Final answer:
Extraglomerular mesangial cells facilitate communication between the macula densa and the juxtaglomerular cells, which are part of the juxtaglomerular apparatus. This interaction is important in regulating the GFR and blood pressure through the renal system's response to changes in fluid osmolarity and Na+ concentration.
Step-by-step explanation:
In the context of the renal system and the regulation of kidney function, the extraglomerular mesangial cells act as intermediaries passing regulatory signals between the macula densa and the juxtaglomerular cells (JG cells). The macula densa, with its cluster of cuboidal epithelial cells, monitors the fluid composition, particularly the sodium (Na+) concentration in the fluid passing through the distal convoluted tubule (DCT). When changes in osmolarity or Na+ concentration are detected, the macula densa cells release paracrine signals like ATP and adenosine to the juxtaglomerular cells, which line the afferent arteriole. These modified smooth muscle cells respond by contracting or relaxing, consequently modulating the glomerular filtration rate (GFR) in response to the body's hydration status, thereby regulating blood pressure.
High osmolarity triggers juxtaglomerular cells to contract, reducing GFR and promoting fluid retention, whereas low osmolarity causes them to relax, increasing GFR and enhancing water loss via urine. This functional coordination ensures the balance of filtration rate, fluid retention, and blood pressure. Notably, the macula densa also has a role in regulating renin release from the juxtaglomerular cells, contributing to the renin-angiotensin-aldosterone system (RAAS) which further influences blood pressure and electrolyte balance.