The tubuloglomerular feedback (TGF) mechanism is mediated by the juxtaglomerular apparatus. Which sequence correctly describes TGF when NaCl delivery to the macula densa is high?

• A High NaCl → JGA releases renin → angiotensin II constricts efferent arteriole → increased GFR
• B High NaCl → macula densa releases NO → afferent vasodilation → increased GFR (positive feedback)
• C High NaCl → sympathetic stimulation via macula densa → efferent arteriolar vasoconstriction → decreased GFR
• D High NaCl → macula densa senses via NKCC2 → releases ATP/adenosine → afferent arteriolar constriction → reduced GFR → reduced NaCl delivery (negative feedback) ✓

Explanation

Tubuloglomerular feedback is a negative feedback mechanism. When macula densa cells detect high luminal NaCl (via NKCC2 cotransporter), they release ATP which is converted to adenosine in the interstitium. Adenosine binds A1 receptors on the afferent arteriole, causing vasoconstriction → reduced glomerular capillary pressure → reduced GFR → reduced tubular NaCl delivery. This prevents excessive solute loss and maintains glomerulotubular balance. Macula densa cells also suppress renin release when NaCl is high.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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