Final answer:
The macula densa is a cluster of cells located in the distal convoluted tubule of the kidney's nephrons, essential for regulating blood pressure and kidney function by monitoring Na+ levels and signaling juxtaglomerular cells to adjust the glomerular filtration rate. The correct answer is option (c)
Step-by-step explanation:
The macula densa is a group of specialized cells found in the wall of the distal convoluted tubule (DCT) of nephrons in the kidney. These cells are a critical component of the juxtaglomerular apparatus (JGA), which is an endocrine structure involved in regulating the function of nephrons and thus, maintaining blood pressure homeostasis. The macula densa cells monitor the sodium ion (Na+) concentration of the fluid flowing through the DCT and respond to changes of Na+ levels by releasing local signaling molecules like ATP or adenosine.
These chemical messengers impact nearby juxtaglomerular cells, which are modified, smooth muscle cells located in the walls of the afferent arteriole. They can contract or relax to regulate blood flow and hence the glomerular filtration rate (GFR) in response to signaling from the macula densa. The macula densa also triggers the release of the enzyme renin, which is crucial for the renin-angiotensin-aldosterone system (RAAS), a hormone system that regulates blood pressure and fluid balance.