How Afferent and Efferent Arteriolar Pressure Changes Affect GFR
Increased afferent arteriolar pressure (dilation) increases GFR by raising glomerular capillary hydrostatic pressure, while increased efferent arteriolar pressure (constriction) initially increases GFR by maintaining glomerular capillary pressure despite reduced renal blood flow, though excessive constriction of both arterioles ultimately decreases GFR. 1
Afferent Arteriole Effects on GFR
Afferent arteriolar dilation increases GFR by allowing more systemic arterial pressure to be transmitted directly into the glomerular capillaries, thereby increasing glomerular capillary hydrostatic pressure (PGC), which is the primary driving force for filtration. 2, 3
Afferent arteriolar constriction decreases GFR by reducing the transmission of systemic pressure to the glomerulus, lowering PGC and reducing the hydrostatic pressure gradient across Bowman's capsule. 1
In hypertension, the glomerular capillary pressure tends to increase because the reduction in afferent arteriolar resistance is greater than the reduction in efferent resistance, allowing excessive pressure transmission to the glomerulus. 3
The afferent arteriole is primarily regulated by the myogenic response and tubuloglomerular feedback (TGF), which maintain GFR constant over a wide range of renal perfusion pressures through autoregulation. 4
Efferent Arteriole Effects on GFR
Efferent arteriolar constriction increases GFR by creating back-pressure that maintains or elevates glomerular capillary hydrostatic pressure, particularly important when renal blood flow is reduced. 1
In states of reduced kidney blood flow (due to declining cardiac output), GFR can be preserved initially by a compensatory increase in filtration fraction from efferent arteriolar constriction as a result of increased renin levels. 1
This mechanism is mediated by angiotensin II, which constricts the efferent arteriole to a greater extent than the afferent arteriole, maintaining glomerular hydrostatic pressure and GFR despite hypoperfusion. 2
Efferent arteriolar dilation decreases GFR by reducing the back-pressure needed to maintain adequate glomerular capillary pressure, thereby decreasing the transglomerular hydrostatic pressure gradient. 5
The Dual Arteriolar Balance
The relationship between afferent and efferent arteriolar resistance is critical for GFR regulation:
When both arterioles constrict excessively, as occurs with frank and persistent reduction in cardiac output, glomerular filtration is reduced, driven by angiotensin II-mediated constriction of both the afferent and efferent arterioles. 1
The filtration fraction (GFR divided by renal plasma flow) increases when efferent constriction predominates, allowing GFR maintenance despite reduced renal blood flow. 1
In essential hypertension, GFR remains normal or elevated while renal blood flow tends to be lower, resulting in an increased filtration fraction due to the balance of these resistances. 3
Clinical Implications
ACE inhibitors and angiotensin receptor blockers cause efferent arteriolar dilation, which decreases transglomerular hydrostatic pressure and thus glomerular filtration rate by reducing the back-pressure effect. 5
This explains why ACE inhibitors can cause acute renal failure in bilateral renal artery stenosis or stenosis to a solitary kidney—the kidney becomes dependent on angiotensin II-mediated efferent constriction to maintain GFR. 2, 5
A 10% to 20% increase in serum creatinine can be anticipated when ACE inhibitor therapy is initiated in patients with chronic renal insufficiency, as this represents reversal of glomerular hyperfiltration through predominant efferent arteriolar vasodilation. 2
Venous Pressure Effects
Increased venous pressure decreases GFR by raising Bowman's space hydrostatic pressure (PB), which opposes the glomerular capillary hydrostatic pressure, thereby reducing the net filtration pressure gradient. 1
- Increased venous pressure transmitted across the tubules leads to increased intratubular pressure, which results in a decreased hydrostatic pressure gradient across Bowman's capsule, with a net effect of reduced single-nephron glomerular filtration rate. 1