Which of the following mechanisms directly regulates the constriction or dilation of the afferent arteriole to control GFR?

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Question restatement: Which mechanism directly regulates the constriction or dilation of the afferent arteriole to control GFR?

Option 1: Tubuloglomerular feedback. This is the intrinsic renal mechanism in which the macula densa cells sense NaCl delivery in the distal tubule and adjust afferent arteriole tone to modify GFR accordingly. If NaCl delivery is high, the afferent arteriole constricts to reduce GFR; if it’s low, it dilates to raise GFR. This direct local control of the afferent arteriole is precisely what tubuloglomerular feedback describes.

Option 2: Renin-angiotensin-aldosterone system. The RAS is a systemic hormonal mechanism that primarily increases efferent arteriolar constriction via angiotensin II and can influence GFR indirectly. However, it is not the direct mechanism that governs afferent arteriole constriction in response to tubular flow; its main effect is on efferent tone and overall blood pressure regulation.

Option 3: Sympathetic nervous system. Sympathetic activation can constrict renal arterioles, including the afferent, thereby reducing GFR during stress or hypovolemia. While this does alter afferent tone, it is a neural regulatory pathway rather than the intrinsic, direct tubuloglomerular feedback mechanism that responds to tubular flow and NaCl delivery. Some texts acknowledge neural influence, but it is not the primary direct mechanism described for local control of the afferent arteriole in response to tubular signaling.

Option 4: Glomerular basement membrane. The glomerular basement membrane is part of the filtration barrier, not a mechanism regulating arteriolar tone. It determines what gets filtered, but it does not constrict or dilate the afferent arteriole to control GFR.

In summary, option 1 provides the direct, intrinsic mechanism that links tubular flow to afferent arteriolar tone and thereby regulates GFR, whereas the other options describe regulatory influences that are either indirect, neural, systemic, or structural rather than direct arteriolar regulation.

6529 SEM-2 2025 BRUCE ON-CAMPUS AND 7896 SEM-2 2025 BRUCE ON-CAMPUS Lab Exam 2 Practice Quiz (2nd)

Which of the following mechanisms directly regulates the constriction or dilation of the afferent arteriole to control GFR?

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