final answer:
The macula densa cells respond to changes in sodium ion concentration, playing a crucial role in the tubuloglomerular feedback mechanism that regulates renal blood flow and glomerular filtration rate to maintain fluid and electrolyte balance.
Step-by-step explanation:
The macula densa cells are specialized cells located in the distal convoluted tubule (DCT) of the nephron, forming part of the juxtaglomerular apparatus (JGA). They are primarily responsible for sensing changes in sodium ion (Na+) concentration in the forming urine. When sodium levels are high, the macula densa cells trigger the release of chemical messengers that cause arteriolar vasoconstriction, thus reducing the glomerular filtration rate (GFR) to promote sodium retention. When sodium levels are low, they facilitate vasodilation, increasing GFR to allow more sodium to be excreted. This system is part of the tubuloglomerular feedback mechanism, which is critical for regulating renal blood flow and glomerular filtration, thereby maintaining body fluid homeostasis and electrolyte balance.