Question

Tubuloglomerular feedback control is useful for the regulation of which one of the following?

• A. GFR
• B. Plasma sodium
• C. Plasma volume
• D. Determining tubular secretion

Hint:

Think of the macula densa and juxtaglomerular apparatus and the one renal variable they keep constant.

Answer 1

The Correct Option is A

Step 1: Tubuloglomerular feedback is an intrinsic autoregulatory mechanism of the kidney that keeps the glomerular filtration rate (GFR) stable despite swings in systemic blood pressure.
Step 2: The macula densa cells of the distal tubule sense the sodium chloride load in the tubular fluid. When flow and salt delivery rise, signals are sent back to the afferent and efferent arterioles of the same nephron.
Step 3: Through the afferent limb, adenosine is released and opens \(Ca^{2+}\) channels, constricting the afferent arteriole and lowering filtration. Through the efferent limb, the renin-angiotensin system generates angiotensin II, which constricts the efferent arteriole. Together these adjust filtration pressure and hold GFR constant.
Step 4: Because the entire loop is built to stabilise filtration, the correct answer is GFR. Plasma sodium, plasma volume and tubular secretion are downstream consequences, not the variable that this feedback directly regulates.