Final answer:
The juxtaglomerular apparatus is where the distal convoluted tubule (DCT) intersects with the afferent and efferent arterioles next to Bowman's capsule, with the macula densa being the specific DCT cells responsible for monitoring and responding to changes in fluid composition.
Step-by-step explanation:
The juxtaglomerular apparatus is formed where the distal convoluted tubule (DCT) projects between the afferent arteriole and efferent arteriole next to Bowman's capsule. This complex structure plays a crucial role in regulating renal blood flow and glomerular filtration rate (GFR). The part of the DCT that comes into direct contact with the arterioles is known as the macula densa. These specialized cells monitor the fluid composition, particularly the concentration of Na+, and can trigger the release of renin by the juxtaglomerular cells of the afferent arteriole, influencing blood pressure and electrolyte balance.