Auto-regulation of Glomerular Filtration and the Efferent Arteriole

Glomerular filtration rate (GFR) is kept relatively constant despite changes in renal perfusion pressure through auto-regulatory mechanisms acting on the afferent and efferent arterioles. The correct answer is Angiotensin II mediated vasoconstriction, which is the component that predominantly acts on the efferent arteriole.

Why Angiotensin II is Correct

When renal perfusion pressure falls (e.g., in hypotension or reduced renal blood flow), the juxtaglomerular apparatus releases renin, which generates Angiotensin II via the Renin-Angiotensin-Aldosterone System (RAAS). Angiotensin II preferentially constricts the efferent arteriole at low-to-moderate concentrations. This selective efferent constriction increases glomerular hydrostatic pressure and helps preserve GFR even when renal blood flow is reduced. Because this vasoconstrictor response is triggered by, and directly compensates for, changes in perfusion pressure to keep GFR stable, it is correctly classified as part of the auto-regulatory machinery of the glomerulus, with its principal site of action being the efferent arteriole.

Analysis of Options

• Autonomous myogenic reflex: This mechanism responds to stretch caused by increased arterial pressure by constricting smooth muscle. It acts primarily on the afferent arteriole, not the efferent arteriole, so this option is incorrect.
• Tubuloglomerular feedback (TGF): The macula densa senses NaCl concentration in the distal tubule; increased delivery signals a rise in GFR and triggers release of adenosine/ATP, which constricts the afferent arteriole to bring GFR back down. Its site of action is the afferent, not efferent, arteriole, so this option is incorrect.
• Angiotensin II mediated vasoconstriction: As explained above, this is the correct answer — it selectively constricts the efferent arteriole, raising glomerular capillary pressure to maintain GFR when renal perfusion falls.
• None of the above: Incorrect, since Angiotensin II mediated vasoconstriction is a well-recognized auto-regulatory component that specifically targets the efferent arteriole.

Conclusion

Among the intrinsic mechanisms of renal auto-regulation, the myogenic reflex and tubuloglomerular feedback act on the afferent arteriole, whereas Angiotensin II mediated vasoconstriction acts on the efferent arteriole, making it the correct answer to this question.