Tubuloglomerular feedback (TGF) is a key autoregulatory mechanism. Which cell type and which ion are central to TGF, and what is the net effect when tubular flow increases?

• A Juxtaglomerular cells sense increased NaCl; efferent arteriole dilates, increasing GFR
• B Macula densa cells sense decreased NaCl; afferent arteriole dilates, increasing GFR
• C Podocytes sense increased flow; mesangial cells contract, reducing filtration surface area
• D Macula densa cells sense increased NaCl delivery; afferent arteriole constricts, reducing GFR ✓

Explanation

Macula densa cells in the thick ascending limb sense luminal NaCl concentration via the NKCC2 cotransporter. When GFR rises, more NaCl reaches the macula densa, which signals via adenosine and thromboxane to constrict the afferent arteriole, reducing glomerular capillary pressure and thereby lowering GFR back toward normal. This negative feedback loop is essential for single-nephron autoregulation.

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

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