What is the significance of juxta glomerular apparatus (JGA) in kidney function ?

Explanation

Macula densa together with JG wells form juxtaglomerular apparatus (JAG). JAG plays a complex regulating role. A fall in glomerular blood flow or glomerular blood pressure or GFR can activate JG cells to release an enzyme called renin into the blood. This catalyses the conversion of angiotensinogen into angotensin-I which is further converted into angiotensin-II by angiotensin converting enzyme. Angiotensin-II, being a powerful vasoconstrictor increase the glomerular blood pressure and there by GFR. Angiotensin-II also activates the adrenal cortex to release aldosterone. Aldosterone causes reabsorption of `Na^(+)` and water from distal convoluted tubule and collecting duct. To reduce loss through urine, and also promote secretion of `K^(+)` ions into distal convoluted tubule and collecting duct. It leads to increase in the blood pressure and GFR. This complex mechanism is generally known as renin - angiotensin - aldosterone system (RAAS).