Final answer:
Macula densa cells in the juxtaglomerular apparatus monitor sodium ion (Na+) concentration in the tubular fluid to regulate kidney function through tubuloglomerular feedback.
Step-by-step explanation:
The macula densa cells continuously monitor the sodium ion (Na+) concentration in the tubular fluid. These cells are located in the distal convoluted tubule (DCT) and form part of the juxtaglomerular apparatus (JGA) in the kidney. These specialized cells are located in a part of the distal convoluted tubule known as the juxtaglomerular apparatus (JGA) and they play a critical role in the regulation of kidney function through a process known as tubuloglomerular feedback. This feedback mechanism modulates the glomerular filtration rate (GFR) by causing vasoconstriction or vasodilation of the afferent and efferent arterioles based on changes detected in sodium concentration, and thereby, balances body fluid homeostasis.