Expansion
Afferent constriction lowers GFR; efferent constriction raises it
Expansion
| Change | Effect on GFR | Effect on renal blood flow |
|---|---|---|
| Afferent constriction | Down | Down |
| Afferent dilatation | Up | Up |
| Efferent constriction | Up (then down if severe) | Down |
| Efferent dilatation | Down | Up |
Two hormonal controls dominate:
- Angiotensin II preferentially constricts the efferent arteriole, raising glomerular pressure and defending GFR when renal perfusion falls
- Prostaglandins dilate the afferent arteriole, defending renal blood flow in hypovolaemia
This explains two important drug hazards. ACE inhibitors and angiotensin receptor blockers remove efferent constriction, so in bilateral renal artery stenosis or hypovolaemia the GFR collapses. NSAIDs remove afferent dilatation, with the same effect. Combining them with a diuretic gives the “triple whammy” of acute kidney injury.
Sympathetic stimulation constricts both arterioles, reducing flow and filtration, which is the mechanism of prerenal failure in shock.