Excretory Products And Their Elimination

The juxtaglomerular apparatus, or JGA, is a specialised region formed by cellular modifications where the distal convoluted tubule contacts the afferent arteriole of the same nephron. NCERT Class 11, Chapter 19 (Excretory Products and their Elimination) explains that a fall in glomerular blood pressure or blood flow activates the JG cells to secrete the enzyme renin, setting off the renin-angiotensin mechanism. Renin acts on angiotensinogen to form angiotensin I, which is converted to angiotensin II, a powerful vasoconstrictor that raises arteriolar blood pressure and the glomerular filtration rate. Aldosterone is the adrenal cortex hormone whose secretion is stimulated downstream by angiotensin II; it is not the substance released by the JGA itself. Atrial natriuretic factor is released by atrial walls in response to high blood pressure and opposes this system by causing vasodilation, so it is the antagonist rather than the answer. Antidiuretic hormone from the posterior pituitary chiefly controls water reabsorption, not the initial enzyme release in this pathway. Hence renin is correct. A regulatory plausibility check supports it: restoring low blood pressure requires an initiating enzyme from the kidney, and renin precisely launches the angiotensin cascade.